2848
Journal of the American Chemical Society
same conclusion-both 7 and 8 are unstable, gas-phase carbanions. However, when the inversion angles, 8, at Cp in 7 and 8 were maintained a t their approximately tetrahedral 4-31G values, the C&, and the C,-F bond distances could be optimized (Table I). A rigid Cp inversion barrier of 7.60 kcal/mol (relative to 8) was calculated when all other geometric parameters were maintained constant (4-31G).
Conclusion Our theoretical considerations prompt us to generalize that the rate of both syn and anti elimination will be increased b y maximizing overlap of the developing carbanion at Cp with the backside of the C, leaving group. Thus, the mechanism for syn elimination is in effect an ElcB process where expulsion of the leaving group occurs concomitantly with inversion at Cp. I n a syn E2 (or ElcB) elimination, rehybridization a t Cp will occur until the best balance between orbital overlap a t C, and loss of counterion bonding between the developing carbanion and the departing hydrogen-base pair is attained. This suggestion is consistent with existing experimental data,3 where El cB-like transition states have been invoked for syn elimination, and provides a rational explanation for the failure to observe hydrogen-deuterium exchange in E l c B reactions. W e also extend these concepts to include pyrolytic syn elimination (Cope) and a',P (ylide) elimination where intramolecular syn 1,2 elimination should also be enhanced by inversion of configuration a t the carbanionic carbon. Acknowledgment. Support of this work by the National Science Foundation ( C H E 76 21992) is gratefully acknowledged. We also wish to thank Professor N. A. LeBel for helpful discussions on the interpretation of these results. R.D.B. would like to thank Professor Roald Hoffmann for his generous hospitality during his sabbatical leave a t Cornell, where the manuscript was written.
/
1Ol:l I
/ M a y 23, 1979
References and Notes (1) E. D. Hughes, J. Am. Chem. SOC., 57, 708(1935). (2) C. K. ingoid, Proc. Chem. SOC.,London, 265 (1962). (3) (a) W. H. Saunders, Jr., and A. F. Cockerill, "Mechanisms of Elimination Reactions", Wiley-Interscience. New York, 1973; (b) J. Sicher, Angew. Chem., lnt. Ed. Engl., 11, 200 (1972); (c) N. A. LeBel, Adv. Alicyclic Chem., 3, 195 (1971); (d) J. F. Burnett, Surv. Prog. Chem., 5, 53 (1969). (4) K. Fukui, Tetrahedron Lett., 2427 (1965); K. Fukui and H. Fujimoto, ibid., 4303 (1965); Bull. Chem. SOC.Jpn., 40, 2018 (1967);K. Fukui, H. Hao, and H. Fujimoto, ibid., 42, 348 (1969); H. Fujimoto, S. Yamabe, and K. Fujiu, ibid., 44, 971 (1971). (5) (a) I. Tvaroska, V. Klimo, and L. Valko, Tetrahedron 30, 3275 (1974); (b) P. C. Hiberty, J. Am. Chem. SOC.,97, 6975 (1975). (6) (a) J. P.Lowe. J. Am. Chem. SOC.,94, 3718 (1972); (b) J. Sediacek, Collect. Czech. Chem. Commun., 42,2027 (1977); (c) K . Fukui and H. Fujimoto, "Mechanisms of Molecular Migrations", Vol. 2, B. S. Thyagarjan, Ed., interscience, New York, 1969, p 117; R. G. Pearson, Acc. Chem. Res., 4, 152 (1971). (7) Calculations employed the GAUSSIAN 70 series of programsawith standard STO-3G and 4-31G basis sets. The geometry of both conformers of ethyl fluoride was first optimized with an STO-3G basis set and then exhaustively minimized with a 4-31G basis set using 0.02 A and 2' increments. The geometries of the ethyl fluoride carbanions were similarly optimized by STO-3G (0.01 A and 1' increments) and then with a 4-31G basis set but excluding C-H bond distances. A similar procedure was used with the geometries of the hydride-induced elimination except that 0.1 A and 5' increments were used. (8) W. J. Hehre, R F. Stewart, and J. A. Pople, J. Chem. Phys., 51,2657 (1969); J. W. Hehre, R. Ditchfield, and J. A. Pople, ibid., 54, 724(1971). (9) The 4-31G total energies of H- and H2 are -0.422 44 and -1.126 82 au, respectively. The STO-3G, 4-31G, and 6-31G total energies of F- are -97.613 32, -99.247 83, and -99.350 18 au, respectively. The fully optimized STO-3G and 4-31G total energies of ethylene are -77.073 92 and -77.922 15 au, respectively, while the 6-31G energy of the 4-31G optimized ethylene molecule is -78.004 42 au. The 6-31G total energies of the 4-31G optimized syn (7) and anti (8) ethyl fluoride anions are -177.320 88 and -177.329 06 au, respectively. (10) The hvdride ion is a relatively strona base. Its Droton affinitv in the aas - .Dhase exceeds the affinity of F- b y more than 5 0 ' k ~ a V m o l . ~ ~ . A. Dedieu and A. Veiilard, J. Am. Chem. SOC.,94, 6730 (1972). H. B. Burgi, J. M. Lehn, and G. Wipff, J. Am. Chem. SOC., 98, 1956 (1974). The 90' conformer has a minimum energy when the inversion angle I) was 62' (total energy -177.129 40 au). The planar carbanion (C#)had a total energy of -177.118 32 au with all variables optimized including the C-F bond distance. The geometries used to calculate the rotational barrier were optimized in a similar fashion with the exception of the syn (I) = 0 ' ) and anti (0 = 180') conformers. C. H. DePuy, R. D. Thurn. and G. F. Morris, J. Am. Chem. SOC.,84, 1314 (1962).
The Unusual Structures, Energies, and Bonding of Lithium-Substituted Allenes, Propynes, and Cyclopropenes Eluvathingal D. Jemmis,' Jayaraman Chandrasekhar, and Paul yon Ragu6 Schleyer" Contribution from the lnstitut f u r Organische Chemie der Unicersitat Erlangen-Nurnberg, 8520 Erlangen, West Germany. Receiced August 13. 1978
Abstract: Propyne and allene undergo sequential hydrogen-lithium exchange experimentally to give C3H3Li (two isomers), C3HZLi2, C3HLi3, and C3Li.+ Mono- and dilithiated cyclopropenes are also known. The structures and bonding of hypothetical isolated monomers with these compositions, investigated by a b initio molecular orbital methods, reveal remarkable features. While acetylide bonding, e.g., i n propynyllithium ( l ) ,is the most favorable energetically. bridging lithiurns are preferred over conventional placements. Allenyllithium (11). the simplest bridged molecule of this type, has a bent carbon skeleton ( K C C = I57.6', STO-3G optimization) which better accommodates simultaneous Li bonding to C- I and C-3. Even though Li is closest to C-2, bonding to that atom is indicated by the overlap populations to be negligible. The vinyl hydrogen of cyclopropene. with its enhanced acidity, exhibits a larger energy of replacement than that of ethylene. The methylene hydrogen behaves i n the opposite way owing to the antiaromatic character of 3-cyclopropenyllithium ( 5 ) . Examination of a number of structural possibilities indicated the most favorable forms, e.g., 17 for C3HzLi2, with an acetylide and a bridging lithium. Two bridging lithiurns in CLiC planes roughly at right angles are found i n the lowest energy structures of C3HLi3 (23)and of C3Li4 ( 3 6 ) , which in addition have one and two acetylide-type C-Li bonds, respectively. The orthogonal H systems, each involved in bonding a bridging lithium, are responsible Tor this arrangement. Numerous other forms of these isomers were considered. With few exceptions, classical structures based on the corresponding hydrocarbons were much less stable than alternative arrangements in which the multicenter bonding capabilities of lithium could be better utilized. Another example is I ,2-dilithiocyclopropene, which prefers the doubly bridged (4a) over the classical (4) structure.
New theories of bonding evolve from a knowledge of structural characteristics of sets of related molecules. The 0002-7863/79/ 1501-2848$01 .OO/O
development of the bonding principles governing boranes, by Lipscomb and others, has brought order to a field where every
0 1979 American Chemical Society
Jemmis, Chandrasekhar, Schleyer / Lithium-Substituted Allenes, Propynes, and Cyclopropenes
2849
malismI2 was used throughout. Consequently, the designation structure was once considered to be rule breaking.2 Another “RHF” is taken to be understood. The minimal STO-3G basis group of electron-deficient molecules can be obtained by replacing one or more hydrogens of hydrocarbons by l i t h i ~ m . ~ ?with ~ the standard exponentst3was employed for all geometry optimizations. Single-point calculations using the split-valence Whatever the nature of the bonding in such organolithium 4-31G (5-21G for lithium) basisI4 were carried out on these compounds, ionic or c o ~ a l e n ttheir , ~ ~ structures ~ are proving optimized structures to check the relative energies. Complete to be remarkable when compared with those of the hydrocarmapping of the potential energy surface, even though desirable, bons from which they are formally d e r i ~ e d .This ~ . ~ is also true is not yet practical. In the absence of such a systematic apof the lithium derivatives of the C3H4 isomers, propyne, allene, proach, we hope that the diversity of symmetries selected for and cyclopropene, the subject of the present investigation. each species includes the most stable geometry. Previous exExperimentally, propyne can be lithiated sequentially, giving perience also provided a g ~ i d e .All ~ . calculations ~ were carried C3H3Li (l),C3H2Li2, C3HLi3, and CjLi4.7$8Spectroscopic out using the G A U S S I A N 70 series of programs;15 a single evidence is available which characterizes these products as precision Telefunken TR440 version written by Dr. H.-U. being allene-like or propyne-like, but no structural details of Wagner was used a t Erlangen. An S C F damping routine, any of these compounds are known e ~ p e r i m e n t a l l yAllene .~~~ written by Dr. D. Poppinger, overcame convergence probbehaves similarly, but with one important difference. An allems. Ienyl- (or propargyl-) lithium (2) is formed first, rather than Cyclopropene, the third C3H4 isomer, is rather acidic; the Results high I3C-H vinyl coupling constant, 243 Hz, indicates very C3H3Li. All possible conventional monolithiated structures, high s ~ h a r a c t e rBoth . ~ mono- (3) and dilithiocyclopropene (4) 1, 3, 5, 9, and 10, were constructed from their hydrocarbon can be prepared easily, but their geometries and energies are parents, propyne, allene, and cyclopropene, using standard unknown.1° The methylene hydrogens are less acidic; basegeometries.I6 The STO-3G energies are presented in Table I. catalyzed exchange of these hydrogens occurs only when As expected, the acetylide structure, 1, is found to be the most the vinylic positions are substituted.” 3-Lithio- ( 5 ) and 3,3stable form. The acetylenic hydrogen is acidic, and can readily dilithiocyclopropenes ( 6 ) (or substituted analogues) are exbe replaced by a metal. The Li-C(sp) bond length is the LI-C =C-CH [H CCCH2 I L I shortest 3 Geometry optimization of the other isomers within each specified symmetry led in some cases to substantial reductions in energy (Table I ) and changes in structure (Table 11). Optimization of allenyllithium (9) leads to a bent structure, 11, in which the CCC unit no longer is linear. Li bridges the carbon Li Li skeleton, and the CCH2 group becomes nonplanar, increasing C-Li bonding. The electronic structure of 11 is similar to that 3, c, of allyllithium (S),5dwhere Li is bonded mainly through the
4,i
A
LI LI
5
c,
xi
9
Calculational Methods Single-determinant self-consistent field molecular orbital (SCF-MO) theory within the restricted Hartree-Fock for-
c,
Lr\
H-
t-c=C-H
H
10,
perimentally unknown. The anionically enhanced n-donor hyperconjugative character of C-Li bonds would be expected to destabilize 5 and 6 antiaromatically.” With 6, another interesting feature emerges. Our published calculations suggest 7, with a planar tetracoordinate carbon, to be more stable than the traditional “tetrahedral” form!5h This paper presents the results of an a b initio study of various C3H3Li, C3H2Li2, C3HLi3, and C3Li4 geometries. An attempt has been made to locate the local and absolute minima (isomers) on the potential energy surfaces. Since conventional bonding does not govern the structure of these molecules, the results obtained often seem surprising. This is shown by our calculated structures of C2Li2,5a CH2=CLi2,Sb C4Li4,5c C3H5Li (8),5dC J L ~( ~C ,H ~~ ~L ~ Z e t )c 5~ Such , ~ ~calculations refer to isolated, hypothetically motion-free molecules. In condensed phases, lithium compounds tend to be associated and to be solvated. The reader should keep these differences i n mind while evaluating the results we report.
I
c, H\
*%-c-c-
Ll L l
13, c,
H\
,*LI
H,c=c=c’
LI
’ 4 , C&
interaction with nonbonding ir orbitals (see Discussion). According to the STO-3G Mulliken population analysis, the negative charge in 11 is mainly centered on the two end carbons ( C I = -0.13, C3 = -0.19). The central carbon is nearly neutral with a charge of -0.028. The C I - C bond ~ length, 1.264 %.,is shorter than that in allene (1.288 8,at STO-3G)I8 while C2-C3 is somewhat longer ( I .356 A). The standard geometry propargyl structure, 10, also gives the same bridged form, 11,
Journal of the American Chemical Society / 1Ol:l I
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/ May 23, 1979
Table 1. Calculated Total and Relative Energies of Lithium-Substituted Propynes, Allenes, and Cyclopropenes' STO-3G structure
point group
re1 energy (STO-3G opt geom)
4-31Gi5-21G STO-3G opt geometry total re1 energy energy
standard geometry
STO-3G opt geometry
-121.205 15 -121.147 63 -121.151 80
-121,20647
0.0
-122.568 59
0.0
-121.187 58 -121.141 94 -121.092 51
11.7 40.5 71.5
- 122.548 30 - 122.494 54 -122.451 48
12.7 46.5 73.5
-127.911 64 -127.954 18 - 127.940 96 -127.934 1 I -127.893 I O -1 27.872 55 -127.887 99 -127.797 50 -127.819 52
26.7 0.0 8.3 12.6 38.3 51.2 41.5 98.3 84.5
- 129.359 92 -129.401 21 -129.388 13 - 129.380 68 - 129.377 09 -129.327 58 -129.311 43 - 129.25 1 97 -129.198 02
25.9 0.0 8.2 12.9 15.1 46.2 56.3 93.7 127.5
C3H3Li 1 9 10 11 3
-121.129 69 -121.072 23
5 C3H2Li2 12 14 15
-
127.899 92 -127.884 66 - 127.873 07
16 17 18
19 20 4 4a
- 127.884 69
- 127.764 27
6
- 127.777 52 - 127.865 73
7 13 C3HLi3 23 21 22 25 C3Li4 21 28 29 30 31
- 134.692 25
-136.233 08
-134.615 48 - 134.623 64 -134.612 45 -141.384 -141.387 -141.418 -141.416 -141.405 -141.441
36
93 16 87 00 64 66
35.6 34.2 14.3 16.1 22.6 0.0
-143.049 83 -143.050 63 -143.060 35 - 143.028 64 - 143.073 48 - 143.080 1 1
19.0 18.5 12.4 32.3 4.2 0.0
Total energies in hartrees. Relative energies in kilocalories/mole.
Table 11. STO-3G Optimized Geometries of C3H3Li Isomers'J,e molecule 1
II
3
5
c31.
c',
c',
c,
parameter ClC2 ClLi LHCC ClC2 ClLi C3Li CIH LOC3C2 icI 2d ClC2
c2c3
C2H LL~CIC: fOC3C2 ClC2 C3Li CIH LHClC3 LHC~CI LHC3Li
value 1.193 (1.216) 1 .8286 (1.896) 110.8(ll1.4) 1.264 (1.277) 1.914 (2.000) 2.246 (2.469) 1.080 (1.068) 69.5 (73.2) 156.9 (167.4) 1.299 (1.304) 1.466 (1.480) 1.074 ( 1.063) 147.5 ( 1 44.2) 156.5 ( I 57.6) 1.272 1.999 1.075 145.3 1 11.9 118.1
parameter
c2c3 C3H
c2c3 C2Li C3H LCIC2C3 LHClC2 LHC3H CIC3 ClLi C3H LHC2C3 LHC3H CIC3 C3H LCIC3C2 LLiC3C I S H C I C2C3
