Intramolecular homolytic substitution of sulfinates and sulfinamides – a computational study

Abstract

Ab initio and density functional theory (DFT) calculations predict that intramolecular homolytic substitution by alkyl radicals at the sulfur atom in sulfinates proceeds through a smooth transition state in which the attacking and leaving radicals adopt a near collinear arrangement. When forming a five-membered ring and the leaving radical is methyl, G3(MP2)-RAD//ROBHandHLYP/6-311++G(d,p) calculations predict that this reaction proceeds with an activation energy (ΔE(1)(‡)) of 43.2 kJ mol(-1). ROBHandHLYP/6-311++G(d,p) calculations suggest that the formation of five-membered rings through intramolecular homolytic substitution by aryl radicals at the sulfur atom in sulfinates and sulfinamides, with expulsion of phenyl radicals, proceeds with the involvement of hypervalent intermediates. These intermediates further dissociate to the observed products, with overall energy barriers of 45-68 kJ mol(-1), depending on the system of interest. In each case, homolytic addition to the phenyl group competes with substitution, with calculated barriers of 51-78 kJ mol(-1). This computational study complements and provides insight into previous experimental observations.

Stereochemical aspects and the synthetic scope of the SHi at the sulfur atom. Preparation of enantiopure 3-substituted 2,3-dihydro-1,2-benzoisothiazole 1-oxides and 1,1-dioxides

“…Concerning the first aspect, the inversion of the sulfur configuration at 1 was assumed on the basis of the behavior of sulfoxides 5 and sulfinates, 6 but it was not unambiguously proven (the absolute configuration of 2 was never established). ROBH and HLYP/6-31++G(d,p) calculations suggest this inversion to occur via hypervalent intermediates 7,8 (A in Scheme 1a), susceptible of giving pseudorotation processes prior to its dissociation. It would explain the slight racemization observed in the formation of 2 (94% ee) from optically pure 1.…”

7-Selenabicyclo[2.2.1]heptane

“…1 reveals that transition state 7 (R¼Me), with BHandHLYP/6-311þþG(d,p) calculated key distances of 2.066 and 2.212 A, like its sulfur counterpart, 16 is 'late' in the direction of reaction depicted in Scheme 3. Similar data are provided at the other levels of theory employed in this study (Table 1); indeed there appears to be little variation in r 1 and r 2 with theoretical level.…”

“…However, when the leaving group was replaced with the phenyl radical, an intermediate (11) was located on the C 13 H 11 O 2 Se potential energy surface, in a similar manner to that previously observed for the corresponding sulfinate. 16 Fig . 3 depicts the ROBHandHLYP/6-311þþG(d,p) calculated transition states 9 (R¼Me, t-Bu) and reveals that these structures, with calculated values of 2.293 A (Me) and 2.358 A (t-Bu) for r 1 , are 'earlier' than the analogous structures 7 located for alkyl radical attack.…”

“…This study aimed to provide a comparison with the analogous 'parent' sulfinate system, 16 while at the same time affording valuable benchmarking data and confidence in the computational methods used.…”

“…16 With an inherent interest in homolytic substitution reactions involving selenium and their further development, 17 we chose …”