Alkenes reaction with carbenes, stereochemistry

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... 

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. 

A singlet carbene adds stereospecifically, meaning that reaction with a cis-alkene gives only ds-cyclopropane, and reaction with a frans-alkene gives trans-cyclopropane. The reaction of the triplet carbene is not stereospecific, however, and the product is a mixture of isomers. The difference in stereochemistry arises because the singlet carbene can add in one step (equation 5.39),... 

Alkenes react with a triplet carbene (i.e., diradical) in a stepwise fashion. A concerted reaction is not possible for triplet carbenes because of the spins of the electrons involved. After the carbene adds to the alkene in a radical reaction, the diradical (triplet) intermediate must wait until one of the spins inverts so that the second C—C bond can be formed with paired electrons. This intermediate also lives long enough for C—C bond rotation and thus loss of stereochemistry. 

The triplet carbene, with its two nonbonding electrons of the same spin, must produce an intermediate that cannot close to a cyclopropane until a spin is somehow changed. Such spin flips are not impossible, but they are usually slow on the time scale of molecular processes. In particular, they are slow compared with the very rapid rotations about carbon-carbon single bonds. Therefore the stereochemistry of the original alkene need not be maintained in the eventual product of the reaction, the cyclopropane (Fig. 10.48). If rotation about the carbon-carbon bond is faster than the rate at which the spin quantum number of one electron changes (spin flip), it will not matter whether cis or trans alkene is used in the reaction with triplet methylene. The product will be a mixture of cis and trans 1,2-dialkylcyclopropanes in either case. 

Reactions of cyclopentyne with alkenes gives [2+2] cycloadduct with complete retention of stereochemistry (Scheme 24) [120], Laird and Gilbert observed the expected [2+2] cycloadduct along with the polycyclic adduct in the reaction of norbomyne with 2,3-dihydropyran (Scheme 24) [121], and located a cyclopropyl-carbene intermediate [122],... 

In marked contrast, we used the cyclopropanation reaction of (phosphino)(silyl)-carbene (la) with methyl acrylate to demonstrate the carbene nature of our compound. More recently, we studied in detail the stereochemistry of this type of reaction.The (phosphino)(silyl)carbene (la) reacted efficiently with 3,3,4,4,5, 5,6,6,6-nonafluorohex-l-ene and (Z)- and ( )-2-deuteriostyrene giving the corresponding cyclopropanes in good yields. The stereochemical outcome was such that all monosubstituted alkenes gave exclusively the syn isomer (with respect to the phosphino group), and the addition of disubstituted alkenes was totally stereospecific (Scheme 8.16). 

Diphenylcyclopropenylidene (391) reacts with electrophilic olefins such as dimethyl fumarate and maleate only. The fact that the same spiropentene 393 (stereochemistry unknown) is isolated from reaction of 391 with both alkenes (equation 111) supports the intervention of zwitterion 392 and augments the concept of cyclopropenylidenes being very nucleophilic carbenes. 

The reaction of 3-chloro-3-(2-thienyl)-3//-diazirine with alkyl-substituted alkenes gave predominantly the trans-isomer of 5 while with acrylates, crotonates and styrene, the c/s-isomer predominated. The reaction of chloro(2-thienyl)carbene with (Z)- and ( )-but-2-ene proceeded with the retention of alkene stereochemistry. 

This reactivity series and the stereochemistry of the cyclopropane adducts. Contrasting with that observed in the case of pentacarbonyldiphenyl carbene, indicate that the reaction first involves an electrophilic attack of the alkylidene complex on the alkene without prior formation of an alkene-metal complex. A transition state (see Fig. 1) involves the formation of a bond from the alkylidene carbon atom to the less substituted end of the alkene and interaction of the positively polarized, more substituted end of the alkene with the carbon atom of the phenyl group. 

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