Carbenoid mechanism

Copper(II) triflate is quite inefficient in promoting cyclopropanation of allyl alcohol, and the use of f-butyl diazoacetate [164/(165+166) = 97/3%] brought no improvement over ethyl diazoacetate (67/6 %)162). If, however, copper(I) triflate was the catalyst, cyclopropanation with ethyl diazoacetate increased to 30% at the expense of O/H insertion (55%). As has already been discussed in Sect. 2.2.1, competitive coordination-type and carbenoid mechanisms may be involved in cyclopropanation with copper catalysts, and the ability of Cu(I) to coordinate efficiently with olefins may enhance this reaction in the intramolecular competition with O/H insertion. 

The reaction also takes place with other bases (e.g., LiH,213 Na in ethylene glycol, NaH, NaNH2) or with smaller amounts of RLi, but in these cases side reactions are common and the orientation of the double bond is in the other direction (to give the more highly substituted olefin). The reaction with Na in ethylene glycol is called the Bamford-Stevens reaction,214 For these reactions two mechanisms are possible—a carbenoid and a carbocation mechanism.215 The side reactions found are those expected of carbenes and carbocations. In general, the carbocation mechanism is chiefly found in protic solvents and the carbenoid mechanism in aprotic solvents. Both routes involve formation of a diazo compound (34) which in some cases can be isolated. 

As shown by calculations, the carbenoid mechanism of dichlorogermylene insertion in ordinary C—Cl bond begins by electrophilic attack of the vacant p-orbital of the Ge atom on the electrons of the bond . The vacant p-orbital does not interact with the unpaired electrons of the chlorine atoms. When the distance between the reactants decreases and becomes close to the value of the C—Cl bond length, inactivated transfer of Cl to the germylene center occurs with formation of a radical pair, followed by its recombination. The closer the components of the radical pair, the smaller the probability of radicals movement away from one another. The theoretical interest concerning the details of the germylene insertion mechanism is continuing. Thus, a quantum-chemical examination of... 

Examples of alkane functionalization reactions of the type shown in equation (1) are first considered, in which the atom X to which the new C—X bond is form comes from metals in Group I, followed by subsequent groups in the Periodic Table. Within each section, radical, electrophilic and carbenoid mechanisms are (Uscussed. 

The reaction probably proceeds througli a carbenoid mechanism. In fact, both heterogeneous and homogeneous catalysts are known to promote an H-D exchange between methane (or other alkanes) and DjO in the DjO-HOAc system. In the homogeneous phase, catalysis by (PtClj)-, DCIO4 and an arc-inatic additive (for example, pyrene) are used to stabilize the active species. 

Zicgler-type catalysts also promote the H-D exchange of CD with methyl aluminum groups and a carbenoid mechanism has been proposed in (his case, too, with the carbenoid resulting from a elimination of methane [40]. 

Here again a carbenoid mechanism may be invoked. An attempt to form a trihexylaluminum adduct of the ylid was unsuccessful presumably due to the increased steric requirement of the hexyl group. Similar polymerization reactions have been observed by Tufariello and Lee 149> using organoborane adducts of dimethyloxosulfonium methylide. 

The oxymercuration of 1-substituted (i,e, H, Me, and C02Me) tricyclo[4,l,0,0 ]-heptanes with mercuric acetate affords norcaranyi- and norpinyl-mercury compounds. A synthesis of 3,4-benzotricyclo[4,l,0,0 ]heptene (692) has been reported in which the usual cyclopropylcarbene C—H insertion process is employed. Isomerization of (692) with silver perchlorate gave benzocycloheptatriene which is also formed in the thermal isomerization of (692). Reaction of (692) with n-ally 1 palladium(ii) chloride dimer yielded 2-methylene-l T-naphthalene which rearranged readily to 2-methylnaphthalene at room temperature a carbenoid mechanism appears to be involved. 

---