Carbene direct insertion mechanism

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

There are three general classes of mechanism most often encountered in alkane reactions (i) radical (ii) electrophilic and (iii) carbenoid. The C—bond-lneaking st in (i) and (ii) are shown in equations (2) and (3). Carbenoid reactions can go either by direct insertion into the C—bond (equation 4), which tends to happen when the carbene in question has singlet character, or by a two-stqi process (equations 5 and 6), in which H-atom abstraction precedes collapse of the radical pair, a pathway which is characteristic of triplet carbenes. ... 

Direct insertions into C—H bonds have been postnlated for reactions that proceed through metal carbenes and nitrenes (12/13). " In these mechanisms, the unsat-nrated M=X intermediates 12 or 13 directly attack the C—H bond and form the prodnct in one concerted step. Often, a preference for cleaving weak C—H bonds is observed however, C—H functionalizations of alkanes are known that have been proposed to follow this so-called C—H insertion pathway. Stereoselective and even enantioselective methodologies have been developed based on this strategy using snitable enantiomerically pure ancillary ligands." ... 

The chemistry of alkenyl- and alkyl-substituted cyclopropylidenes has been explored. They were generated from a. gm-dihalide by lithium-halogen exchange, and were shown to give both allenes and 1,5-CH bond insertion reactions. A new mechanism for the insertion of carbenes into OH bonds was proposed on the basis of a computational study. A cyclic five-membered transition state, composed of the car-bene and two hydroxyl-containing molecules, was shown to be lower in energy than proton transfer or direct insertion into an OH bond. ... 

The second example is a rhodium(II)-catalysed decomposition of an a-diazo ketone (71). This interaction generated a metal-bound carbene that appears to undergo CH-insertion when tethered to thiophenes the product (72) was an aromatic tricycle. The direct CH-insertion mechanism is probably excluded by the reaction of (73). Treatment of (73) with a rhodium catalyst gave rise to a bicyclic system (75) with a pendant ketene group. The ketene appears to have migrated the authors suggested a mechanism via ketene (74), being formed by nucleophilic attack of the enol ether on the electrophilic carbene. ... 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

The reactions of / -ketoethynyl- and ) -amidoethynyl(phenyl)iodonium triflates, 17 and 18, with sodium / -toluenesulfinate illustrate the synthetic potential of alkynyliodonium salts33. Although the direct attachment of a carbonyl group to the / -carbon atom of the triple bond in alkynyliodonium ions might be expected to facilitate alkynyl sulfone formation via the Ad-E mechanism, this mode of reactivity has not been observed. Instead, the MC pathway with carbenic insertion dominates and affords sulfones containing the... 

The direct carbene insertion scheme in Fig. 4.8 is thought to be the most common cyclopropanation mechanism, but in some cases the reaction may occur in a stepwise fashion through a metallacyclobutane intermediate. Once formed by... 

The mechanism of 0-H bond insertion by carbenes remains an intense held of investigation. In the case of ether formation, three distinct pathways can be proposed (i) abstraction of protons from the alcohol forming an intermediate ion pair, (ii) reaction with the oxygen atom of an alcohol forming an intermediate ylide, and (hi) direct (concerted) insertion into the O-H bond. In that context, the carbene - alcohol ylide resulting from the reaction between carboethoxycarbene and MeOD has been experimentally detected for the first time, thus corroborating the viability of the ylide pathway. ... 

Certain dinuclear Rh(II) carbene complexes react with alkanes to generate products from insertion of the carbene imit into the alkane C-H bond with high diastereo- and enantioselectivity (Equation 6.59). ° These reactions occur by mechanisms distinct from those of the reactions of C-H bonds witti the tungsten alkylidene and alkylid5me complexes just described. The reactions of the dinuclear Rh(II) carbene complexes appear to occur by a mechanism that involves direct reaction of the carbene at the C-H bond without coordination of the alkane and addition across the M=C bond of the carbene. Such rhodium carbene complexes have not been isolated, but the absence of an open coordination site cis to the carbene ligand in the accepted carbene intermediate is thought to preclude initial reaction of tire substrate at the metal center to form a new metal-carbon bond. The catalytic chemistry that occurs via these carbene complexes is presented in more detail in Chapter 18 (catalytic C-H bond functionalization). 

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