Cyclopropane singlet carbenes

A second characteristic reaction of carbenes is addition to olefins to yield cyclopropanes. Singlet carbenes might react as either nucleophiles or electrophiles triplets may be expected to behave like free radicals- The data in Table 5.9, showing the increase in rate of addition on substitution of electron-donating... 

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. 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

The species identified as XA reacts with styrene to give the expected cyclopropane. The rate constant for this reaction is ca 200 times less than the corresponding rate constant for 3BA (Table 6). Also, use of the deuterium-labeled a-methylstyrene reveals that the cyclopropanation occurs with essentially total retention of stereochemistry. Moreover, precisely the same result is obtained when this carbene is formed by triplet sensitization rather than by direct irradiation. These findings also point to a reaction originating from a singlet carbene. 

The singlet carbene adds stereospecifically, because it can correlate with the lowest singlet configuration of a trimethylene diradical, and thus also with a ground state of cyclopropane 1° ). 

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... 

The photolysis of diphenyl-diazomethane in cis-jS-deutero-styrene, on the other hand, can be affected by dilution with hexafluorobenzene. The amount of trans-cyclopropane 49 is slightly larger, indicating that about 12% of the singlet carbene are still present in very dilute solutions (see Table 9). 

The sensitized reaction yields the cyclopropanes 67 as the only product, whereas direct photolysis results in a Wolff-rearrangement yielding the cyclo-pentaneester 77 AWolff-rearrangement is only given by the singlet carbene... 

Photolysis of 21d (see earlier 21) in alkenes (cyclohexene, 1,1-diphenylethylene) always gave the cyclopropane derivatives 75 by a cis addition of the singlet carbene (64JOC3577) (Scheme 21). 

Singlet and triplet carbenes exhibit different properties and, to great extent, show markedly different chemistry . For example, a singlet carbene will add to a c/ -disubstituted alkene to produce only cis-disubstituted cyclopropane products (and to a rara-disubstituted alkene to produce only /rara-disubstituted cyclopropane products), while a triplet carbene will add non-stereospecifically to produce a mixture of cis and trans products. 

The stereospecificity observed with (Z)- and ( )-2-deuteriostyrene presents convincing evidence for the concerted nature of the cyclopropanation reaction, and therefore the genuine singlet carbene nature of stable (phosphino)(silyl)carbenes (I). The diastereoselectivity is at first glance surprising. Indeed, it is clear that steric factors cannot govern the observed selectivity since the bis(amino)phosphino group... 

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). 

Carbenes are much more reactive toward carbon-carbon double bonds than toward single bonds. Without doubt the most useful feature of a elimination is that it provides a practical route to cyclopropanes and cyclopropenes by [2 + 1 ] cycloaddition of carbenes to double or triple bonds. These additions are stereospecific suprafacial additions if they involve singlet carbenes, but can give mixtures with triplet carbenes ... 

Analysis of the product distributions arising from both sensitized and non-sensitized irradiation of 2-allyloxyphenyldiazo species (8) showed that the C—H insertion product and much of the cyclopropanation arise from the triplet carbene.16 For the singlet carbene, intermolecular 0—H insertion with methanol is about 50 tunes faster than intramolecular addition to the double bond, hi this system, intramolecular reactions and intersystem crossing of the triplet carbene proceed at similar rates, hi the closely related indanyl system (9), the smaller RCR angle stabilizes the singlet state relative to the triplet and the intramolecular reactivity is dominated by the singlet state.17... 

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... 

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