Carbene complexes methoxycarbene

While the first transition metal carbene complex was reported in 1964134, the first cyclopropylcarbene complex salt [(CO)5Cr=C(chromium hexacarbonyl, followed by tetramethylammonium bromide135. Subsequent reaction with trimethyloxonium fluoro-borate gave methoxycarbene complex (CO)5Cr=C(OMe)(c-Pr) (equation 62)136. 

For most methoxycarbene complexes a one pot modification of the above method is utilized. This involves direct alkylation of the initially formed lithium acylate carbene complex with trifluoromethanesulfonate or with methyl fluorosulfonate. The method is successfully employed for preparation of chromium140 as well as molybdenum and tungsten monocyclic uation 65), bicyclic (equation 66) and tricyclic carbene complexes... 

We quickly established that these complexes are not particularly stable. They tend to split off the carbene ligand with a simultaneous hydrogen shift thereby liberating aldehydes, a fact that Japanese investigators also discovered (13). Very recently, we learned how to prepare these hydroxy-carbene complexes analytically pure (14) Previously, these complexes, without isolation, had been successfully converted to the substantially more stable methoxycarbene compounds by treatment with diazomethane (12). 

The methoxycarbene complex Cr(CO)5[C(OCH3)C6H5], Figure 6-4, illustrates some important characteristics of bonding in transition metal carbene complexes.11 Evidence for double bonding between chromium and carbon is provided by X-ray... 

In Fischer s original synthesis, carbyne complexes were obtained fortuitously as products of the reactions of carbene complexes with Lewis acids. For example, the methoxycarbene complex Cr(CO)5[C(OCH3)C6H5] reacts with the Lewis acids BX3 (X = Cl, Br, or I). 

Iron and cobalt carbene complexes are capable of much more selective furan synthesis. Reactions of cobalt methoxycarbenes with internal alkynes give good to excellent yields of 2-methoxyfurans and have been applied to a synthesis of a natural product, bovolide [75 b]. Both terminal and internal alkynes are viable substrates in the preparation of 2-aminofurans from iron (dimethylamino)carbenes [78], although the nature of the rearrangement of the heteroatom-containing substituent is also as yet unclear [Eq. (33)]. The inclusion of elevated carbon monoxide pressure can divert this reaction to the production of pyrones [75c, 79]. 

The alkoxycarbene complex, [FeGp (G0)2 =G(0Me)Me ][GF3S03], undergoes substitution of one or two carbonyl ligands to produce various mono- and disubstituted methoxycarbene complexes, providing a convenient method for the generation of cationic iron-carbene complexes. 

The vast majority of work exploring the reactivity of ruthenium viny-lidene complexes has focused on the attack of alcohols at the electrophilic a carbon of monosubstituted vinylidenes, resulting in the formation of ruthenium alkoxycarbene complexes. Bruce and co-workers have determined, for example, that the phenylvinylidene complex 80 is slowly transformed in refluxing MeOH to the methoxycarbene complex 82 in good yield (73,83). The mechanism for this reaction must involve initial attack of the alcohol at the electrophilic Ca to form a transient vinyl intermediate 81 which is rapidly protonated at the nucleophilic Cp, generating the product carbene 82 [Eq. (79)]. In contrast to monosubstituted vinylidene complexes, disubstituted vinylidene complexes are generally unreactive to nucleophiles even the relatively small dimethylvinylidene complex 83 shows no reaction with MeOH after 70 hours at reflux [Eq. (80)]. 

Alkylation of iron acyl complexes also provides access to iron carbenes. Thus, the neutral iron acyl complex will react with acid, with alkylating agents, or with trifluoromethanesul-fonic anhydride to afford cationic hydroxy- or aUcoxycarbene complexes (52) and (53) or the cationic vinylidene complex (54, L = CO) (Scheme 20). The vinylidene complex can be used to prepare a more substituted analog of (51) by treatment with a thiol. The enantiomerically pure iron acyl complex (R)-(45a) can be converted to the corresponding enantiopure methoxycarbene complex with Me30Bp4 as well. Finally,... 

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