Carbenic character

As a tool to improve the regio- and stereoselectivity of C-H insertion, activation of a specific C-H bond of substrates to be inserted seems to be appropriate in conjunction with the manipulation of carbene character. These two tools for the improvement of insertion selectivity will provide us with useful tools of the C-C bond formation by carbenes and carbenoids. 

In 1981, Appel et al. postulated the transient formation of the (diphenyl-phosphino)(trimethylsilyl)carbene 2e to explain the formation of the phos-phaalkene 4e, in the thermolysis of the P-chloromethylene phosphorane 3e.29 At that time, the authors did not recognize the carbene character of 2e and simply named the intermediate a A5-phosphaalkyne. 

Formamides are usually not deprotonated a to nitrogen but at the formyl group [227-231], The resulting carbamoyl lithium derivatives (R2NCOLi), which can also be generated from deprotonated amines [351] or amides [352] and carbon monoxide, react with electrophiles E+ to yield the expected products (R2NCOE), despite the carbene character and consequent low stability of these intermediates [179, 351] (Scheme 5.39, see Section 5.4.7). Palladium-catalyzed versions of the reaction have been reported [353, 354],... 

In contrast, the addition of amines results in nucleophilic addition/elimination at the opposite a-position, resulting in substitution of the a-oxygen substituent, a reaction consistent with the Fischer carbene character embedded into this architecture (Equation 2) <2004JOM2000>. This regioselectivity difference almost certainly arises from the reversible nature of addition reactions using such weakly nucleophilic reagents. 

However, the carbene character of Bertrand s phosphinocarbene 89 was initially doubted, as its chemical reactivity appeared to resemble moreaphosphaacetylene90 (Dixon etal. 1991 Soleihavoupetal. 1992 Bourissou and Bertrand 1999), but recent studies seem to support its carbene character (Scheme 20) (Despagnet et al. 2002). 

The crystallographic data also stresses the important carbenic character of the aminocarbyne-metal bond (Re=C=NHR) inasmuch as the NHR group is planar and the C-N bond distance is appropriate for a C=N bond. The chemistry of aminocarbynes has been thoroughly reviewed. ... 

Insertion reactions of carbon monoxide and isonitriles. The carbenic character of carbon monoxide and isonitriles, which favors initial donation of an electron pair to the 16-electron Zr atom, is certainly responsible for the success of these processes. 

The triplet dimer diradical DR2(Ti) finally will relax into thermal equilibrium (kT) with its singlet ground state DR2(So). As we have seen from the ESR spectra (see Fig, 10) the energy separation between the singlet and triplet diradical states is very low and thermally activated transitions occur even at low temperatures. Furthermore the ESR spectra have revealed an admixture of about 10% carbene character with the diradical intermediates. This carbene character may be important in determining the probability x of the side reactions (see Eq. (19)) for the DR -+ AC chain termination reaction. It surely is not, however, the only essential factor, otherwise there should be no difference in the optical and thermal termination reaction steps. Up to now a direct observation of the metastable triplet state Ti(M) has been possible only in two specific crystals where the polymerization reactions are very weak. 

The crystal structure of a lithiated thiazole showed the carbene character of the compound (see Section 3.06.5.7.1) <95AG(E)487>. 

Carbonyl compounds and imines are the electrophilic partner of isocyanides in the Passerini-3CR and the Ugi-4CR, respectively. Other highly electrophilic multiple bonds, such as in dimethyl acetylenedicarboxylate (DMAD), are also known to react with isocyanides. The initially formed 1 1 zwitterionic species having carbanion, carbene character can undergo further reaction with DMAD and isocyanides in different molar ratios, leading to a variety of heterocyclic systems (Scheme 5.32) [69]. 

These carbenes have not been trapped by alkenes. Presumably the presence of an electron-rich phosphorus atom next to an electron-deficient carbon atom results in some delocalization of the phosphine non-bonded electron pair, tending to produce a dipolar species as shown above, and reducing the carbene character of the intermediate. 

A related carbene formed in the coordination sphere of a gold(I) complex has been identified as a potential intermediate in the cycloisomerization of an alkynylindole/ The complex exhibits both carbocationic and a carbenic character. 

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