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The Effect of Adjacent Unshared Electron Pairs on Ease of Carbanion Formation' Jack Him* and Philip D. Dalsin Contribution f r o m the Department of Chemistry, The Ohio State Unioersity: Columbus, Ohio 43210. Receiced February 2, 1972 Abstract: The kinetics of sodium methoxide catalyzed deuterium exchange of methyl cyclopentanecarboxylate, two of its oxa derivatives, and their acyclic analogs were studied in methanol-0-d solution. At 35" methyl cyclopentanecarboxylate exchanged 13 times as fast as methyl a-ethylbutyrate, methyl tetrahydrofuran-2-carboxylate exchanged half exchanged four times as fast as methyl a-methoxybutyrate, and methyl 1,3-dioxolane-2-carboxylate as fast as methyl dimethoxyacetate. From these results it was concluded that rates of carbanion formation may be decreased by repulsions between the unshared electron pairs of the carbanionic carbon atom and those of oxygen
atoms attached directly to it. Such repulsions may be minimized by appropriate rotation around the carbonoxygen bond in acyclic compounds but not as easily in cyclic compounds. This electron repulsion effect operates in addition to the electronegativity effect, by which rates of formation of sp2-hybridized carbanions are slowed by highly electronegative atoms attached directly to the site of carbanion formation. This interpretation of the results is analogous to that required to explain recent observations on rates of pyramidal inversions at nitrogen and phosphorus.
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t has been suggested that highly electronegative stituents on carbanion formations8 Attributing desubstituents on unsaturated carbon tend to make a stabilization of carbanions by a-fluorine to a positive compound less stable than analogous species in which mesomeric effectY or to conjugative destabilization lo the substituents are attached to saturated c a r b o n 2 To may be equivalent. Such a n explanation for the effect test this hypothesis in cases where stabilizing r-resoof a-methoxy and -fluor0 substituents on the rate of nance interactions between the substituents and the deuterium exchange of methyl esters2 had been conunsaturated carbon are minimized, the effect of a subsidered, but it seemed to offer no explanation for why stituents on the rates of sodium methoxide catalyzed the deviation from the Taft equation correlation obdeuterium exchange of methyl esters was ~ t u d i e d . ~ served with a fluoro substituent (104.4-fold) was so a-Fluorine and a-methoxy substituents were found t o much larger than that observed with a methoxy subdecrease the reactivity or to increase it by much smaller stituent (102.'-fold)." It is true that the smaller coamounts than would be expected from a Taft equation valent radius of fluorine would bring the orbitals concorrelation of the effect of a number of other substitutaining the unshared electrons closer to the negatively ents. Similar effects of a-fluorine and a-alkoxy subcharged carbon than the analogous orbitals on oxygen stituents had been observed in all the cases of formation in a methoxy substituent. However, it seemed unlikely that this would d o much more than compensate of sp?-hybridized carbanions upon which literature data were found. In the smaller number of cases in for the more diffuse character of the orbitals bearing which the formation of a-fluoro- and a-alkoxy-subthe unshared electrons on oxygen, resulting from the stituted sp3-hybridized carbanions was studied, these smaller nuclear charge of oxygen (cj: the larger van substituents were found t o increase the reactivity der Waals radius of oxygen). A potentially impormarkedly (although perhaps not by as large a factor tant factor that had been overlooked was the possibility that interactions between the 7r electrons of the carbas would be expected from the inductive effect). An alternative (or additional) explanation for the deacanion and the unshared electrons of a methoxy subtivating effect of cy-fluorine and a-alkoxy substituents stituent would be minimized by appropriate rotation on the formation of sp2-hybridized carbanions is that around the bond between the sp2 carbon and oxygen there is a repulsive interaction between the unshared atoms. To obtain experimental data concerning this possibility, we have studied the deuterium exchange electron pairs of the substituent and the T electrons of of methyl esters of cyclopentanecarboxylic acid and the sp?-hybridized carbon atom of the carbanion. The two of its oxa derivatives. Some reference compounds slower carbanion formation at the 2 position than at were also studied to give an idea of the significance of the 3 and 4 positions of pyridine has been explained steric effects. in such terms,4-' as have the effects of a-halogen sub(1) (a) This a o r k was supported in part by Research Grants GP7629 and GP-14697 from the National Science Foundation. (b) Part XVI in the series Structurai Effects on Rates and Equilibria. For part XV sec J. Hine, J . Amer. Chem. Soc., 94, 5766 (1972). ( c ) Abstractzd in part from the Ph.D. Dissertation of P. D, Dalsin, 1970. ( 2 ) J. Hine and P. R . Langford, J . Amer. Chem. SOC.,78, 5002 (1956). (3) J. Hine, I.. G. Mahone. and C. L . Liotta, ibid., 89, 591 I (1967); 91, 1043 (1969). (4) R. A . Abramovitch, G. M. Singer, a n d A. R. Vinutha, Chem. Commrin., 5 5 (1967). ( 5 ) I . F. Tupitsyn, N. N . ZatLepina, A . W. I
