Skell-Woodworth rule

The other carbene reactions shown in Scheme 8-15 will be discussed in Section 8.5. There again, we will see that violations of the Skell-Woodworth rule can be explained if the size of the singlet-triplet gap and the rates of forward and reverse intersystem crossing of carbenes are taken into account. In addition, it should be... 

Griller, D., Nazran, A.S., and Scaiano, J. C., Flash Photolysis Studies of Carbenes and Their Impact on the Skell-Woodworth Rules, Tetrahedron, 41,1525, 1985. 

Despite closely reasoned counter-arguments (DeMore and Benson, 1964), the commonly held view, due to Skell, is that singlet carbenes add to olefins in a stereospecific cia-manner, whereas attack by triplet carbenes leads to non-stereospecific addition (Skell and Woodworth, 1956). The rationale of this view is that a singlet carbene should react with the olefin to form a cyclopropane in a one-step, concerted process because in this way it could occur with conservation of spin (equation 23) the addition would thus be stereospecifically cis. On the other hand, a concerted addition of a triplet carbene would violate the rule of spin conservation in consequence, a multistep reaction, in which spin inversion of an intermediate 1,3-diradical constitutes a discrete process... 

The relationship between the reactivities of singlet and triplet carbenes in some of these processes has, however, attracted the interest of many investigators. Chemical evidence from such studies led to the conclusion that the singlet state can be intercepted due to its higher energy and that it displays a different chemistry from that of the triplet, since Skell and Woodworth developed their rule in the late 1950 s and indicated that it may also be applied to carbene reactions other than cyclopropanation. 

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