ABSTRACT
Probably the –rst di-π-methane examples [2,3] are depicted in Equations 21.2 and 21.3. But these “Type-A” and “Type-B” rearrangements involve excitation of the enone moiety to an n-π* triplet state rather than the more common π-π* excitation; and we will arbitrarily not include these in our present discussion. Nevertheless, these rearrangements, involving a pair of π groups bonding to an sp3-hydridized carbon led us to look for generality of the reaction. Another early example of the di-πmethane rearrangement is illustrated in Equation 21.4 [4]. In contrast to the Barrelene example, this acyclic di-π-methane reactant, the Mariano diene, utilizes the singlet excited state (i.e., S1). Again due to symmetry there is only one available mode of ring opening of the cyclopropyldicarbinyl diradical species. However, in the case of the unsymmetrical diene in Equation 21.5 there are two a priori ways, a and b, for cyclopropyldicarbinyl ring opening [5]. e cyclopropyldicarbinyl diradical species selects only one of these (i.e., b). e reaction regioselectivity results from the demand for maintenance of benzhydryl odd-electron delocalization; thus the benzhydryl odd-electron is not used by the cyclopropyldicarbinyl diradical. is was an early example of regioselectivity. One of two examples by GriŸn and coworkers [6] is shown in Equation 21.6. Although no mechanisms were advanced at the time, we can see that the usual di-π-methane mechanism suŸces to lead to the observed photoproducts using the mechanism we write in Equation 21.6.
