Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1497 161

24. Advances in the metathesis of olefins

1507

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21 24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0 60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9:1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct 1H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms.

O	O	O	O
	CO2 Me	CF3	CN
	CO2 Me	CF3	CN
(21)	(22)	(23)	(24)

In some cases the metallacyclobutane complexes can be isolated and their crystal structure determined. Thus, for 25 the geometry about the molybdenum atom is squarepyramidal (with NAr at the apex), the CMe2Ph substituent is trans to the norbornene ring with the phenyl group directed towards this ring and the MoC3 ring is planar. The distance between Mo and O (3.32 A)˚ indicates that there is no significant bonding between them. In the reaction of 24 the 1H NMR spectrum shows that two square-pyramidal transoid metallacycles are formed, in one of which the cyano group closest to CMe2Ph is in the endo position (65%), see 26, while in the other (not shown) it is in the exo position (35%).

	Ar	CMe2 Ph	Ar	CMe2 Ph
	N
			N
	Me3 CO		Me3 CO
	Mo		Mo
	Me3 CO		Me3 CO	CN
			CF3
		O		O
		CF3		CN
	(25)		(26)

The half-life for the rearrangement of 25 to the metal carbene adduct in C6D6 at 35 °C is 22 h, with an activation energy of 97 kJ mol 1. When the substituent is CMe3 in place of CMe2Ph the rate constant of rearrangement increases five-fold. The rate goes up by another order of magnitude if the CF3 groups are replaced by CO2Me, and by yet another order of magnitude for the rearrangement of the metallacyclobutane derived from 21 (half-life ca 1 h at 0 °C). The stabilization brought about by the CF3 groups is attributed to their inductive effect. Replacement of the Me3CO ligands by Et3CO ligands reduces the rate of rearrangement of the metallacyclobutane (R D Ph) derived from 22 by a factor of six115.

Square-pyramidal metallacycles are not observable at 25 °C upon adding 7-oxa- norbornadiene derivatives to Mo(DCHMe2R)(DNAr)(OCMe2CF3)2 except as a transient

1508	K. J. Ivin

red colour, rapidly changing to the characteristic orange colour of the living carbene complexes. However, the metallacycles can be observed at low temperature115.

4. Detection of propagating metal carbene olefin complexes

It has generally been assumed that in olefin metathesis reactions the olefin first coordinates to the metal carbene complex, en route to the formation of the intermediate metallacyclobutane complex, and that after cleavage of this intermediate the newly formed double bond is temporarily coordinated to the metal centre. A number of stable metal carbene olefin complexes are known; see elsewhere116,117 for earlier references. They are mostly stabilized by chelation of the olefin and/or by heteroatom substituents on the carbene, although some have been prepared which enjoy neither of these modes of stabilization118,119.

The only direct evidence for the presence of metal carbene olefin intermediates in catalytic metathesis systems comes from a study of the interaction of the tungsten cyclopentylidene complex 27 with cycloalkenes such as cycloheptene 28 in CD2Cl2. When these are mixed at 96 °C and the temperature raised to between 53 and 28 °C, no polymerization occurs but the 13C NMR spectrum contains additional resonances which may be assigned to the metal carbene olefin complex 29. The line intensities show that the equilibrium 7 moves to the right as the temperature is lowered120.

				OR	Br	2
OR Br			GaBr4 −	+W		3
						1
W	+		7	7′		3′
						2′
Br3 Ga Br		8		RO	8′
OR				9′
		9
				10	4	5 6
(27) OR = OCH2 CMe3	(28)		(29) OR =
					OCH2 CMe3 (7)

The nature of 29 is established by the presence at 73 °C of a signal at υ 355.3 (1JWC D 142 Hz), which is assigned to the carbene carbon (C-1) of the cyclopentylidene unit, and of two signals at υ 124.4 and 107.8 (1JCH D 165 Hz) corresponding to two non-equivalent olefinic carbons, upfield from the olefinic carbon signal of 28 at υ 132.4 (1JCH D 154 Hz). The assumption that GaBr4 has left the coordination sphere of tungsten to yield a cationic complex is based on the large low-field shift of the alkylidene carbon and on the substantial decrease of 1JWC, which suggest that the tungsten centre is significantly more electrondeficient in 29 than in 27 (υ 335.6 and 1JWC D 169 Hz). The olefin occupies an apical site of the trigonal bipyramidal geometry with its CDC axis aligned with the WDC axis and with its ‘plane’ parallel to that of the cyclopentylidene ligand. This particular conformation is the one that would most easily lead to a metallacyclobutane.

The 1H NMR spectra are in keeping with this interpretation, showing that the two hydrogens of each ˇ-methylene group of the cyclopentylidene ligand, as well as those of the neopentoxy methylene groups, are non-equivalent. However, the two signals for the olefinic hydrogens give only one multiplet at υ 5.64 which is little shifted from the corresponding signal (υ 5.76) in 28. Such small changes, compared with those for nonalkylidene d2 olefin complexes, tend to show that the complex 29 should be viewed as a

24. Advances in the metathesis of olefins

1509

d0 metal complex in which the olefin is bound mainly through donation of its electrons into an empty orbital of tungsten. A weak additional interaction of the Ł orbital with theelectrons of the WDC bond may, however, be at the origin of the parallel configuration and of the substantial barrier to rotation (see below).

The equilibrium constant for the formation of 29, determined from the NMR spectrum at 38 °C, is 4.5 M 1; the temperature variation gives H° D 57 kJ mol 1 andS° D 230 J K 1 mol 1. Raising the temperature to 33 °C leads to the coalescence of the olefinic carbon signals C-7/C-70 (υ 124.4, 107.8), reversible on cooling. Likewise the pairs of peaks for C-3/C-30 and C-5/C-50 coalesce to singlets at 58 and 63 °C respectively, while the two AB (2H) patterns due to the pairs of geminal protons attached to C-2 and C-20 simplify to a single AB (4H) pattern above 48 °C, and the same applies to the two AB patterns due to the two OCH2 groups. Consideration of the splittings as a function of coalecence temperature shows that they result from a single intramolecular dynamic process with an energy barrier of 44 kJ mol 1. In this process, equilibration of the two neopentoxo ligands and of the two sides of both cycloheptene and cyclopentylidene ligands occurs, leaving their CH2 protons non-equivalent. This can only be reconciled with the occurrence in 29 of olefin ligand rotation about the tungsten olefin axis (equation 8). Such rotational barriers with one rotamer favoured at low temperature could clearly be of importance in determining the stereochemistry of propagation reactions in olefin metathesis.

	OR Br		OR
GaBr4	− +	W	Br

	GaBr4 −	+	(8)
RO			W
		RO

When the temperature is raised above 18 °C, the ROMP of cycloheptene begins to occur, indicating that 29 can be considered as a true intermediate in this system. No further intermediates are, however, detected and hardly any of the initiator is consumed showing that propagation is very much faster than initiation.

Similar observations have been made on replacing bromo ligands by chloro ligands, cyclopentylidene by cyclohexylidene or cycloheptene by cyclooctene. However, when one bromo ligand is replaced by neopentoxo, or cyclopentylidene by neopentylidene, no intermediate can be detected, even though the ROMP of 28 occurs at 33 °C. No interaction is found between 27 and cyclohexene, and no ROMP occurs, suggesting that ring-strain relief is involved in the formation of the cycloalkene adducts as well as in their actual ROMP. Replacing the cyclopentylidene ligand by n-pentylidene leads to a metallacyclobutane complex as the main observable intermediate and the ROMP of 28 starts at even lower temperature ( 53 °C), this behaviour being similar to that for norbornene and its derivatives, probably arising from further ring-strain relief on conversion of the metal carbene olefin complex into the metallacyclobutane120.

These results show that subtle changes in the nature of the metal carbene initiator or of the substrate can lead to important modifications in the relative energy levels of the three types of intermediates involved in catalytic olefin metathesis reactions.

5. Structures; barriers to rotation about MtDC

The structures of 8,	10, 12 and 20a (R D p-ClC6H4, R0 D PCy3) are
known. The geometries	around the metal centres are approximately tetrahedral (12e),

1510 K. J. Ivin

trigonal bipyramidal (12e), octahedral (14e) and square pyramidal (16e), respectively.

		Ar							Ar
		N	H						N		CM e2 Ph
		M o	C	ka/s					M o	C	(9)
	(CF3 )2 M eCO					(CF3 )2 M eCO
				ks/a
	(CF3 )2 M eCO		CM e2 Ph			(CF3 )2 M eCO					H
	(8a) anti		K = ka/s / k s/a					(8b) syn
											Cl
	Me3 CCH2 O Br		H				CHCMe3			H
	W C				Cl	W		OEt2			C
					O		Ph
	Me3 CCH2 O
		Br	CMe3			O			Cy3 P		Ru
										Cl	Cl
						Ph					PCy3
	(10)		(12)								(20a)

As indicated, the complex 8 exists in two forms, 8a (anti) and 8b) (syn), in equilibrium (equation 9). The syn rotamer is the dominant form in toluene at 25 °C (K D ka/s/ks/a D 1450); the anti form is difficult to detect in routine NMR spectra. However, the equilibrium can be displaced by UV-irradiation (366 nm) of the solution for several hours at 80 °C to yield a mixture containing about 33% of the anti rotamer as determined from the H˛ resonances: syn, υ 12.11, JCH D 120.3 Hz; anti, υ 13.30, JCH D 153.3 Hz. On adding 0.33 equiv of 2,3-bis(trifluoromethyl)norbornadiene to this solution and running the spectrum again at 80 °C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product (equation 10), while the syn rotamer has scarcely reacted at all. It is estimated that the anti rotamer is at least 100 times more reactive than the syn rotamer121,122. The possibility of the presence of different active species having different reactivities must clearly be borne in mind in other metathesis systems and may be of profound importance for the mechanism of formation and structure of the product. The new CDC double bond formed in reaction 10 is trans as shown by the value of 15.4 Hz for JHH. Further irradiation of the reaction mixture (containing 0.67 equiv of syn initiator and 0.33 equiv of syn first-addition product) results in the isomerization of some of each to the anti isomers. In THF the coordination of the solvent alters the position of equilibrium (K D 23 for 8 at 25 °C) and reduces the rate of attainment of equilibrium, but qualitatively the same effects are observed as in toluene.

Ar

CF3

CF3

CF3

Ar

H

N

H

H

N

RO

Mo

C

CF3

RO

Mo

C

RO

CMe2 Ph

CMe2 Ph

RO

H

anti initiator

syn first-addition product

(10)

24. Advances in the metathesis of olefins

1511

While certain ethers, such as THF and DME, may be strongly bound and have a moderate or strong retarding effect on metathesis, as for 6, 15 and 19, other solvents may be less strongly bound and have no serious effect on the rate of metathesis, as for 11 and 12 where the loosely bound ether in the octahedral complex is readily displaced by the substrate.

In the complex 10 the neopentoxy ligands are non-equivalent and no rotation of the carbene ligand can be observed below the decomposition temperature (>150 °C)123, cor-

responding to a barrier ( G‡) of >96 kJ mol 1. In the complex 20a the structure is close to square-pyramidal, the bond angles being Cl Ru Cl 167.6°; P Ru P 161.1°; Cl Ru P 87.2°, 90.8°, 91.5° and 86.5°; P RuDC 97.5° and 101.2°; Cl RuDC 88.7° and 103.7°. The aryl ligand is only slightly twisted out of the Cl2RuDC plane60.

The ease with which the geometry of the metal carbene complexes can adjust to accommodate the incoming olefin may be an important factor in determining the rate and stereoselectivity in a given metathesis reaction124.

C. Initiator Efficiencies
Initiation	I C M ! P1	ki
Propagation	Pn C M ! PnC1	kp

When the initiator (I) is very efficient, e.g. kp/ki < 0.3, and an excess of monomer (M) is used, the initiator disappears according to a near-first-order law before much monomer has been consumed. Once it is all used up the monomer disappears exactly according to a first-order law since the concentration of living propagating species (P) is now constant and equal to the original concentration of initiator. Hence, provided that both initiator and monomer concentrations can be followed, say by 1H NMR, both ki and kp may be

determined60.

On the other hand, if the initiator is not very efficient, e.g. kp/ki > 3, the monomer may be used up before the initiator. An expression for kp/ki in terms of the fraction of initiator remaining can be obtained by dividing the rate of reaction of the monomer by that of the initiator, substituting [P] D [I]0 [I], and integrating between the limits [M] D [M]0 and 0, and [I] D [I]0 and [I]1 , where [I]1 is the final concentration of initiator. This leads first to equation 11 and, after integration, to equation 12.

d[M]/d[I] D 1 C kp/ki ([P]/[I]) D [1 kp/ki ] C kp/ki ([I]0/[I])	(11)
kp/ki D f([M]0/[I]0) C ([I]1 /[I]0 1g/fln ([I]0/[I]1 ) C ([I]1 /[I]0) 1g	(12)

In order to determine values of kp/ki experimentally from this relationship, presented graphically in Figure 1, it is best to adjust [M]0/[I]0 so that the measured value of [I]1 /[I]0 is in the middle range, preferably near 0.5. When P1 and Pn (n > 1) give separate signals in the NMR spectrum, as is often the case, kp/ki can be determined directly from the value of [I]/[P1] at the maximum concentration of [P1], since at this point the rate of formation of P1, (ki[I][M]), is equal to its rate of disappearance (kp[P1][M]). Some values are collected in Table 3. In most cases kp > ki, presumably because the substituent on the carbene ligand of the initiator offers more steric hindrance to the reacting monomer than the substituent on the carbene ligand of the propagating species.

D. Theoretical Treatments

Extended Huckel¨ MO calculations on Ti(DCH2)L2, where L D H, Cl, Cp, have shown that the completely planar molecule is easily distorted into a flattish pyramid with Ti at the

1512	K. J. Ivin

FIGURE 1. Relationship between kp/ki and [I]/[I]0 when [M]0/[I]0 is (a) 20, (b) 10, (c) 7, (d) 5 or

(e) 3; see equation 12. Reprinted with permission from Ref. 129. Copyright (1995) American Chemical Society

TABLE 3. kp/ki values determined mainly from equation 12a

Monomer	Catalystb	Solvent	kp/ki	Reference
O
O	W-1	CD2Cl2	0.26	125
O
CF3
	Mo-1	C6D6	0.72	126
CF3
COOMe	W-1	CD2Cl2	1.0	127
COOMe
O
CF3
	Mo-2	C6D5CD3	2.4	115
CF3
COOMe
	Mo-1	C6D6	3	126

COOMe

Mo-1	C6D5CD3	7	10

		24. Advances in the metathesis of olefins					1513
TABLE 3. (continued)
Monomer		Catalystb	Solvent	kp/ki			Reference
	CN	Mo-1	C6D5CD3	7			126
	Me
		Mo-1	C6D6	9			128
		Mo-1	C6D6	12			126
		Mo-3	C6D5CD3	30			129
O
	CF3
		Mo-4	C6D5CD3	40			115
	CF3
		Mo-5	C6D6	270			10
		Ru-1	CD2Cl2/C6D6	v. large			110
		Ru-2	CH2Cl2	170		0.8c	60
		Ru-3	CH2Cl2	0.1			60,91

aAmbient temperature.

bMo-1: Mo(DCHCMe3)(DNC6H3-i-Pr2-2,6)(OCMe3)2

Mo-2: Mo(DCHCMe2Ph)(DNC6H3-i-Pr2-2,6) (OCMe2CF3)2 Mo-3: Mo(DCHCMe2Ph)(DNC6H3-i-Pr2-2,6) (OCMe3)2 Mo-4: Mo(DCHCMe2Ph)(DNC6H3-i-Pr2-2,6) [OCMe(CF3)2]2

Mo-5:Mo( CH	CH CHCMe3 )( NC6 H3 -i-Pr2 -2 ,6 )(OCMe3 )2
	CMe2
W-1: W(	)(Br)2 (OCH2 CMe3 )2

Ru-1: Ru(DCHCHDCPh2)(Cl)2(PCy3)2; (Cy D cyclohexyl)

Ru-2: Ru(DCHCHDCPh2)(Cl)2(PPh3)2

Ru-3: Ru(DCHC6H4X-p)(Cl)2(PPh3)2

c p-X in Ru-3 (kp/ki values): H (0.11), F (0.21), Me (0.34), NMe2 (0.38), OMe (0.38), NO2 (0.43), Cl (0.83).

1514 K. J. Ivin

apex, ready to receive the incoming donor olefin130. Similarly, SCF-X˛-SW calculations on Mo(DCH2)(DNH)(OMe)2 show that the reaction with ethene at the COO face to form the metallacyclobutane is facilitated by twisting the DCH2 ligand about the MoDC bond131.

Structural parameters and other data have been calculated by ab initio MO methods for various other models of the intermediates in the olefin metathesis reaction, for example Mo(DCH2)(CH3)(Cl)(OAlH3)132; Mo(DCH2)(Cl)4133 135; Mt(DCH2)(DO)(Cl)2 where Mt D Mo, W136,137; Mt(DCHR)(DNH)(OH)2 where Mt D Mo, W137; Mo(DCH2)(X)(L)2 where X D O, NH and L D Cl, OMe, OCF3138; Mo(DCH2)(DNH)(OR)2 where R D H, Me131; and W(DCR1R2)Cl4139. These MO treatments mostly involve model compounds somewhat removed from real life. Nevertheless, they reveal factors and trends which are likely to be valid in real situations. The more recent force field (METMOD) treatments deal with actual metal carbene initiators and the intermediates derived therefrom and give remarkably accurate predictions of geometry, rotational barriers etc140 143.

On the question of the transitory existence of metal carbene olefin intermediates, for which there is kinetic evidence in one system144 and spectroscopic evidence in another (see Section III.B.4)120, MO calculations do not reveal a potential-energy-well intermediate between the reactants Ti(DCH2)(Cl)2 C CH2DCH2 and the product metallacyclobutane, although the metal carbene olefin configuration does have an intermediate energy in

the overall exothermic reaction134,145; similarly for the reaction of Mo(DCH2)(Cl)4 with CH2DCH2133.

The interconversion of metal carbene olefin complexes with corresponding metallacycles are formally 2 C 2 processes. It might have been expected, from the Woodward Hoffmann and other rules (see elsewhere146,147 for summaries), that these would have large activation barriers. Yet they generally proceed with remarkable facility. The reason, as determined from a detailed consideration of the reaction of Ti(DCH2)(Cl)2 with CH2DCH2, is that the participation of a 3d orbital allows the Pauli principle constraints to be satisfied in a unique way that avoids the unfavourable transition-state bonding interactions that are usually the source of the high barrier137,146.

The unsubstituted metallacyclobutane formed from Ti(DCH2)(Cl)2 C CH2DCH2 is calculated to have a planar but easily puckered ring. Even a substituent in the 2-position (opposite to Ti which is numbered 4) is known to cause very little puckering124. However, in 1,3-disubstituted tungstacyclobutanes, extended Huckel¨ calculations show that the ring has a puckered ee configuration, as required by the interpretation of the cis/trans stereoselectivity in the metathesis reactions of alk-2-enes148 (see Section IV).

IV. ACYCLIC MONOENES NOT CONTAINING FUNCTIONAL GROUPS

With terminal alkenes, degenerate metathesis (equation 13), competes with productive metathesis (equation 14), to an extent which depends very much on the catalyst.

RCH

CH2

RCH

CH2

(13)

+

+

CH2

CHR

CH2

CHR

RCH

CH2

RCH

CH2

(14)

+

+

RCH

CH2

RCH

CH2

24. Advances in the metathesis of olefins

1515

Isotopic labelling experiments have demonstrated that for the reaction of propene on MoOx /TiO2 or on photoreduced MoO3/TiO2 the chain carrier for the degenerate metathesis is [Mo]DCHMe rather than [Mo]DCH2149 151. As a general rule in olefin metathesis a substituted carbene is less reactive than an unsubstituted carbene, and so the former tends to build up to a higher steady state.

With proper choice of catalyst, high yields for reaction 14 can be obtained with all terminal olefins. When propene is passed over Re2O7/Al2O3/Et4Sn at 20 °C/1.5 bar, equilibrium conversion can be achieved with 100% selectivity at a throughput of 25 dm3 h 1

(g catalyst) 144 . The reaction of propene on a catalyst made from Mo(CO)6/Al2O3 is somewhat stereoselective at 25 °C. In the early stages the product but-2-ene has a cis/trans ratio of 67/33, but this very quickly moves towards the equilibrium ratio of 24/76 as a result of secondary metathesis152.

With liquid olefins the product ethene can be allowed to escape so enhancing the yield. For the substrates RCHDCH2 one can obtain yields of RCHDCHR as follows: pent-1- ene 64% (using WCl6/Et2O/Bu4Sn in CHCl3)153, 3-methylbut-1-ene 58%153, vinylcyclopropane 65%154, hex-1-ene 99% (using catalyst 8, R D Ph)97, oct-1-ene 86% (by refluxing through a column of Re2O7/Al2O3)153, dec-1-ene 77%153, styrene 95% [using Mo(DCHCMe2Ph)(DNC6H3-Me2-2,6)(OCMe2CF3)2]97, allylbenzene 77%153. All these reactions occur with good selectivity.

Alkenes of the type RCHDCHR readily undergo cis/trans isomerization in the presence of metathesis catalysts. With unsymmetrical alkenes R1CHDCHR2 interest centres on the stereoselectivity. In the case of cis-pent-2-ene catalysed by 12 in chlorobenzene at 25 °C the reaction is highly stereoselective giving initially 99% cis-but-2-ene and 100% cis-hex-3-ene. Similarly, starting from trans-pent-2-ene the initial products are 99.6% trans-but-2-ene and 99.6% trans-hex-3-ene; trans/cis isomerization only begins to occur as the but-2-ene approaches its equilibrium proportion of 25%101. Such high selectivity can be explained in terms of a puckered metallacyclobutane intermediate (see Section III.D) in which the interaction of the substituents (Me or Et) in the 1,3-positions is the dominant factor (the metal is numbered 4)26,27. In the metathesis of pent-2-ene with other catalysts the initial trans content of the hex-3-ene is always higher than that of the but-2-ene regardless of whether one starts from the cis or trans reactant; see Ivin26 for a summary. This may be taken as evidence of an effect of the substituent at the 2-position, a 2-ethyl group exercising a stronger influence than a 2-methyl group. Where this exists it favours the 1,2-aa and -ee structures, accounting for the trans bias in the hex-3-ene relative to but-2-ene. This effect becomes completely dominant in the metathesis of 4-methylpent- 2-ene (i-PrCHDCHMe) where the product 2,5-dimethylhex-3-ene (i-PrCHDCH-i-Pr) is entirely trans for the reaction of both cis-and trans-substrates155. In line with these arguments the trans content of the products increases with the size of R1 and R2 in the Re2O7/CsNO3/Al2O3-catalysed metathesis of linear olefins (C5 C9). The general order of reactivity on this catalyst is alk-2-ene > alk-3-ene > alk-4-ene > alk-1-ene156.

The metathesis of 1,1-disubstituted alkenes is not so easy to achieve because the equilibrium lies on the side of the reactant. However, if the ethene is allowed to escape the reaction can proceed in certain cases, e.g. for 2-methylbut-1-ene157,158, 2-methylpent- 1-ene and 2-methylhept-1-ene159; also for methylenecyclobutane160,161, but surprisingly not for methylenecyclopropane nor for methylenecyclopentane154. Methylenecyclohexane will exchange with W(DCHCMe2Ph)(DNAr)[OCMe(CF3]2]2 to give CH2DCHCMe2Ph but the reaction does not proceed further162. It will also exchange methylene groups with isobutene in the presence of a Ti-based catalyst163,164. For a number of 1,1-disubstituted alkenes metathesis catalysts first bring about conversion to isomers which then undergo cross-metathesis with the remaining substrate161,165,166.

1516	K. J. Ivin

The metathesis of some trisubstituted ethenes has recently been reported. The reaction of ethylidenecyclobutane over Re2O7/Al2O3 at 35 °C gives 30% dicyclobutylidene in 4 h, but if promoted with Bu4Sn the yield is increased to 75%154,167. 2-Methylbut-2-ene undergoes metathesis in the presence of 8 (R D Ph), equation 15, reaching equilibrium (16% conversion) in 1 week or less. All attempts at bringing about the metathesis of trisubstituted ethenes containing an alkyl group larger than methyl have failed with this catalyst162.

8				MeCHDCHMe
2Me2CDCHMe Me2CDCMe2			C		15
With the catalyst Re2O7/CsNO3/Al2O3 at 20		70 °C 2-methylpent-2-ene (Me2CDCHEt)

undergoes slow isomerization to 2-methylpent-1-ene (MePrCDCH2) which then rapidly cross-metathesizes with the starting olefin to yield 4-methylhept-3-ene (MePrCDCHEt) with 75% selectivity. Small amounts of the self-metathesis products, Me2CDCMe2 and EtCHDCHEt, are also formed. Similar behaviour is observed with 2-methylhex-1-ene (Me2CDCHPr) and 3-methylpent-2-ene (EtMeCDCHMe)166.

The simplest example of a cross-metathesis reaction is that between ethene and but-2- ene; the equilibrium mixture then consists of a mixture of four compounds, counting both cis and trans isomers. For the reaction of R1CHDCHR2 and R3CHDCHR4, with R1, R2, R3, R4 all different, twenty different compounds will be present in the equilibrium mixture. If cross-metathesis reactions are to be used for synthetic purposes it is usually possible to simplify the situation by choosing at least one symmetrical olefin (R3 D R4) or one with R4 D H and by taking one olefin in excess. Some examples are the following.

(1) The reaction of isobutene with hexa-1,5-diene catalysed by Re2O7/Al2O3/Bu4Sn at 40 °C gives >20% yield of 6-methylhepta-1,5-diene, an intermediate in the synthesis of vitamins and carotenoids168.

(2)The reaction of isobutene with higher terminal alkenes or symmetrical internal

alkenes catalysed by Re2O7/Al2O3/Me4Sn leads to 2-methylalk-2-enes with conversions of 70 80%169.

(3)In the presence of Mo(CO)6/ZrCl4/h, pent-1-ene undergoes substantial isomerization to pent-2-ene which then cross-metathesizes with pent-1-ene without itself undergoing appreciable self-metathesis. In the reaction of pent-1-ene with 4-methylpent-2-ene, the cross-products, hex-2-ene and 2-methylhept-3-ene, predominate over the products of self-

metathesis170.

(4)Cross-metathesis of hex-1-ene with a four-fold excess of tetradec-7-ene, catalysed by WCl6/Et2O/Bu4Sn at 50 °C, results in a 90% conversion of hex-1-ene and a selectivity of 90% for the cross-metathesis product dodec-5-ene153.

(5)3,3-Dimethylbut-1-ene (neohexene), which is inactive to self-metathesis, undergoes

cross-metathesis with internal alkenes to high conversion when catalysed by WCl6/Et2O/Bu4Sn153.

(6) Cross-metathesis of equimolar amounts of styrene with symmetrical alkenes occurs on Re2O7/Al2O3 at 50 °C. 85 90% of the styrene is converted to 1-phenylalk-1-enes and only 10% to the self-metathesis product stilbene171. Likewise, the reaction of styrene with 0.5 equiv oct-1-ene catalysed by 8 gives >85% of the cross-metathesis product (>95% trans) and <4% of the self-metathesis product of oct-1-ene172.

(7) A number of insect sex pheromones are long-chain internal olefins or their epoxides and can be prepared via metathesis reactions, for example the reaction of dec-1-ene with pentadec-1-ene to give tricos-9-ene, the cis isomer of which is a sex pheromone of the housefly (Musca domestica)173,174; see also Kupper¨ and Streck175,176.