An MNDO study of substituent effects in carbocations: the unimportance of inductive effects

Abstract

. 64, 532 (1986).Stabilization effects for both saturated and unsaturated substituents were examined using MNDO calculations. (By "saturated substituents" we mean to imply that the substituent is attached to C+ by sp3 carbon, whether or not the substituent has a site of unsaturation at some point remote from the carbocationic center.) Relative gas phase stabilities of regioisomeric L carbocations have been calculated. For simple groups, directly bound to C, a donation, a donation, and polarization effects are found to be important. Saturated-substituent stabilization effects are examined by means of reaction enthalpies for 64, 532 (1986).On utilise des calculs MNDO sur des carbocations choisis pour dCterminer les effets de stabilisation de substituants tant saturCs que non saturks. (Par "substituant saturC", on entend un substituant qui est attache au C+ par un carbone sp3 mCme si le substituant porte un site d'insaturation en un autre point CloignC du centre carbocationique.) On a calculC les stabilitCs relatives + de carbocations rCgioisom&res. Pour des groupements simples qui sont directement liCs au C , on a trouve que les effets de donation TI ou a ainsi que de polarisation sont importants. On a CtudiC les effets de stabilisation des substituants saturCs a l'aide + des enthalpies de rCactions isodesmiques entre les molCcules neutres appropriCes et le CH3. Les analyses traditionnelles des effets de substituants qui sont uniquement bakes sur les effets inductifs des groupements saturCs n'ont que peu ou pas de signification. Les effets inductifs n'ont pas d'importance spCciale dans la dktermination des effets de stabilisation des substituants saturCs des carbocations.[Traduit par le journal] Introduction As part of our work in organosulfur chemistry, we have recently initiated a program of theoretical studies. Our initial study (1) has raised two questions: (i) how does sulfur compare to saturated carbon as a carbocation stabilizer? and (ii) how useful is the concept of inductive stabilization in anticipating substituent-stabilization effects in carbocations? Recent studies authored by Topsom et al. (2, 3) have further stimulated our interest in problems associated with inductive effects.Inductive effects accompany a shift of electron density in a u bond toward the more electronegative atom involved in the bond. Predictions may be improved by utilizing group electro-

A MNDO study of planar thioacyl-substituted carbocations

“…There is a good inverse correlation between the stabilizing ability of the substituent and the weakening of the C,-C, n bond. These results are consistent with those obtained for thionium ions (2) and carbocations (7).…”

“…CF3 was also found to be a polarization destabilizer for carbocations (7). Table 6 presents ionic bond orders (8, 9a), involving C, for the destabilizing substituents.…”

“…Comparison of AAH,'s, a-bond orders, and a-electron populations at S leads to the conclusion that the hyperconjugative effects of BH2 and CH, have the same order of magnitude. Substituent effects it1 propyl carbocations (7). thionium ions (2), and thioacyl carbocations…”

Chemoselective reaction of a sulfonate ester with methoxide ions in preference to benzyl mercaptide anions

“…'O~hese calculations were carried out as part of a previously 1 published study. Details may be found in that paper (16).…”

Dications of Fluorenylidenes. Electronic Effects on the Paratropicity/Antiaromaticity of Diphenylmethyl-Substituted Fluorenyl Cations

“…Because the small sample size is the greatest limitation on conclusions that can be drawn, we have begun to examine these substituent effects using higher level calculational methods. We anticipate that calculations at the density functional theory level will allow us to discern the effects of σ and π donation …”