Carbene from acyl complexes

Alkylation of an anionic acylmetallate is the second step in the classic Fischer synthesis of carbene complexes. The same type of reaction can be performed with stable neutral acyl complexes, producing cationic carbene compounds. Compound 17 has been prepared from a ruthenium acetyl complex by direct methylation and by protonation with subsequent methylenation (45) ... 

A. CARBENE COMPLEXES OBTAINED FROM ACYL, AND CARBAMOYL METALLATES... 

A different strategy is utilized to prepare Fischer-type cyclopropylmethoxycarbenium complexes. [LM C=(OMe)(c-Pr)]+ by alkylation of cyclopropylcarbonyl complexes c-PrC(0)ML (vide supra). Thus, treating acyl complex FpCO(c-Pr), obtained from the reaction of NaFp [Fp = Cp(CO)2] and c-PrCOCI, with Me,0+BF4 in CH2C12 gives cyclopropylmethoxycarbenium fluoroborate complex salt [Fp=C(OMe)(c-Pr)]BF4 (equation 68) which can be further reduced to the corresponding Schrock-type cyclopropyl carbene complex (vide infra, Section II.C.2)142. 

These Fischer-type carbene complexes can be readily converted to Fischer-type carbyne complexes, and this material has been reviewed. Methods to remove the a-alkoxy group include treatment with BBrs, oxide abstraction from acyl ligands, and electrophihc attack on the jS-carbon of... 

Cyclopropanation reactions of nonheteroatom-stabilized carbenes have also been developed. The most versatile are the cationic iron carbenes that cyclopropanate alkenes with high stereospecificity under very mild reaction conditions. The cyclopropanation reagents are available from a number of iron complexes, for example, (9-alkylation of cyclopentadienyl dicarbonyliron alkyl or acyl complexes using Meerwein salts affords cationic Fischer carbenes. Cationic iron carbene intermediates can also be prepared by reaction of CpFe(CO)2 with aldehydes followed by treatment with TMS-chloride. Chiral intermolecular cyclopropanation using a chiral iron carbene having a complexed chromium tricarbonyl unit is observed (Scheme 61). 

By reaction of cationic carbonyl complexes with lithium carbanions, neutral acyl complexes are prepared. Whereas treatment of [> -CpFe(CO)3]BF4 with (a) PhLi gives the expected > -CpFe(CO)2 [C(0)Ph] in 80% yield, with (b) MeLi only traces of > -CpFe(CO)2 [C(0)Me] can be detected . This complex and other phosphane-substituted acyl compounds of the type f -CpM(CO)L[C(0)Me] [M = Fe, Ru L = CO, PPh3, P(hex)j], as well as >/ -CpMo(CO)2P(hex)3[C(0)Me] (prepared by different routes), are protonated with and alkylated with [R3 0]BF4 reversibly, yielding cationic hydroxy- and alkoxy(methyl)carbene complexes, respectively . The formation of the ( + )- and ( —)-acetyl complex / -CpFe(C0)(PPh3)[C(0)Me] from the ( + )-and ( —)-conformers of optically active > -CpFe(C0XPPh3)[C(0)0-menthyl] and MeLi occurs with inversion of configuration at the asymmetric iron atom . 

The electrophilicity of the alkylidyne carbon in cationic complexes of manganese and rhenium such as 154 or 157 is well established. A study by Chen et al. established that the carbonyl ligands are also potential sites for nucleophilic attack (157,158). Reaction of 154 with the bulky carborane anion LiCjBioH, results in the formation of two products the carbene complex 156, resulting from attack at the alkylidyne carbon, and the alkylidyne acyl complex 155, resulting from attack at a carbonyl ligand [Eq. (135)]. At room temperature complex 155 transforms into complex 156. 

The product from nucleophilic attack at the carbonyl carbon undergoes, besides the reverse course of the attack, some transformations useful for the purpose of utilizing the new hgand group. These include alkene insertion and reductive elimination, as in Schemes 8.3, 8.6 and 8.7. Decarboxylation of M-COOH was also described (Scheme 8.8). The acyl complex reacts with carbon electrophile to give carbene complex (Eq. 8.5) [22]. 

The CO insertion mode regarding early transition metals and f-elements is different from that of late transition metals. Thus, carbene-like dihaptoacyl bonding mode is observed with these acyl complexes formed in the first CO insertion. The subsequent course of the reaction with the second CO may differ considerably from that of late transition metals. " ... 

Related acetoxy-substituted carbene complexes have been prepared by acylation of anionic metal acyl complexes with acetyl chloride (Connor and Jones, 1971a). The silylation of anionic acyl complexes was used to prepare (CO)5CrC[OSi(CH3)3]CH3 (Fischer and Moser, 1968a). Phenoxy-substituted carbene complexes have been obtained from the reaction of anionic acyl com-... 

Labeling studies provided some evidence that aldehydes are not intermediates in the formation of heptanol from 1-octene but a hydroxy carbene-like intermediate (Scheme 5.50) [63]. The latter derives from the protonation of the relevant Rh-acyl complex by ethanol and benefits from the high electron density at the metal center, which is caused by trialkylphosphines. The reaction with hydrogen (here D2) produces the alcohol. A similar mechanism was suggested for the tandem reaction with 2-propen-l-ol as a substrate [64]. 

Hydroxy carbene complexes 161 and 162, formed in the same reaction mixture by the abstraction of chloride from an acyl complex, are reported (Scheme 39). X-ray structures of both complexes show crystallographically imposed symmetry, and hence bond lengths that are intermediate between the extremes of Pt-C and Pt=C (and G-O and C=0). 

Scheme 21). It involves the initial formation of anionic acyl complexes by addition of Li[GH2R] reagents to the carbonyl ligand, which, by reaction with acetyl chloride, generate the vinylidenes via spontaneous loss of MeC02H from intermediate acyl(oxy) carbenes. " ... 

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