Stepwise Photochlorination

All investigations into l,l,3 3-tetrachloro-l,3-disilacyclopentane 351 are related to the work done on (Cl2Si-CHj)3. The is accessible by reaction of CljSi-CH - CHj SiClj CHjCl with Mg [147], Photochlorination of 351 proceeds through a number of stages  

The perchlorinated 1,3-disilacyclopentane 354 was obtained in approximately 70% yield. Progressive chlorination of 3M was carefully monitored using NMR spectroscopy, and in the course of doing so the appearance and disappearance of various compounds in the reaction mixture was determined. By intercepting the chlorination reaction at points along the way, compounds 3 and 3 could be isolated. 

Radical chlorination of 351 always begins on C atoms 4 or 5, while chlorination of C atom 2 will only proceed when all available positions on C atoms 4 and 5 are substituted. Therefore, if was not possible to synthesize 1,1,2,2,3,3-hexachloro-1,3-disilacyclopentane in this way [148]. 

---

The most innovative photohalogenation technology developed in the latter twentieth century is that for purposes of photochlorination of poly(vinyl chloride) (PVC). More highly chlorinated products of improved thermal stabiUty, fire resistance, and rigidity are obtained. In production, the stepwise chlorination may be effected in Hquid chlorine which serves both as solvent for the polymer and reagent (46). A soHd-state process has also been devised in which a bed of microparticulate PVC is fluidized with CI2 gas and simultaneously irradiated (47). In both cases the reaction proceeds, counterintuitively, to introduce Cl exclusively at unchlorinated carbon atoms on the polymer backbone. 

The process of chlorination takes place in a stepwise manner, methyl chloro-formate being formed first and then, successively, the monochloro-, dichloro-, and trichloromethyl chloroformates. Methyl formate reacts readily with chlorine in the dark, giving methyl chloroformate. Further chlorination in the dark without the aid of a catalyst gives some chloromethyl chloroformate, but the reaction is slow and the yield is poor. Complete conversion can only be achieved by photochlorination at near the boiling point of the liquid [51, 70]. 

---