Addition reactions singlet carbenes

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

The mechanism for the addition of singlet carbenes to alcohols has been studied in some detail (Bethell et al, 1971 Kirmse et al, 1981). By and large, the evidence supports two routes. The first, more common, sequence features initial formation of an ylid. Under some circumstances this reaction is reversible (Zupancic et al., 1985 Liu and Subramanian, 1984 Warner and Chu, 1984). Next, proton transfer, either intramolecularly, which may be slowed by symmetry constraints, or by a pair of intermolecular protonation and deprotonation steps, gives the ether. These reactions are outlined in (7). 

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 addition of a triplet carbene to an alkene 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 3 5 you should be able to classify it as a [1 -h 2 ] cycloaddition, addition to alkenes of triplet carbenes is a radical reaction addition of singlet carbenes is a [1-h2] cycloaddition... 

The most important cheletropic reaction is the addition of singlet carbenes to olefins to make cyclopropanes. Only singlet carbenes will be considered here the pericyclic selection rules cannot be applied to triplet states. The electronic structure of a singlet carbene involves an empty p orbital and a roughly sp hybrid that has two electrons (see, for example, the two lone pair orbitals of the water molecule in Appendix 3). We know from Chapter 10 that singlet carbenes add stereospecifically to olefins, and that the olefin stereochemistry is retained in the cyclopropane product. As such, in the present context, the reaction would be described as suprafacial on the olefin. 

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. 

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

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. 

The experimental ratio of ds- to trans-cyclopropane 43 46, i.e. the stereo-specifity of the reaction cannot be considered as a simple indication of singlet or triplet percentage of RaC , since the stereochemistry of the cyclo-addition depends on many factors. Photolysis produces the exdted 5i-state of the diazoalkane 41. This compound can lose nitrogen and form the singlet carbene 42 (So-state). 42 can add directly in a stereospecific manner if ki is large. If, however, intersystem crossing 42 45 (Aisc is large) competes favorably with... 

A deeper understanding of carbenic philicity requires a more detailed representation of the addition reaction transition state than that afforded by structure 4. Early MO calculations furnished structure 6 as representative of the transition state for addition of a singlet carbene to an alkene (Fig. 7.6). " ... 

Finally, addition reactions of the isolable phosphasilylcarbenes (13) to such electron-poor substrates as methyl acrylate, C4F9CH=CH2, and styrene afford cyclopropanes. The additions of 13a to (E)- or (Z)-p-deuteriostyrene are stereospecific, and the competitive additions of 13b to ring-substituted styrenes exhibit nucleophilic selectivity, consistent with singlet, nucleophilic carbene addition (Fig. 7.8). ... 

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