Carbenoids, electrophilic

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazo ketones and esters.215 In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into methine hydrogen is preferred to a methylene hydrogen. Intramolecular insertion can be competitive with intramolecular addition. Product ratios can to some extent be controlled by the specific rhodium catalyst that is used.216 In the example shown, insertion is the exclusive reaction with Rh2(02CC4F9)4, whereas only addition occurs with Rh2(caprolactamate)4, which indicates that the more electrophilic carbenoids favor insertion. 

The benzylic C-H activation has been effectively applied to the enantioselective synthesis of (+)-imperanene (Equation (16)).80 The key step was the Rh2(i -DOSP)4-catalyzed functionalization of the benzylic methyl C-H bond in arene 2. An impressive feature of this transformation was that both the carbenoid and substrate contained very electron-rich aromatic rings, which were compatible with the highly electrophilic carbenoids because they were still sterically protected. 

The proposed mechanism is as follows First, the a-sulfonyl lithium carbanion attacks the electrophilic carbenoid carbon atom to give the vinyhnagnesium intermediate (158). As the sulfonyl moiety is a good leaving group, a /-elimination takes place to afford the allenes (159). 

The most common method to prepare cyclopropenyl derivatives is the reaction between an electrophilic carbenoid and an alkene. On the other hand, sp3-geminated organodimetal compounds possess two nucleophilic sites on the same carbon, so should lead to nucleophilic [2+ 1] reaction with 1,2-diketones. Indeed, the reaction of bis(iodozincio)methane (3) with 1,2-diketones shows a novel [2+1] reaction to form c -cyclopropanediol diastereoselectively as shown in Scheme 3467. 

The intramolecular reaction between diazo ketones and benzenes is an effective way to generate a range of bicyclic systems.7 The earlier copper-based catalysts have largely been superseded by rho-dium(ll) salts. Unlike the case in the intermolecular reactions, rhodium(ll) acetate is the catalyst that has been most commonly used. Studies by McKervey,133 136 however, indicated that rhodium(II) mandelate, which would be expected to generate a slightly more electrophilic carbenoid than rhodium(ll) acetate, often gave improved yields. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

Considering the selectivity of this reaction (terminal vs. 1,2-disubstituted alkenes) and the fact that an electron-rich alkene such as isobutyl vinyl ether does not undergo cyclopropanation, it seems that the reactive species formed from the lithiated sulfone and the nickel catalyst does not behave as an electrophilic carbenoid. In this respect, one should note that the Simmons-Smith reagent is electrophilic whereas the methylene transfer reagent arising from treatment of dibromomethane with nickel(O) can achieve cyclopropanation of electron-deficient alkenes only. ... 

The cyclopropanation is initiated by the interaction of the electrophilic metal-carbene species with the jr-system of the olefin (Scheme 4). Two different mechanisms have been proposed for the formation of the cyclopropane ring a concerted pathway (a) or a two-step process via a metallacyclobutane (b). The first pathway (a) resembles the mode of addition of free carbenes to (C=C) double bonds [33] and has been proposed for reactions of metal carbenoids by various authors [7,11]. The principal bonding interaction in this case initially develops between the electrophilic carbenoid C-atom and the Ti-system of the C-C double... 

Transition metal-catalyzed Buchner reactions of arene substrates proceed via electrophilic carbenoids. In addition to cyclopropanation of the arene double bond, these a-diazoketones possessing an aromatic ring can also participate in C-H insertion reactions. As shown in the decomposition of diazomethyl ketone 53, the benzylic C-H insertion product 56 is obtained as a minor product (vide supra). The rhodium(II) acetate-catalyzed reaction of diazoketone 71 also affords cycloheptatriene derivative 73 along with the benzylic C-H insertion product, y-lactam 72, in a ratio of 1 2. Treatment of 71 with the more electron-rich rhodium(II) caprolactamate [Rh2(Cap)4] favors more C-H insertion, but the cycloaddition pathway is still significant the ratio of 73 to 72 is 1 3.5. 

The previous highly reactive, electrophilic carbenoid [Rh-Rh] =C intermediate, formed during these catalytic reactions, remained elusive until a recent seminal study by Berry and Davies et al. 

Bromination was carried out using bromine in presence of catalytic amount of aluminum chloride or bromine-dioxane complex at 0-20 °C to form 3-bromoderivative 173 in high yield [63], Electrophilic carbenoid species, generated at elevated temperature, reacted with polyfluoroindole to form indolyl carboxylic esters 174 after treatment with formic acid [58c],... 

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