Cycloaddition reactions of unsaturated sulfones

The reaction of a series of allyl-substituted bis(phenylsulfony1)methanes or dimethyl malonates with 2,3-bis(phenylsulfonyl)l13-butadiene in the presence of base afforded alkenyl-substituted allenes in good yield. The reaction proceeds by initial attack of the soft carbanion onto the terminal position of the diene and subsequent PhS02elimination to give the phenylsulfonyl substituted allene. The thermal reactions of these phenylsulfonyl allenes gave [2+2]-cycloadducts. Only the Cl-C2 double bond of the alleneparticipates in the [2+2]-cycloaddition. Stepwise bonding prefers to occur in a 1 ,&ex0 manner rather than in a 1,7-endo fashion. The formation of all products can be rationalized by a mechanism which includes an initial carbon-carbon bond formation involving the central allene carbon to give a diradical intermediate. The product distribution is then determined by the substitution pattern of the alkene and the fate of the diradical intermediate. [2+2]-Cycloaddition reactions between allenes and ethylene derivatives have frequently been employed for the preparation of methylene cyclobutane derivatives1 Many of these reactions proceed by way of photochemical initiation, in which case the mechanistic pathway involves stepwise ring closure via diionic or diradical intermediates2 Not only is there considerable regiochemical regularity in the [2+2]-photoaddition, but the products are also easily transformed into useful ring systems by one of several general methods3 making this a very synthetically useful reaction. While well represented in the literature, these photochemical protocols are not the sole choice for allenic [2+2]-cycloadditions. Certain examples involve Lewis acid catalysis, where ionic intermediates are clearly involved.4~5 Still others proceed under strictly thermal condition^.^^^ The mechanistic details associated with these [2+2]-reactions constitute a topic of much study and debate.8 Substitution on the allene not only enhances its reactivity but also allows for the formation of a mixture of regioisomers. In most cases it has not been unequivocally established whether the cyclizations are concerted or stepwise in nature. A concerted reaction would require an antarafacial-suprafacialorbital interaction and result in a stereospecific cyclization. On the other hand, a stepwise mechanism would proceed by an initial rate-determining carboncarbon bond formation, most likely involving the central carbon of the allene, due to ally1 stabilization in the resulting diradical, followed by a subsequent radical coupling. This latter step has been termed the 'product-determining steps since there are two possible sites for secondary ring closure. If this process is rapid enough, the entire cyclization will occur with stereospecificity. Thus, the distinction between a concerted and stepwise mechanism is not necessarily apparent in product distribution. Furthermore, molecular orbital calculations indicate that the two mechanisms are comparable energetically.10 Intramolecular [2+2]cycloaddition of allenes has also been studied, and this process constitutes a particularly versatile method for the stereocontrolled synthesis of a variety of functionalized polycyclic compounds.1 1