Supplement A3: The Chemistry of Double-Bonded Functional Groups. Edited by Saul Patai Copyright 1997 John Wiley & Sons, Ltd.
ISBN: 0-471-95956-1
CHAPTER 24
Advances in the metathesis of olefins
K. J. IVIN |
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The Queen’s University of Belfast† |
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I. ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1499 |
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II. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1499 |
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III. CATALYSTS, INTERMEDIATES, INITIATOR EFFICIENCIES . . . . . |
1502 |
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A. First-generation Catalysts (Non-carbene Catalysts) . . . . . . . . . . . . |
1502 |
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B. Second-generation Catalysts (Carbene Catalysts) . . . . . . . . . . . . . . |
1505 |
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1. |
Fischer complexes (18e) . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1505 |
2. |
Complexes with less than 18e around the metal centre . . . . . . . |
1505 |
3. |
Detection of propagating metallacyclobutane complexes . . . . . . |
1506 |
4. |
Detection of propagating metal carbene olefin complexes . . . . |
1508 |
5. |
Structures; barriers to rotation about MtDC . . . . . . . . . . . . . . |
1509 |
C. Initiator Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1511 |
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D. Theoretical Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1511 |
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IV. ACYCLIC MONOENES NOT CONTAINING FUNCTIONAL |
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GROUPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1514 |
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V. ACYCLIC MONOENES CONTAINING FUNCTIONAL GROUPS . . . |
1517 |
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VI. THE CARBONYL OLEFINATION REACTION . . . . . . . . . . . . . . . |
1519 |
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VII. ACYCLIC DIENE METATHESIS (ADMET) . . . . . . . . . . . . . . . . . . |
1522 |
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A. Ring-Closing Metathesis (RCM) . . . . . . . . . . . . . . . . . . . . . . . . |
1522 |
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1. |
Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1522 |
2. |
Ethers and sulphides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1523 |
3. |
Alcohols, aldehydes, acids, esters and ketones . . . . . . . . . . . . |
1525 |
4. |
Rings containing N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1526 |
5. |
Rings containing P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1530 |
6. |
Rings containing Si, Ge and Sn . . . . . . . . . . . . . . . . . . . . . . |
1531 |
B. ADMET Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1531 |
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C. ADMET Copolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1534 |
† Emeritus Professor of Chemistry. Present address: 12, St. Michael’s Gardens, South Petherton, Somerset, TA13 5BD, UK.
1497
1498 |
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K. J. Ivin |
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VIII. RING-OPENING METATHESIS POLYMERIZATION (ROMP) OF |
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CYCLOALKENES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1534 |
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A. General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1534 |
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1. |
Ring |
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chain equilibria . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1534 |
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2. |
Use of transfer agents; telechelic polymers . . . . . . . . . . . . . . . |
1534 |
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3. |
Cis/trans blockiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1535 |
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4. |
Head |
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tail bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1536 |
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5. |
Tacticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1539 |
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B. Monocyclic Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1545 |
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1. |
Four-membered rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1545 |
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2. |
Five-membered rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1547 |
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3. |
Six-membered rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1549 |
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4. |
Seven-membered rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1549 |
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5. |
Eight-membered rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1550 |
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6. |
Larger rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1553 |
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C. Polycyclic Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1554 |
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1. |
Monomers containing a fused cyclobutene ring . . . . . . . . . . . . |
1554 |
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2. |
Benzvalene and deltacyclene . . . . . . . . . . . . . . . . . . . . . . . . |
1557 |
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3. |
Norbornene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1558 |
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4. |
Monosubstituted alkylnorbornenes . . . . . . . . . . . . . . . . . . . . |
1561 |
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5. |
Norbornenes with a 5-(Si-containing) substituent . . . . . . . . . . . |
1562 |
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6. |
Norbornenes with a 5-(COOR) substituent . . . . . . . . . . . . . . . |
1562 |
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7. |
Norbornenes with a 5-(OCOR) substituent . . . . . . . . . . . . . . . |
1564 |
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8. |
Norbornenes with 5-substituents containing OH or OR . . . . . . . |
1565 |
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9. |
Norbornenes with 5-substituents containing CN or halogen . . . . |
1565 |
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10. |
Other 5-substituted norbornenes . . . . . . . . . . . . . . . . . . . . . . |
1565 |
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11. |
5,5-Disubstituted norbornenes . . . . . . . . . . . . . . . . . . . . . . . |
1566 |
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12. |
5,6-Disubstituted norbornenes . . . . . . . . . . . . . . . . . . . . . . . |
1567 |
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13. |
Other polysubstituted norbornenes . . . . . . . . . . . . . . . . . . . . |
1571 |
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14. |
Norbornadiene and its monosubstituted derivatives . . . . . . . . . |
1572 |
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15. |
Polysubstituted norbornadienes . . . . . . . . . . . . . . . . . . . . . . |
1574 |
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Dicyclopentadienes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1577 |
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17. |
Bicyclo[2.2.1] compounds containing heteroatoms in the ring |
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system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1579 |
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18. |
Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1583 |
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IX. COPOLYMERIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1584 |
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A. Direct Metathesis Copolymerization . . . . . . . . . . . . . . . . . . . . . . |
1584 |
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B. Block Copolymers by Sequential Addition of Monomers |
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to Living Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1586 |
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C. Block Copolymers by Modification of Homopolymers . . . . . . . . . . |
1587 |
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D. Comb and Graft Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . |
1588 |
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Comb copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1588 |
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2. |
Copolymers with short grafts . . . . . . . . . . . . . . . . . . . . . . . |
1588 |
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3. |
Copolymers with long grafts . . . . . . . . . . . . . . . . . . . . . . . . |
1589 |
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E. Copolymers by ROMP in Conjunction with Radical Reactions . . . . |
1589 |
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X. POLYMERIZATION OF ACETYLENES BY OLEFIN |
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METATHESIS CATALYSTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1590 |
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A. Proof of Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1590 |
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B. Metathesis Polymerization of Acetylene . . . . . . . . . . . . . . . . . . . |
1591 |
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C. Metathesis Polymerization of Monosubstituted Acetylenes . . . . . . . |
1591 |
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D. Metathesis Polymerization of Disubstituted Acetylenes . . . . . . . . . |
1592 |
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24. Advances in the metathesis of olefins |
1499 |
E. Metathesis Polymerization of Diynes; Cyclopolymerization . . . . . . . |
1593 |
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F. Copolymerization of Acetylenes . . . . . . . . . . . . . . . . . . . . . . . . |
1595 |
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XI. INTRAMOLECULAR METATHESIS REACTIONS OF ENYNES |
1596 |
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AND DIENYNES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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XII. METATHESIS REACTIONS OF ALKYNES INVOLVING |
1597 |
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TOTAL CLEAVAGE OF THE C C BOND . . . . . . . . . . . . . . . . . . . |
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A. Acyclic Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1597 |
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B. ROMP of Cycloalkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1598 |
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C. Acyclic Diynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1599 |
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XIII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
1599 |
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I. ABBREVIATIONS |
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ADMET |
acyclic diene metathesis |
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COSY |
correlation spectroscopy |
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CTA |
chain transfer agent |
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DBC |
double bond cleavage |
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HT |
head tail |
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MO |
molecular orbital |
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MWD |
molecular weight distribution |
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RCM |
ring-closing metathesis |
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ROMP |
ring-opening metathesis polymerization |
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SAXS |
small-angle X-ray scattering |
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TBC |
triple bond cleavage |
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TEM |
transmission electron microscopy |
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ZNP |
Ziegler Natta polymerization |
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II. INTRODUCTION
The olefin metathesis reaction was so named by Calderon1 in 1967 following the discovery that it involved the total cleavage of the CDC bond and the apparent exchange of alkylidene moieties between two alkene molecules (equation 1).
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Such reactions are chain reactions catalysed by compounds of many of the transition elements, particularly Ti, Nb, Mo, Ru, Ta, W, Re, Os and Ir, and occasionally V, Co and Rh. A cocatalyst such as EtAlCl2 or Me4Sn is often, though not always, required. It was first postulated2 in 1971 and later proved beyond doubt that both the initiating and propagating species of the chain reaction were metal carbene complexes, the initiation and propagation reactions being two-step processes, represented by the framework in equation 2, involving the intermediate formation of a metallacyclobutane complex. (Mt denotes the transition metal centre; M will later denote monomer.)
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1500 |
K. J. Ivin |
Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators.
With cycloalkenes the metathesis reaction leads to long-chain unsaturated polymers together with unsaturated cyclic oligomers formed by intramolecular metathesis reactions of the propagating species. These are termed ring-opening metathesis polymerizations (ROMP). Dienes can undergo either intramolecular metathesis with ring closure, or intermolecular metathesis leading to a high polymer with the elimination of a small olefin (a type of condensation polymerization). The intramolecular reaction dominates when it is thermodynamically favoured (at low substrate concentration, and/or with conformational restrictions to bring the two double bonds into close proximity). Such ring-closing metathesis (RCM) reactions have proved of great synthetic value. Alkynes are also polymerized by olefin metathesis catalysts and it is now established that here too the reactions are propagated by metal carbene complexes. Equation 3 shows the framework for the addition of the first molecule of alkyne to the metal carbene initiating species, with the intermediate formation of a metallacyclobutene complex.
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The alkyne thus behaves as a quasi-cycloalkene, two of the three C C bonds being broken in the propagation step.
Alkynes can also undergo total metathesis, with cleavage of all three C C bonds, catalysed by metal carbyne complexes at room temperature and proceeding through metallacyclobutadiene intermediates as indicated by the framework in equation 43 6.
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For a cycloalkyne the product is a ring-opened polymer containing C C bonds; cyclic oligomers can also be formed7,8.
Scheme 1 summarizes the interrelationships of the various types of olefin and acetylene metathesis reaction that are observed for single substrates. When there are two substrates present then so-called cross-metathesis reactions occur, for example between ethene and a higher olefin (ethenolysis), or between a cycloalkene and an acyclic olefin (which acts as a chain transfer agent and reduces the molecular weight of the polymer formed), or between two cycloalkenes (giving a copolymer). With metal carbene complexes as initiators one can often obtain ‘living’ systems (free from termination reactions) allowing the preparation of well-defined homopolymers or block copolymers of very narrow molecular weight distribution (Mw/Mn < 1.1). Cross-metathesis reactions can also occur between alkenes and alkynes, and the intramolecular metathesis reactions of enynes and dienynes can sometimes provide a valuable step in a synthetic strategy.
Before passing to more detailed considerations it should be noted that the reactions of metal carbenes are part of a much wider family of [2 C 2] reactions between multiplybonded compounds in which one of the reactants involves an MtDX bond. Equation 5
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24. Advances in the metathesis of olefins |
1501 |
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OLEFI N and ACETYLENE METATHESES |
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Alkenes |
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Acyclic |
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ADMET |
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(RCM) |
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ROMP |
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SIMPLE |
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METATHESI S |
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METATH. |
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METATH. |
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SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage; TBC, triple bond cleavage; ADMET, acyclic diene metathesis; RCM ring-closing metathesis; ROMP ring-opening metathesis polymerization
TABLE 1. Some types of observed [2 C 2] reactions involving metal atoms attached to double bonds (other than those involved in olefin metathesis); cf. equation 5
Multiple bondsa |
Type of compound |
Notes |
References |
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MtÐ Ð ÐX |
YÐ Ð ÐZ |
containing YÐ Ð ÐZ |
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TiDC |
CDO |
ketone |
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9 |
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MoDC |
CDO |
aldehyde |
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10 |
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ˇ- or -hydroxyketone |
b |
11 |
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WDC |
CDO |
ketone, aldehyde |
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13 |
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WDC |
CDN |
carbodiimide |
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WDP |
PDP |
imine |
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diphosphene |
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16 |
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OsDC |
SDO |
sulphur dioxide |
c |
17 |
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TaDC |
C O |
carbonyl complex |
d |
18 |
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ZrDN |
CDN |
imine |
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19 |
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WDN |
CDN |
carbodiimide |
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14 |
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ZrDN |
C C |
alkyne |
e |
19 |
aThe products correspond to the formation of an initial metallacycle MtXYZ. bThe hydroxyl group is essential for the reaction to succeed.
c Reaction stops at the metallacycle.
dProduct formed by rearrangement rather than cleavage of the metallacycle. eProduct is an unsaturated metallacycle.
1502 |
K. J. Ivin |
indicates the reacting framework and Table 1 gives some examples.
Mt |
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There are similar [2 C 2] reactions involving W X and Mo X bonds20 |
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25. |
26 |
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For a recent comprehensive account of the subject, see the book by Ivin and Mol |
, |
which updates an earlier book27. An extensive collection of papers concerning metathesis catalysts may be found in the proceedings of a NATO Advanced Study Institute28.
III.CATALYSTS, INTERMEDIATES, INITIATOR EFFICIENCIES A. First-generation Catalysts (Non-carbene Catalysts)
The first metathesis catalysts were discovered by chance, arising out of observations on (i) the heterogeneously catalysed reactions of olefins using MoO3 supported on Al2O329 31, and on (ii) the attempted Ziegler Natta polymerizations of cycloalkenes using TiCl4/LiAlH4, MoCl5/Et3Al, or WCl6/Et3Al32,33. The Calderon catalyst WCl6/EtOH/EtAlCl2 (1/1/4) was later found to be extremely effective for both ROMP of cycloalkenes34 and metathesis of acyclic alkenes1. The EtOH converts some of the chloride ligands at the tungsten centre to ethoxy ligands thereby making the system more active; see Schrock35 for a recent discussion of the role of OR ligands in olefin metathesis reactions.
The Calderon catalyst has two disadvantages: (i) the Lewis acids in the system are liable to cause double-bond shift reactions, and (ii) it is vulnerable to destruction by polar groups in the substrate. These problems were overcome by using R4Sn (R D Me, Bu, Ph) in place of EtAlCl2 as cocatalyst with WCl6; the metathesis of unsaturated esters such as methyl oleate can then be achieved36. In the system WCl6/(13CH3)4 Sn one can observe the initiating metal carbene species by both 1H and 13C NMR, but the initiation efficiency for the ROMP of norbornene is low (<0.7%) and the propagating species cannot be detected37.
When there is no cocatalyst the initiating metal carbene complex must result from a reaction between the catalyst and the substrate olefin. In the case of Re2O7/Al2O3 (activated at 550 °C) its interaction with but-2-ene to form [Re]DCHCH3 can be clearly demonstrated by the so-called ‘chemical counting’ method. After removing the excess but- 2-ene by evacuation the catalyst is treated with ethene. This reacts with [Re]DCHCH3 to form propene, the amount corresponding to about 1.8% of the total Re in the catalyst38. At low Re content (<3% Re2O7) the activity of this catalyst is extremely low, but rises very rapidly as the Re content is increased (>7%). The explanation for this behaviour is based on an examination of the surface OH groups by FTIR. At low loadings ReO4 ions have reacted mainly with the basic surface alumina OH groups during deposition of the rhenium compound, while at higher loadings the neutral and more acidic OH groups have
also reacted, the latter resulting in the most active sites39. Similar behaviour is observed for MoO3/Al2O340.
R4Sn or R4Pb causes a spectacular improvement of the catalytic performance of Re2O7/Al2O3, raising the rate of propene metathesis by 10 100 and also making possible the metathesis of functionalized alkenes where previously no reaction was observed41 43. When R D Me some methane is produced, suggesting that the reaction [Re](CH3)2 ! [Re]DCH2 C CH4 is involved in the production of the initiating species.
24. Advances in the metathesis of olefins |
1503 |
The active centres generated in this way appear to be different from those present in the unpromoted catalyst44,45.
Another example of enhancement of catalytic activity of a heterogeneous catalyst by appropriate pretreatment of the catalyst is observed with MoO3/SiO2 and MoO3/SiO2 Al2O3. Best results for the metathesis of propene are obtained if the calcined catalyst is first photoreduced in CO at room temperature, using a mercury lamp or laser ( D 308 nm), and then exposing the catalyst to cyclopropane followed by heating at 350 °C. Molybdenum carbenes are formed, as shown by both IR and UV/vis spectra46,47, and are assumed to result from the sequence of reactions shown in Scheme 248 50. A small proportion (<5%) of the molybdacyclopropane complexes yields propene by reductive elimination at 350 °C. The reaction of methylcyclopropane yields both [Mo]DCH2 and [Mo]DCHCH351,52.
The nature of the initiating complex can sometimes be deduced from the product of its reaction with a carbene trap. For example, the seven-coordinate, 18e complex WCl2(CO)3(AsPh3)2 catalyses the ROMP of norbornene in benzene at 80 °C, presumably through loss of a CO ligand followed by coordination of the norbornene, and rearrangement to a tungsten carbene complex which then propagates the ROMP. In the presence of benzaldehyde as carbene trap, polymerization is inhibited and the main product is 2- benzylidenenorbornane 3. One may therefore conclude that its precursor 2, produced by a 2,3-hydrogen shift in the tungsten olefin complex 1, is the initiating tungsten carbene complex53.
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PhCH
(3)
Among the more novel non-carbene metathesis catalysts recently reported are the following.
(1) The organoisopolymolybdate [(C13H27)3NH]4[Mo8O26] is soluble in endo- dicyclopentadiene and, with Et2AlCl as cocatalyst, can be used to effect smooth ROMP without added solvent. This is a considerable advantage when using the reaction injection moulding technique54.
(2) WOCl3(OAr), WOCl2(OAr)2 or WOCl(OAr)3 in conjunction with R3SnH are reported to be very effective for the ROMP of dicyclopentadiene, the activity increasing with the electron-withdrawing power of the OAr ligand55; also for the metathesis of pent-2-ene when brought onto a support56,57. The related imido complexes, W(DNAr)(Cl)4 x (OAr)x are also effective58.
(3) Cobalt neodecanoate or acetylacetonate in combination with R3Al (R D Et, i-Bu) in heptane at 20 °C initiates the ROMP of norbornene, giving an 80% yield of an all-cis polymer in 3 days. This is the first report of a cobalt-based metathesis catalyst59.
1504 |
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K. J. Ivin |
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O− |
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− CO2 |
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20˚C |
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350˚C |
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SCHEME 2. Generation of molybdenum carbenes by the reaction of cyclopropane with Mo(IV)/SiO2
(4) Metal carbenes have long been proposed as intermediates in the reactions of diazoalkanes; see Schwab and coworkers60 for references extending back to 1952. A recent example in olefin metathesis is Ru(H2O)6(OTs)2 the activity of which is enhanced by the addition of ethyl diazoacetate to the extent that it will then bring about the ROMP of less strained monomers such as cyclopentene and cyclooctene61. Likewise RuCl2(p-cymene) (PCy3), when treated with Me3SiCHN2, gives a very active catalyst for promoting the ROMP of functionalized norbornenes and cyclooctenes (Cy D cyclohexyl); both the initiating species [Ru]DCHSiMe3 and the propagating species can be detected by NMR62. Unassisted non-carbene compounds of ruthenium, such as RuCl3, do not generally catalyse the ROMP of low-strain cycloalkenes because of the difficulty of generating the initial metal carbene complex by reaction with the substrate.
(5)An extensive series of molybdenum nitrosyl complexes have been prepared which are very active in the presence of a cocatalyst, e.g. [Mo(NO)2(OEt)2]/EtAlCl263 73.
(6)The polystyrene-bonded complexes Pol CH2 ( 5-C5H4) W(CO)3R (R D H, Cl, Me), when activated by i-BuAlCl2, are reported to be highly active for the metathesis of
internal olefins, though accompanied by double-bond migration reactions74.
(7) Three reports have appeared recently in which it is claimed that catalysts not containing a transition metal can bring about metathesis; first, that magnesium chloride can effect the ROMP of norbornene and other strained monomers75; second, that Al2O3/Me4Sn brings about the metathesis of propene, hex-1-ene and pent-2-ene76; third, that SiO2 alone, activated by evacuation at high temperature, catalyses the metathesis of deuterated ethene with propene under photoirradiation77. The mechanisms of these reactions need further investigation.
24. Advances in the metathesis of olefins |
1505 |
The first-generation catalysts are still preferred for many synthetic or commercial applications, e.g. RCM reactions using trans-WOCl2(OAr)2/Et4Pb (1/2) where Ar D 2, 6- dibromophenyl78. However, the metal carbene catalysts provide much more detail about the mechanism and are being used increasingly for synthetic applications.
B. Second-generation Catalysts (Carbene Catalysts)
1. Fischer complexes (18e)
Among the first 18-electron (18e) Fischer-type metal carbene complexes to be used as part of an olefin metathesis catalyst system were W[DC(OMe)Et](CO)5 with Bu4NCl (for pent-1-ene)79, and W[DC(OEt)Bu](CO)5 with TiCl4 (for cyclopentene)80. These complexes may also be activated thermally, e.g. for the polymerization of alkynes81, or photochemically, e.g. for the ROMP of cycloocta-1,5-diene82. The essential requirement is that a vacancy be created at the metal centre to allow the substrate to enter the coordination sphere. Occasionally the substrate may itself be able to displace one of the CO ligands.
2. Complexes with less than 18e around the metal centre
Much more effective are metal carbene complexes that are both electron-deficient (<18e) and coordinatively unsaturated (usually with 4 or 5 ligands). Since 1980 a large number of these have been prepared, thanks to the pioneering work of the groups of Schrock, Osborn, Grubbs and Basset. The main examples are listed in Table 2. Some but not all can bring about the metathesis of pent-2-ene and low-strained cycloalkenes. For molybdenum and tungsten carbenes the activity of the initiator can be much enhanced by the use of electron-withdrawing ligands, making the metal centre more attractive towards the olefinic substrate. Thus for the family of complexes related to 7, in which the alkoxy ligands are progressively substituted, the reactivity increases in the order OCMe3 < OCMe2(CF3) < OCMe(CF3)2 < OC(CF3)3. The analogous tungsten complexes are more reactive but more prone to side reactions87. With 10 the reactivity is greatly enhanced by the presence of an equivalent of GaBr3 which essentially removes a bromide ion to form an ion-pair. Mo and W complexes that are sufficiently electrondeficient are capable of bringing about the metathesis of pent-2-ene, whereas the less electron-deficient complexes can only cause the metathesis of strained olefins such as norbornene. On the other hand, with the ruthenium carbene complexes 18 and 19 it is the one with the better -donating alkylphosphine ligands (PCy3) that is the more active and able to metathesize pent-2-ene. This has been explained in terms of the increased stability conferred on the intermediate metallacyclobutane by the better -donor, the metal centre being formally Ru(IV)110.
The isolation of the ruthenium carbene complexes of the type 20 represents a major step forward for olefin metathesis. Not only are these complexes easy to prepare but they are stable to air and water, unlike the molybdenum and tungsten carbene complexes which must be handled in a dry box. Furthermore, they are highly efficient initiators. Thus for the ROMP of norbornene (M) initiated by Ru(DCHC6H4X-p)(Cl)2(PPh3)2 the ratio of the initiation to propagation rate constants ki/kp ranges from 9 for X D H to 1.2 for X D Cl in C6D6 at 17 °C. This means that for [M]0/[Ru]0 D 20, the initiator is all converted to propagating species at a very early stage in the reaction, i.e. the initiator is 100% efficient. In this respect they are better than 18 and 19 which are 20 1000 times less active60,91. The one-pot synthesis of Ru(DCHPh)(Cl)2(PCy3)2 is shown in equation 6. The second stage must be carried out immediately after the first. The product is obtained as a purple
1506 |
K. J. Ivin |
TABLE 2. Examples of metal carbene complexes with a count of less than 18 electrons and their effectiveness as initiators of olefin metathesisa
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Complexb |
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Metathesis reactivity |
References |
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pent-2-ene |
norbornene |
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4 |
Nb(DCHCMe3)(Cl)(OCMe3)2(PMe3) |
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yesc |
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d |
92 |
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5 |
Ta(DCHCMe3)(OAr)3(THF) |
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yes |
yese |
93,94 |
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6 |
Ta(DCHCMe3)(TIPT)3 |
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no |
yes |
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93,94 |
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7 |
Mo(DCHCMe2R)(DNAr)(OCMe3)2 (R D Me, Ph) |
no f |
yes |
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95 |
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8 |
Mo(DCHCMe2R)(DNAr)[OCMe(CF3)2]2 |
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yes |
yes |
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9 |
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W( CHCMe |
3)(O)(Cl)2(PEt3) |
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10 |
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W(DCHCMe3)(Br)2(ONp)2 |
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yes |
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11 |
D |
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W( CHCMe3)(Cl)(Np)(OAr0 )2(O-i-Pr2) |
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12 |
[W]DCHCMe3 |
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yes |
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D |
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yes |
yes |
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W[ CHC6H4(OMe)-2]( NAr00 )[OCMe(CF3)2 |
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14 |
W(DCHSiMe3)(DNPh)(CH2SiMe3)L |
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no k |
yes |
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15 |
Re(DCHCMe3)( CCMe3)[OCMe(CF3)2]2 |
l |
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yes |
yes |
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105 |
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16 |
Re(DCHCMe3)( CCMe3)(CH2CMe3)(OTf)L |
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yesm |
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17 |
Re( CHCH CPh )(O)[OCMe(CF ) ] (THF) |
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107 |
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18 |
D |
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2 |
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n 3 2 3 |
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no p |
yes |
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108,109 |
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Ru(DCHCHDCPh2)(Cl)2(PPh3)2 n |
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19 |
Ru(DCHCHDCPh2)(Cl)2(PCy3)2 |
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yes |
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20 |
Ru( |
CHR)(Cl)2 |
(PR0 )2 |
; R0 |
D |
PPh3, PCy ; |
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R D H, Me, Et, Ph, p-ClC6H4
aFor synthetic routes to metal carbene complexes see elsewhere60,83 91.
bThe W counterparts of 7 and 8 are denoted as 7W and 8W, respectively, in the text. Ar, C6H3-i-Pr2-2,6;
Ar0 , C6H3-Ph2-2,6; Ar00 , C6H3-Me2 |
-2,6; TIPT, S C6H2-i-Pr3-2,4,6; Np, CH2CMe3; Cy, cyclohexyl; OTf, triflate. |
c Short-lived. |
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dAt 50 °C, via isolable metallacyclobutane complex. |
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eReaction inhibited in THF. |
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fAlso active for terminal olefins. |
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gBecomes very active in the presence of a Lewis acid such as AlBr3 or GaBr3, which removes a bromide ion from the complex.
hBecomes much more active in the presence of GaBr3. iSee text.
jL is a ligand such as 8-quinolinolate containing a nitrogen chelated to the tungsten. k Much retarded in THF.
lL D MeCN; short-lived (<1 h). mWhen activated by GaBr3.
nMixture of isomers in which the phosphine ligands are either cis (20%) or trans (80%).
pRate dependent on solvent: CD2Cl2 > C6D6 > THF-d8 (relative rates 103:26:11, respectively).
microcrystalline solid by precipitation in methanol (99% yield).
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2.0 equiv PhCHN2 |
2.2 equiv PCy3 |
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RuCl2 |
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pentane |
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D |
CHPh)(Cl)2 |
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6 |
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! ! |
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°C to |
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3. Detection of propagating metallacyclobutane complexes
When catalysts of the type 10/GaBr3 are used to initiate the ROMP of norbornene derivatives at low temperature ( 50 °C) the intermediate transoid metallacyclobutane complexes (the precursors to the formation of trans double bonds) may be observed. The corresponding cisoid complexes are not stable enough to be detected. No metallacyclobutane complexes are observed in the absence of GaBr3100,112 114.