Concerted addition triplet carbenes

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

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 addition of a triplet carbene to an aikene can be considered to be rather like a radical addition to a double bond. The concerted addition of a singlet carbene, on the other hand, is a pericyclic reaction, and from Chapter 35 you should be able to classify it as a [ 1 + 2] cycloaddition. 

The stereochemistry of these cycloadditions is so specific that Skell used it as a diagnostic test for distinguishing between singlet and triplet carbenes. According to Skell, the addition of singlet carbene to an olefin occurs in a concerted manner and is therefore stereospecific. However, in the case of triplet carbene, both the unpaired electrons cannot form a new covalent bond because of their parallel spins. Therefore, in the latter case the reaction will take place in two steps. In the first step a triplet diradical is formed, which undergoes spin inversion and then ring closure. For this the radical has to wait for the appropriate... 

The spin state of the reacting acylcarbene has a decisive influence on the reactivity of this short-lived species (Houben-Weyl, Vol.4/5b, pp 1158-1257 Vol. E19b, pp 1052 and 1231 refs 67,100-103). As far as cyclopropanation is concerned, carbene addition to a (Z)- or ( )-alkene can occur with retention or with loss of the stereochemical relationship present in the alkene (stereospecific and nonstereospecific cyclopropanation, respectively). Singlet carbenes add to C-C double bonds in a concerted manner, and therefore, stereospecifically. In contrast, a triplet carbene undergoes stepwise addition via an intermediate triplet 1,3-diradieal. Sinee spin-in-version must occur before ring closure of the latter species, the extent to which stereospecificity... 

The behaviour of carbenes depends upon whether they are in the singlet or triplet state. Additions of singlet carbenes to unsaturated systems are generally concerted and stereospecific. Additions of triplet carbenes are two-step processes they are non-stereospecific. The subject of addition of carbenes to alkynes fits into this chapter because most carbenes and carbenoids behave as electrophiles. 

Insertion into C-H bonds is more probable than insertion into C-C bonds. Insertion into C-C bonds does not appear to occur at all. For example, photolysis of diazomethane in cyclopentane at —75 °C produced only methylcyclopentane, with cyclohexane not being observed. Singlet carbenes are thought to add to C- H bonds by a concerted process, while triplet carbenes can produce net addition through hydrogen abstraction and then recombination of the alkyl radicals. It was found that o-(2-endo-norbornyl)phenylcarbene inserts into the 3-position in such a way as to give a trans-junction (Scheme 5.47). 

The reactivity of the singlet and triplet states should be different, as proposed by SkelL The triplet state is a diradical, and would be expected to exhibit a selectivity in its reactions toward olefins similar to that of other species having unpaired electrons. The singlet state, with its unfilled p-orbital, should be electrophilic and exhibit reactivity toward olefins similar to that of other electrophilic species. In general, carbenes add to alkenes to give cyclopropane derivatives. Dibromocarbene is typical in this respect. This reaction could involve the singlet dibromocarbene in a concerted addition, or the triplet state in a stepwise process ... 

Carbenes come in two flavors —spin-paired singlets and spin-unpaired triplets. Often the triplet is the more stable form, but the singlet state is usually more reactive. Both singlet and triplet carbenes add to alkenes to give cyclopropanes. In the concerted reaction of the singlets, any stereochemistry present in the alkene is preserved. In the nonconcerted additions of triplets, stereochemistry is lost. 

Skell s hypothesis proved to be extremely useful in carbene chemistry even though it was frequently opposed. The principal significance of these rules is represented in the scheme below. The singlet reaction occurs in a concerted step, the cis-addition product being formed in a stereospeciiic manner. In the triplet addition, which is a two-step reaction, rotation is thought to be faster than intersystem crossing (spin inversion) and ring closure, i.e., which would... 

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