Carbene addition, mechanism

In certain cases the process analogous to the isonitrile synthesis for the preparation of phosphaalkenes, showing proton- and halogene-substituted C-bridged atoms, is a successful one. 2,4,6-tri-i-butylphenyl-phosphane can be transferred to the phosphaalkene using a strong alkaline solution of chloroform [Eq. (7)] or methylene chloride [Eq. (8)]. A carbene addition mechanism is involved in this reaction (36, 37). 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

The addition of a particularly useful reagent, bearing two ester functionalities, is obtained by means of the Bingel-Hirsch cyclopropanation [29]. Although the mechanism may not be of carbene-type, the result is formally equivalent to a carbene addition. The presence of the ester groups renders this derivative susceptible for further modifications. 

When 2,2-dichloro-3-phenylpropanal 203 is subjected to standard reaction conditions with chiral triazolium salt 75c, the desired amide is produced in 80% ee and 62% yield Eq. 20. This experiment suggests that the catalyst is involved in an enantioselec-tive protonation event. With this evidence in hand, the proposed mechanism begins with carbene addition to the a-reducible aldehyde followed by formation of activated car-boxylate XLII (Scheme 32). Acyl transfer occurs with HOAt, presumably due to its higher kinetic nucleophilicity under these conditions, thus regenerating the carbene. In turn, intermediate XLin then undergoes nucleophilic attack by the amine and releases the co-catalyst back into the catalytic cycle. 

Problem 9.29 (a) Use the following observations to discuss stereospecificity of carbene addition, (b) Suggest mechanisms for addition of (i) singlet and (ii) triplet carbenes to alkenes. 

Overall, the transformations are equivalent to carbene additions to the styrenes. However, a carbene mechanism can be ruled out since the only alkenes which are successful are those carrying anion-stabilising groups. 

Substitution by the SN2 mechanism and -elimination by the E2 and Elcb mechanisms are not the only reactions that can occur at C(sp3)-X. Substitution can also occur at C(sp3)-X by the SRN1 mechanism, the elimination-addition mechanism, a one-electron transfer mechanism, and metal insertion and halogen-metal exchange reactions. An alkyl halide can also undergo a-elimination to give a carbene. 

The mechanism proposed by Shephard and Rooney (95) involved a (T-7i-l,2,5-triadsorbed species for the dehydrocyclization of o-ethyltoluene and a n-allylic species for the dehydrocyclization of propylbenzene (Scheme 41). We suggest, instead, a carbene-benzene addition mechanism, in better agreement with the general picture we have given for 1-5 and 1-6 ring closure (Scheme 81). 

Singlet carbene (concerted mechanism - stereospecific syn- addition)... 

Hamilton suggested that the model systems (at least some of them) interact with the substrates via a so-called oxenoid mechanism similar to that of monooxygenase functioning. Since the reaction, in many aspects, parallels the processes with carbenes (addition to the multiple bond, insertion into the C-H bond), in the oxenoid mechanism an oxygen atom (oxene) inserts into the C-H bond without an intermediate formation of free radicals ... 

CHEMICAL MODIFICATION The following chemical modifications of cis-1,4-polyisoprene are employed as a convenient way of altering physical and mechanical properties hydrohalogenation, halogenation, oxidation, ozonolysis, hydrogenation, carbene addition, cyclization. ... 

Many of the leading workers in this field participated in the Lyon symposium on the subject.The mechanism of alkene metathesis proceeds through a metallocyclobutane derivative formed by reaction of an alkene with a metal-carbene complex. Retro-carbene addition yields a product molecule and a new carbene... 

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