TY - JOUR

T1 - Disulfide conformational analysis. The nature of the S-S rotation barrier1 1 Organo-Sulphur Mechanisms-9. For Part 8 see L. Carlsen and J.P. Snyder, J. Org. Chem. 43, 2216 (1978).

AU - Jørgensen, Flemming S.

AU - Snyder, James P.

PY - 1979/1/1

Y1 - 1979/1/1

N2 - Previous DNMR measurements for a series of bulky disulfides led to the conclusion that rotation about the S-S bond occurs preferentially through the cis transition state. To investigate this conclusion and to study the conformational properties of disulfides in general, we have applied Allinger's force field to a series of dialkyl disulfides generated by homologating dimethyl disulfide to di-t-butyl disulfide. The optimized ground state geometries evidence a gradual increase in the CS-CS dihedral angle from 83 to 114° and indicate that increased substituent bulk drives the disulfide system in the direction of the trans rotational maximum. Explicit calculation of barrier heights yields ΔE(trans) < ΔE(cis) in every case. Furthermore the energy gap, ΔΔE(cis-trans), increases sharply as substituent size grows. This trend results from a rapid rise in the cis barrier and a small drop in the trans one. A rotation-inversion pathway is ruled out and it is concluded that disulfide conformational isomerization occurs by way of the trans transition state. p ]Torsion about the S-C bonds for several t-Bu substituted disulfides is considered. A strongly coupled alkyl-t-Bu rotation is observed computationally in accord with Nelander and Sunner's speculations concerning a "cogwheel effect." ΔG† trends for S-S rotation are discussed in connection with the latter. p ]Finally a ΔH(S-S) parameter is derived. Heats of formation and strain energies for dialkyl disulfides are calculated.

AB - Previous DNMR measurements for a series of bulky disulfides led to the conclusion that rotation about the S-S bond occurs preferentially through the cis transition state. To investigate this conclusion and to study the conformational properties of disulfides in general, we have applied Allinger's force field to a series of dialkyl disulfides generated by homologating dimethyl disulfide to di-t-butyl disulfide. The optimized ground state geometries evidence a gradual increase in the CS-CS dihedral angle from 83 to 114° and indicate that increased substituent bulk drives the disulfide system in the direction of the trans rotational maximum. Explicit calculation of barrier heights yields ΔE(trans) < ΔE(cis) in every case. Furthermore the energy gap, ΔΔE(cis-trans), increases sharply as substituent size grows. This trend results from a rapid rise in the cis barrier and a small drop in the trans one. A rotation-inversion pathway is ruled out and it is concluded that disulfide conformational isomerization occurs by way of the trans transition state. p ]Torsion about the S-C bonds for several t-Bu substituted disulfides is considered. A strongly coupled alkyl-t-Bu rotation is observed computationally in accord with Nelander and Sunner's speculations concerning a "cogwheel effect." ΔG† trends for S-S rotation are discussed in connection with the latter. p ]Finally a ΔH(S-S) parameter is derived. Heats of formation and strain energies for dialkyl disulfides are calculated.

UR - http://www.scopus.com/inward/record.url?scp=0043228154&partnerID=8YFLogxK

U2 - 10.1016/0040-4020(79)85034-6

DO - 10.1016/0040-4020(79)85034-6

M3 - Journal article

AN - SCOPUS:0043228154

VL - 35

SP - 1399

EP - 1407

JO - Tetrahedron

JF - Tetrahedron

SN - 0040-4020

IS - 11

ER -