Journal of t h e American Chemical Society
1210
treated as separable consecutive first-order reactions and treated as above for the simpler case. Because of the fact that krearrangement = '/20 kisomerization very little error will be introduced by treating the data in this fashion.
Acknowledgment. Financial support from the National Science Foundation (Grants GP-29738X, GP-37701 to V.P.V.) and the Robert A. Welch Foundation (Grant D-326 to J.N.M.) is gratefully acknowledged. Special thanks are due to Professors D. L. Whalen, R. M. Pollack, and M. W. Logue for many helpful discussions. We appreciate the a - D effects for 3a reported in Table IV determined by Toby Wilgis. References and Notes (1) (a) University of Maryland Baltimore County: (b) Texas Tech University. (2) Project CATALYST Student, sponsored by the American Chemical Society, summer 1975. (3) Part 6: V. P. Vitullo and E. A. Logue, J. Am. Chem. SOC., 98, 5906 (1976). (4) V. P. Vitullo and N. R. Gossman, J. Am. Chem. SOC.,94, 3844 (1972). (5) V. P. Vitullo and E. A. Logue, J. Org. Chem., 37, 3339 (1972). (6) V. P. Vitullo and E. A. Logue, J. Org. Chem., 38, 2265 (1973). (7) V. P. Vitullo and M. J. Cashen, Tetrahedron Lett., 4823 (1973). (8) J. N. Marx, J. C. Argyle, and L. R. Norman, J. Am. Chem. SOC.,96, 2121 (1974).
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K. L. Cook and A. J. Waring, J. Chem. SOC., Perkin Trans. 2, 84, 88 (1973). M. S.Newman and L. L. Wood, Jr., J. Am. Chem. SOC.,81,6450 (1959); T. G. Miller, J. Org. Chem., 27, 1549 (1962). ti. Plieninger and G. Keilich, Ber., 91, 1891 (1958). R. E. Rondeau and R. E. Sievers, J. Am. Chem. SOC., 93, 1522 (1971). K. Von Auwers and G. Keil, Ber., 35,4207 (1902). See also A. J. Waring, Adv. Alicyclic Chem., 1, 146 (1966): H. Wynberg, Chem. Rev., 60, 169 (1960). M. S. Newman, J. Eberwien, and L. L. Wood, Jr., J. Am. Chem. SOC.,81, 6454 (1959). C. H. Rochester, "Acidity Functions", Academic Press, New York, N.Y., 1970, pp 109-187. For conformational studies of dihydrobenzenes, see P. W. Rabideau, E. G. Burkkolder, M. J. Yates, and J. W. Paschal, J. Am. Chem. Soc.,99,3596 (1977). Conformational studies in the present system will be published later. P. E. Peterson and F. J. Waller, J. Am. Chem. SOC.,94, 991 (1972). T. W. Bentley, F. L. Schadt, and P. v. R. Schleyer, J. Am. Chem. SOC.,94, 992 (1972). C. D. Ritchie, Acc. Chem. Res., 5, 348 (1972). M. Sczwarc, Ed., "Ions and Ion Pairs in Organic Reactions", Vol. 2, Wiley, New York, N.Y., 1974, p 322 ff. C. J. Collins and N. Bowman, Ed., "Isotope Effects in Chemical Reactions", Van Nostrand-Reinhold, Princeton, N.J., 1970, p 90. T. W. Bentley, S. H. Liggero, M. A. Imhoff, and P. v. R . Schleyer. J. Am. Chem. SOC.,96, 1970 (1974). C. W. Spangler and P. L. Boles, J. Org. Chem., 37, 1070 (1972). C. D. Ritchie, J. Am. Chem. SOC.,94, 3275 (1972). T. H. Fife, Acc. Chem. Res., 5, 264 (1972).
Molecular Orbital Theory of the Electronic Structure of Molecules. 38. Flexible Olefins Preferring Nonplanar Puckered Structures Hans-Ulrich Wagner,*la Giinter Szeimies,Ia Jayaraman Chandrasekhar,Ib Paul v. R. Schleyer,*Ib John A. Pople,lc and J. Stephen Binkleylc Contribution f r o m the Institut f u r Organische Chemie der Universitat Munchen, 8 Munchen 2, West Germany, Institut f u r Organische Chemie der Universitat Erlangen-Nurnberg, 8520 Erlangen, West Germany, and Department of Chemistry, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213. Received August 24, 1977
Abstract: Ab initio molecular orbital calculations indicate that a number of strained olefins prefer nonplanar structures. These include bicyclobutene-Al~3,bicy~lopentene-A',~, and bicy~lohexene-A'-~. In addition, bicycl~hexene-Al,~, although planar, is indicated to have a very weak out-of-plane bending force constant. Model calculations from ethylenes distorted to represent these olefins demonstrate the generality of this behavior and provide a theoretical explanation.
Unless dictated by geometrical constraints (e.g., I)2a,b or by the lack of the appropriate symmetry,2colefins are planar entities. Remarkably, Hehre and Pople's ab initio calculations on bicyclobutene- A'%3 indicated preference for a nonplanar structure even though nothing would seem to preclude Table I. Relative Energies of Distorted Ethylene Modelsa for I I and IV (RHF/STO-3G), kcal/mol
LHCC,
Planar (VIIA)
deg
D2h
120
110 100 90 80 70
0.0 (-77.071 0.0 (-77.034 0.0 (-76.944 0.0 (-76.786 0.0 (-76.518 0.0 (-75.981
21)' 59) 19) 88) 85) 37)
Nonplanar (VIIB)CCzu 11.1 7.3 2.1 -5.5 -17.3 -37.9
Assumed geometrical parameters: r(C=H) = 1.34, r(C-H) = 1.08 A. Total energy (au) of planar form in parentheses. 6a
(H,CCH,) = @(HbCCHd)= O', @(HaCCHd)= 150'.
0002-7863/78/1500-1210$01 .OO/O
planarity. As possible reasons for this behavior were not anal y ~ e da, ~more extensive molecular orbital investigation of I1 and other olefins (111, IV, V, and VI (R = H)) which might also be nonplanar has now been undertaken. IV4 and VI (R = CH3)5 are known, but no experimental structural studies have been reported. Very recently, a tricyclic derivative of I1 has been implicated as a reaction intermediate.6 A platinum Table 11. Relative Energies of Distorted Ethylene Modelso for I l l , V, and VI (RHF/STO-3G), kcal/mol O,c deg
Planar (XIA) c2u
Nonplanar (XlB). C,
I50
0.0 (-76.608 67)' 0.0 (-76.590 17) 0.0 (-76.501 03)
2.5 -1.1 -6.5
120 100
a Assumed geometrical parameters: r(C=C) = 1.34, r ( C - H ) = 1.08 A. Total energy (au) of planar form in parentheses. LH,CC = LH,CC = 70". @(H,CCH,) = Oo, LHbCC = LHdCC = 8, @(HaCCH,j)= 150'.
0 1978 American Chemical Society
Wagner, Schleyer, et al.
/ Flexible Olefins with Nonplanar Puckered Structures
121 1
Table 111. Total Energies" of 11-V
I1 planar nonplanar Difference' 111 planar nonplanar Difference I V planar V planar nonplanar Difference'
STO-3G
4-31G
6-31G*
-151.602 13 -151.650 18 30.2 -190.240 57 - 190.268 30 17.4 -228.947 61 -228.935 02 -228.936 60
-153.274 28 -153.303 46d 18.3 - 192.283 28 -192.305 15 13.8
- 153.557 54 - 153.575 70
Dipole momentb 0.0 0.76e
11.4 0.82 1.20 0.0 0.75 0.86
1.o
" au.
Debye, calculated from the STO-3G wave function. Kcal/mol. Energy of 4-3 IG optimized structure = - 153.305 85 au. D at 4-31G, 1.14 D at 6-31G*. The direction of the dipole is plus (CH2)2C2 minus. ,Hb
,Hd
h'
H,
H
H
v
\,'t.' nonplanar VIIB
An analysis of the MOs indicates the reason for this startling structural switchover. Consider the ethylene 3a, orbital VI11 composed of carbon px orbitals (Figure When LHCC is >looo, favorable overlap with the four hydrogen orbitals is possible (VIIIA). As LHCC is decreased, such favorable CH overlap diminishes in the planar form as the hydrogens move into the nodal regions (VIIIB). If the symmetry is reduced by making the molecule nonplanar, MO VIIIB mixes with the originally pure a MO (IXB). The consequence of this mixing is to produce a new set of MOs (X), one of which (XA) has gained C-H bonding and is considerably more favorable than its counterpart, VIIIB. The other MO of this combination (XB) is only marginally less favorable than IXB. The net result is a substantial lowering in energy on going from planar to nonplanar forms when LHCC < 100'. This behavior is not seen at large HCC angles. When LHCC is around 120°, MO VIIIA already has strong C-H bonding as the carbon px lobes are directed toward the hydrogen atoms. Out-of-plane deformation, and consequent mixing with the a orbital (IXA), does not lead (in contrast to VIIIB vs. XA) to lowering in the orbital energy. Other orbitals (b3u and b2,)1° of ethylene become destabilized on puckering and ethylenes with LHCC
