Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1253 128

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 21

Strained olefins

¨

JAN SANDSTROM

Division of Organic Chemistry 1, Center for Chemistry and Chemical Engineering, University of Lund, P. O. Box 124, S-221 00 Lund, Sweden

Fax: [phone]; e-mail: JAN. [e-mail]

I. BASIC CONCEPTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1254
II. TETRASUBSTITUTED ETHYLENES . . . . . . . . . . . . . . . . . . . . . .	1255
III. PUSH PULL SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1256
IV. BIS-TRICYCLIC ETHYLENES . . . . . . . . . . . . . . . . . . . . . . . . . . .	1263
V. MEDIUM-SIZED trans-CYCLOALKENES AND ANALOGUES . . . .	1272
VI. BIAND POLYCYCLIC BRIDGEHEAD OLEFINS . . . . . . . . . . . . .	1274
VII. MISCELLANEOUS COMPOUNDS . . . . . . . . . . . . . . . . . . . . . . . .	1275
VIII. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1277
IX. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1277

I. BASIC CONCEPTS

The concept of strain in organic chemistry is based on the experimentally and theoretically well founded structure theory, according to which bond lengths, bond angles, dihedral angles involving four atoms joined by three bonds in sequence, and distances between nonbonded atoms have optimal values, which are determined by the atoms involved and their connectivity. A molecule, in which these parameters can attain their optimal values, is free from strain. In many molecules, restrictions due to space-requiring substituents or cyclic structures prevent one or more of the ideal parameter values being attained. The molecule is subject to strain, and its energy is increased accordingly. A quantity named strain energy can be defined as the difference between the heat of formation of an actual strained molecule and that of a hypothetical strain-free molecule with the same atoms in the same bonding arrangement1. Mathematical expressions describing the relation between the degree of deviation from an ideal parameter value and the energy of the molecule have been developed, creating a force field for the molecule. Based on the principle that a physical system strives to attain the lowest possible energy, the deformations are distributed over the different degrees of geometrical freedom in the optimal way. This is the basis of the technique of empirical force-field (EFF) or molecular mechanics calculations2 4, a technique that has had an enormous impact on physical

1253

1254	Jan Sandstrom¨

organic chemistry, permitting highly realistic predictions of the geometries and energies of the different feasible forms of more and less flexible molecules. It is also possible to map the entire energy hypersurface of a molecule and to find reaction itineraries and transition state energies for exchange reactions between different minimum energy forms.

Strong steric strain leads to modifications of the structures and the physical and chemical properties of molecules, and investigations into these changes and the limits for the existence of extremely strained molecules have become important fields of research with challenges for preparative and theoretical chemists alike.

The concept of the steric stability of the CDC bond, manifested in the high barrier to exchange between cis and trans forms of 1,2-disubstituted ethylenes, is one of the most longstanding dogmas of organic chemistry. The underlying theory, based on sp2 hybridized carbon atoms, the separation and the sideways overlap of the p orbitals to form a bond, is one of the early triumphs of the MO model5. In ‘normal’ doublebonded systems the sp2 hybridized carbon atoms and the four atoms bonded to them lie in one plane, the double bond plane. The bond gives rise to a barrier to rotation around the C C bond in ethylene corresponding to an Arrhenius activation energy of

65.0 kcal mol 16 .

However, over the years an increasing number of molecules have been studied, which contain nonplanar CDC bond systems. The deformations are caused by strain, due to bulky substituents or to inclusion of the double bond in cyclic systems, or to both. The deviations can be described as twisting about the CDC bond or as pyramidalization of the carbon atoms, or as a combination of both. In CDC systems with pure twisting the carbon atoms remain sp2 hybridized, and the bonding energy is diminished because of diminished overlap between the p orbitals. On pyramidalization of a carbon atom the hybridization changes in the direction of sp3, and the p orbital, which is a component in the bond, acquires some s character and turns away from the p orbital on the other carbon atom, which leads to diminished overlap and a weakened bond (Scheme 1). The degree of pyramidalization is often measured by the angle , but a more general pyramidalization analysis based on the -orbital axis vector (POAV) has been proposed by Haddon7 9.

Θ

Twisting Φ

Planar olefin

Pyramidalization

SCHEME 1

An experimentally accessible index for strain in bridgehead olefins, olefinic strain energy (OS), has been proposed by Schleyer10,11. The OS is defined as the difference in strain energy between the olefin in its most stable conformation and the corresponding saturated hydrocarbon, also in its most stable conformation. It can be obtained by equation 1, where H°H is the heat of hydrogenation of the olefin and 26.1 is the heat of hydrogenation of an unstrained trisubstituted olefin to the corresponding unstrained saturated hydrocarbon. OS values have been derived both by quantum-mechanical (ab initio and semiempirical) and by EFF calculations, and OS <17 kcal mol 1 in general means that the compound is stable at ambient temperature, 17 < OS < 21 kcal mol 1 indicates stability in the range C20 to 78 °C, while compounds with OS > 21 kcal mol 1 can at

21. Strained olefins	1255
most be studied in low-temperature matrix isolation.
OS D HH° 26.1 (in kcal mol 1)	1

The energetically unfavorable situation with pyramidal carbon atoms in double bonds was early realized by Bredt. His famous rule12,13, which states that a carbon atom in a CDC bond cannot be a bridgehead in a bicyclic system, is based on studies of camphenes and pinenes and is thus intended to be valid only for fiveand six-membered rings.

The field of strained olefins has been the subject of numerous reviews14 18.

II. TETRASUBSTITUTED ETHYLENES

Tetra-tert-butylethylene (1a) can serve as a symbol for this group of compounds. In spite of ingenious approaches to its synthesis19,20 it still eludes its pursuers. However, the most recent EFF calculations by Burkert21 and by Favini and coworkers22 predict a twist angle of ca 45° and a C1 C2 bond length of 137.720 and 136.0 pm21, respectively. The latter authors calculated a strain energy of 89.6 kcal mol 1, which means that 1a should be capable of existence, since other hydrocarbons with much higher strain energies have been prepared, e.g. cubane with 166.9 kcal mol 1. Syn-difenchylidene (2) is a close analogue of 1a, but the cyclic structures remove some nonbonded interactions and the twist angle is only 11.8° and the C1 C2 bond length 134.9 pm23, well reproduced by EFF calculations22.

	Me	Me
		Me
Me3 C	R
	C1 C2
Me3 C	R	Me
	Me Me
(1a)	R = t Bu	(2)
(1a)	R = Ph

1,1-Diphenyl-2,2-di-tert-butylethylene (1b) has been prepared and studied by X-ray crystallography24, and twist angle (24°) and C1 C2 bond length (136 pm) as well as other structural details were well reproduced by EFF calculations22. Gano and coworkers have studied two isomers of 1b, viz. E- and Z-1,2-diphenyl-1,2-di-tert-butylethylene (3a and 3b). The former was obtained in low-valent Ti-induced coupling of phenyl-tert-butyl ketone and was photoisomerized to 3b, which in turn could be thermally isomerized back to 3a with an enthalpy barrier of 31.2 kcal mol 1. The low barrier indicates a high ground-state strain energy, estimated at 27.1 kcal mol 1. The equilibrium mixture con-

tained 0.4% of 3b, corresponding to an energy difference of 4.2 kcal mol 125 . An X-ray crystallographic study of 3b unexpectedly shows an untwisted double bond with a CDC distance of only 134.3 pm and phenyl groups perpendicular to the double bond. The considerable crowding leads to large CDC CMe3 angles (132.7°)26.

Sakurai and coworkers27 30 have prepared a series of strongly crowded 1,1,2,2- tetrasilyl-substituted ethylenes (4a to 4e). The twist angle and the lengths of the C1 C2 and C Si bond lengths increase with increased crowding (Table 1). In 4a and 4b pure twisting occurs, but in 4d C2 and in 4c both C1 and C2, which have

1256

Jan Sandstrom¨

TABLE 1. Twist angles ( ) and bond lengths in some tetrasubstituted ethylenes

Compound

(deg)

rC1 C2 (pm)

rC1/C2 Si (pm)

Reference

1a

45

137.7

21a

1b

45.5

136.0

22a

24

136

24

2

11.8

134.9

23

3b

0

134.3

26

191.5b

4a

29.5

136.8

27

4b

49.6

137.0

28

4c

50.2

136.9

193.0

29

47.1

191.2

4d

c

138.1

197.1

30

4e

0

136.7

191.7

30

a From EFF calculation.

bNormal lengths in unstrained molecules 184

187 pm.

c Strongly twisted, but no angles are specified. C1 with substituents planar, C2 pyramidal.

Me3 C

Ph

Me3 C

CMe3

R1Me2 Si

SiMe2 R3

C

C

C C

C1

C2

Ph

CMe3

Ph

Ph

R2 Me2 Si

SiMe2 R4

(3a)

(3b)

(4a)

R1 = R2 = R3

= R4 = Me

(4b)

R1 = R2 = Me, R3 = R4

= t Bu

(4c)

R1 = R3 = Me, R2 = R4

= t Bu

(4d)

R1 = R2 = R3

= t Bu, R4 = Me

(4e)

R1 = R2 = R3

= R4 = H

nonidentical substituents, are pyramidal. These compounds are red, and some of them show thermochromic behavior, indicating the existence of more twisted, high-energy conformers. However, the less crowded 4e is colorless and has a planar double bond.

III. PUSH PULL SYSTEMS

Twisting is particularly facile when the substituents are of push pull type (5, A1 and/or A2 acceptors, D1 and/or D2 donors). The A1A2C part develops an increasing anion character and the D1D2C part an increasing cation character with increasing twist angle, and the loss of bonding energy in the CDC bond is partly compensated by the delocalization energy of the incipient anion and cation. This stabilization increases and consequently the barrier to rotation about the CDC bond decreases with increasing donor capacity of D1 and/or D2 and with increasing acceptor capacity of A1 and/or A2. Studies by dynamic

A1 A2

C

C

D1 D2

(5)

21. Strained olefins

1257

NMR spectroscopy have shown that push pull ethylenes with acyl groups as acceptors

and amino groups as donors may have barriers to rotation lower than 5 kcal mol 131 . If a steric interaction exists between the donor and the acceptor groups in the planar form of such a system, the strain may be relieved by rotation about the CDC bond. The geometry of the system and therefore the angle of rotation is controlled by the balance between the strain energy and the loss of electron stabilization on rotation. However, the strain may also be released by rotation of the individual donor and acceptor groups around the D C(DC) and A C(DC) bonds, which leads to an increase of the CDC barrier. More clear-cut conditions are obtained if the two donor and/or the two acceptor groups are joined in a small (fiveor six-membered) cyclic system, which diminishes the deformation of this part of the molecule.

The conformations of push pull ethylenes are thus determined by an interplay between-electronic and steric effects, and a schematic subdivision of this group of compounds into three different classes (case 1 case 3) with respect to the two effects has been proposed32,33. The energy contributions are discussed as functions of the dihedral angle,, which describes the twist of the CDC bond. The steric energy (Ester) is assumed to have maxima at D 0° and 180° and the -electronic energy (E ) at D 90° and 270°. During a full 360° rotation, four energy minima are passed.

In case 1 (Figure 1), Ester is small and E relatively large, and the energy minima fall close D 0° and 180°. Most ‘normal’ push pull ethylenes fall in this group, and cis

and trans isomers are likely to be observable, at least with NMR spectroscopy unless the

symmetry is too high. In case 2 (Figure 2), the relation is the opposite with Ester × E . The positions of the energy minima depend on the shapes of the energy curves, but they are

likely to fall closer to D 90° and 270° than to 0° and 180°. If A1 6D A2 and D1 6D D2, the molecules appear as pairs of enantiomers, which may be separated by substantial

− Energy (kcal mol 1)

	D2	Eπ		D2
A1	A2	Ester	A2	A1
		Etot
20	D1			D1

15

10	A1
5	A2

0	90	180	270	360
		Θ (degrees)

FIGURE 1. Schematic potential energy curve for a case 1 push pull ethylene. E × Ester

1258

	25
	20
)
−1
mol(kcal	15
Energy	10
	5

Jan Sandstrom¨

A1

Eπ

Ester

Etot A2

D2

	A1		A2
D2	A2	D1	A1 D2

A1

D1

D1

A2

D2

	A2				A1
D2	A	1	D	1	A	2 D2

A2

D1

D1

A1

0	90	180	270	360
		Θ (degrees)
FIGURE 2.	Schematic potential energy curve for a case 2 push		pull ethylene. E − Ester

− Energy (kcal mol 1)

	Eπ
	Ester				A1
25	Etot	D2
						D2
20	A1		A2		A2	A2	A1
	D2							D2
		D1		1 D2	D2	D1	A2
	A2		A
15						A1
	A1			A2				A1
	D1				A2			D1
				D1	D1
10
5

0	90	180	270		360
		Θ (degrees)
FIGURE 3.	Schematic potential energy curve for a case 3 push			pull ethylene. E ³ Ester

21. Strained olefins

1259

steric barriers. As will be discussed below, enantiomers in systems with sufficiently high barriers have been separated, using a chromatographic technique with chiral stationary phases. The barriers have then been measured by monitoring the rate of racemization. Lower barriers can be measured by a dynamic NMR technique34 if the molecule contains prochiral groups35, even if A1 D A2 or D1 D D2. Cis and trans isomers, on the other hand, are separated by very low barriers and are not likely to be observed separately.

Finally, in case 3 (Figure 3), Ester and E are both of similar and substantial magnitude. The energy minima fall in the neighborhood of D 45°, 135°, 225° and 315°, and in

systems with suitable barriers and substituents the passages past both the steric and the-electronic barriers can be followed by NMR spectroscopy.

In this chapter, only case 2 and case 3 systems are of interest. Representatives of the former group are found among compounds 6 and 7 (Table 2). The structural requirements are bulky substituents and a combination of good donor and good acceptor groups. While compounds 6a 6c can be seen as intermediates between case 1 and case 2 because of their rather weak acceptor groups, compounds 6d and 6e, with good acceptor groups,

clearly belong to case 2. Because of the larger bond angles in six-membered rings, Ester is larger in compounds 7 than in corresponding compounds 6, as shown by the larger

twist angle ( ) in 7c than in 6c and by the higher G#ster in 7d than in 6d.

	A1	A2		A1	A2
				R	R
R	N	N	R	N	N
		(6)			(7)

Rather few case 3 systems have been described. The requirement is a strong steric effect and a combination of rather poor donor and/or acceptor groups. The first chosen candidate, 7b, has good donors and poor acceptors, and it fulfilled the expectations. The 1H NMR spectrum of the benzylic protons showed a singlet at ambient temperature, which changed into an AB system around 70 °C, and into two broad, equally intense AB systems around 130 °C36. Raised acceptor capacity as in 7e leads to an increased

TABLE 2. Free-energy barriers to rotation through the 90° twisted state ( G‡, kcal mol 1) and

through the planar state ( Gster‡ , kcal			mol 1), twist angles ( °) and C1 C2 bond lengths (pm) for
compounds 6 and 7
Compound	A1	A2	R	G‡	G‡	r 1	2	References
					ster	C	C
6a	4-BrC6H4	CN	Me	9.5		41	144.8	38, 39
6b	Ph	CN	CH2Ph	9.2	<5.0			32
6c	CN	CN	Me			20	140.7	37
6d	PhCO	COMe	CH2Ph	<5.0	16.5			32
6e	MeCO	COMe	Me			73	146.8	37
7a	Ph	CN	Me	7.4				36
7b	Ph	CN	CH2Ph	7.3	10.5			36
7c	CN	CN	Me			32	142.9	37
7d	PhCO	COMe	CH2Ph	<5.0	22.0			32
7e	4-O2NC6H4	CN	CH2Ph		13.9			36
7f	4-H2NC6H4	CN	CH2Ph	8.3	9.7			32

1260	Jan Sandstrom¨

steric barrier and a decreased (to below the limit of detectability) barrier, while lowered acceptor capacity as in 7f has the opposite effect.

Coming back to case 2 systems, a number of analogues of 7 with acyl and thioacyl groups as acceptors and with i-Pr and PhCH2 groups as substituents on the nitrogen atoms (8 10, Table 3) have been resolved by chromatography on swollen microcrystalline triacetylcellulose40,41, and the barriers to rotation through the planar state have been determined by monitoring the thermal racemization. As expected, replacement of carbonyl by thiocarbonyl groups as acceptors increases the steric barrier both by lowering E (CDS

is a better acceptor than CDO42) and by raising Ester. It is also clear that Ester is increased and E is decreased compared to the situation in compounds 8 if the acceptor groups are

also included in a ring system. This should lead to an increase in G‡ster, which is borne out by a comparison of the barriers in 8a and 9b.

					R1	R2			R1	R2
R1		R2
X	C		Y−	X	C		Y−	−S		C	SMe
iPr	C			3	C		R4	R	3	C	R4
			CH2 Ph	R
N	+	N			N +		N		N	+	N
	(CH )	3			(CH2 )		n		(CH2 )		n
	2
	(8)				(9)					(10)

Compounds 9a, 9c, 9e and 10c have been subjected to X-ray crystallographic studies43,45 and large twist angles and long C C bonds have been observed (Table 3). Comparison of 9a and 9e shows the effect of the size of the donor ring discussed above, the twist angle being ca 5° larger with a six-membered than with a five-membered

donor ring. The same effect is responsible for the difference in G‡ster between 10a and 10b. The difference in twist angle between 9e and 10c can be ascribed to differences in E . The steric effects should be rather similar, but the acceptor part in 9e contains two and in 10c only one thiocarbonyl group. Therefore E is higher in 10c and the energy minimum falls at a lower value than in 9e. The C C bond lengths in the four compounds are rather similar and show no correlation with the twist angles.

The barriers to passage of the 90° twisted state in case 1 type push pull ethylenes have been shown to correlate roughly with R of the acceptors46 and the nitro group should be a better acceptor than acyl and thioacyl groups. Consequently, push pull systems with two nitro groups as acceptors, like 11 and 12, show practically perpendicular acceptor parts with D 89.0 and 86.9°, respectively47.

Case 2 systems like compounds 8 12, and especially 11 and 12, are not olefins but are best described as zwitterions of carbanions and amidinium ions. Consequently, the polarity of the molecule should decrease when going from the ground state to the transition state to rotation, contrary to the situation in case 1 systems. This has been supported by AM1 and CNDO calculations45. The activation parameters in the two kinds of systems have been studied, and as expected case 1 systems have large and negative, and case 2 systems moderate and positive, activation entropies48. The barriers in case 1 systems are considerably lowered, and those in case 2 systems practically unchanged, by increased solvent polarity48,49.

TABLE 3.

Free-energy barriers to rotation through the planar state (kcal mol 1), C1 C2

bond lengths (pm) and twist angles (deg) for com-

pounds 8

10

Compound

n

R1

R2

R3

R4

X

Y

G‡ster

rC1 C2

Reference

8a

Me

Ph

O

O

25.6

40

8b

Me

Ph

O

S

30.3

41

8c

Me

Me

O

S

30.3

41

8d

Me

Ph

S

S

29.9

41

9a

2

Me

Me

CH2Ph

CH2Ph

S

S

148.2

80.6

43

9b

3

H

Ph

i-Pr

CH2Ph

O

O

27.8

44

9c

3

Me

Me

CH2Ph

CH2Ph

O

O

147.2

78.8

45

9d

3

Me

Me

i-Pr

CH2Ph

O

S

>30.4

41

9e

3

Me

Me

CH2Ph

CH2Ph

S

S

146.6

85.1

45

10a

2

Me

Me

i-Pr

CH2Ph

28.9

44

10b

3

Me

Me

i-Pr

CH2Ph

30.4

44

10c

3

Me

Me

CH2Ph

CH2Ph

147.6

72.5

45

1261

1262		Jan Sandstrom¨
		O−			O−
	O2 N	N+		O2 N	N+
		O−			O−
	Me	Me		Me	Me
	N	+ N		N	+ N
				Pr	H
		(11)			(12)
		O		O−
		Me		Me
		Me		Me
		N	+	N
Me	O−	Me		Me	O	Me
			ZZ
O	Me				Me	O−
Me	Me				Me	Me
N	+ N				N	+ N
Me	Me	Me		Me	Me	Me
	EZ			O−		ZE
		O
		Me		Me
		N	+	N

Me Me

EE

SCHEME 2

The delocalization in the donor and acceptor parts of case 2 systems leads to considerably hindered rotations around the formal single bonds. The acceptor part of a compound like 13 is similar to the anion of a ˇ-diketone. While the dynamics of the latter one is difficult to study without the intervention of a chelated cation, the acceptor part of 13 is more similar to an isolated ˇ-diketonate ion. A study of 13 by dynamic 1H NMR revealed exchange between three rotamers (EZ, ZE and EE, Scheme 2). The fourth rotamer, ZZ,