Singlet carbene reaction with alkene

Before we close the discussion of carbene reactions with alkenes we mention tetrahedral boron hydride-substituted diazomethanes (8.11-8.13) which were obtained by the group of Jones (Li and Jones, 1992 Li et al., 1993) by manipulating substituted o-carboranes, as shown in (8-19). Reaction of 8.13 with ( )-but-2-ene yielded the pure (jE )-derivative, i.e., the product of singlet addition. With (Z)-but-2-ene 3 % triplet reaction product was observed. The percentage of triplet products was higher with 8.13 (22 and 18 7o with (E)- and (Z)-but-2-ene, respectively) (Huang et al., 1992). 

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. 

Carbene protonation has been amply demonstrated by product studies, time-resolved spectroscopy, and kinetic measurements. The ability of singlet carbenes to accept a proton is not adequately described by the traditional scale of carbene philicities, which is based on addition reactions with alkenes. In particular, aryl- and diarylcarbenes excel as proton acceptors but would traditionally be classified as electrophiles. 

By analogy with the mechanism proposed for the reaction with alkenes, C—H insertion product formation can be explained in terms of a H abstraction-recombination process of triplet arylcarbenes. The observations that ground-state singlet carbenes, for example, chlorophenylcarbene (67), produce only O—H insertion... 

The monosubstituted (trifluoromethyl)carbene has only been generated by irradiation of 2,2,2-trifluorodiazocthane, which is prepared by nitrosation of 2,2,2-trifluoroethylamine.1,3 175 The course of reactions with alkenes is dependent on concentration and pressure conditions. Thus, insertion products and pyrazolines may be obtained in competition with [2+1] cycloaddition giving the cyclopropane system. Yields of the latter are often moderate and, depending on singlet or triplet carbene formation, the reaction is not always stereoselective (Table 14). 

Singlet and triplet fluorenylidenes have been generated and observed spectroscopically as transient products of irradiation of diazofluorene in acetonitrile. As a result, it has proved possible to calculate the rate of intersystem crossing and the rate constants for reaction of both carbenes with alkenes and alcohols. Reaction of singlet fluorenylidene (67) with alkenes affords cyclopropanes as the major product, as shown for methyl methacrylate in Scheme 9. Surprisingly,... 

The addition reaction with alkenes is the best-studied reaction of carbene intermediates, both from the point of view of understanding carbene mechanisms and for synthetic applications. The usual course of reaction of a carbene with an alkene results in the formation of a cyclopropane, an observation that is true for both the singlet and the triplet state of most carbenes. The alternative electronic states... 

Product Studios. Alkenes are known as diagnostic reagents for spin state of reacting carbenes. Thus, the reaction of a singlet carbene with an olefin usually results in the formation of a cyclopropane through stereospecific addition to the C—C double bond, while a triplet carbene gives rise to a nonstereospecific addition product (see Section 7). 

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