REINVESTIGATION OF THE S(N)I REACTION - THE IONIZATION OF CHLOROSULFITES

The decomposition of alkyl chlorosulfites (ROSOCl) has been investigated both computationally and experimentally. Semiempirical (AM1 and PM3) as well as ab initio (HF/3-21G(*), HF/6-31G*, and MP2(full)/6-31G*//MP2(full)/6-31G*) methods were employed, and the results were confirmed experimentally by NMR spectroscopy. The computations indicated that certain alkyl sulfinyl cations (ROSO+) are stable and might be involved in the decomposition of chlorosulfites. Detection of these ions by H-1 and C-13 NMR spectroscopy in polar solvents such as acetone-d6 and acetonitrile-d3 as well as kinetic studies allowed important conclusions to be drawn about the mechanistic details of the S(N)i reaction. We conclude that primary alkyl chlorosulfites ionize to yield a sulfinyl cation (ROSO+) and Cl-, whereas tertiary chlorosulfites preferentially give a carbenium ion and a chlorosulfinyl anion (OSOCl-). The generation of these ion pairs is facilitated in polar solvents where the rates of decomposition of chlorosulfites are largely accelerated. The decomposition of neopentyl chlorosulfite without rearrangement and the substitution at the bridgehead position of 7,7-dimethylbicyclo[2.2.1]-heptyl 1-chlorosulfite show that the loss Of SO2 from ROSO+ must be accompanied by the attack of the chloride ion from the front side.