1-Methoxycarbene

Carbyne complexes were first made In 1973 by the unexpected reaction of methoxycarbene... 

Ikegami s successful synthesis of racemic 720 materialized by initial conversion of 701 to 725 via a 1,2-carbonyl transposition sequence (Scheme LXXVIII) Treatment of 725 with methoxycarbene, deprotection, and oxidation provided 72 6. Acid-promoted cyclopropane ring cleavage and added functional group manipulation led to 727 which could be allylated stereoselectively. The tricyclic enone 724 was subsequently produced conventionally. 

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... 

Preparation of b/s(5-(methoxy(pentacarbonylchromium(0) carbene))pentyl) zinc and its iodolysis preparation of pentacarbonyl ((5-iodopentyl)methoxycarbene) chromium(O)6... 

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 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)]. 

As would be anticipated, the unsubstituted vinylidene species [(i75-C5H5)(PPh3)2Ru=C=CH2]+ (84) is correspondingly more reactive than the monosubstituted complexes. In fact, the reaction of ethyne with 1 in methanol at room temperature affords only the methoxycarbene 85, the methanol addition reaction being so rapid that isolation of the unsubstituted vinylidene 84 is impossible [Eq. (81)] (78). As the alcohol must... 

Consiglio et al. (88) have reported that the reaction of LiAlH4 with the methoxycarbene complex 99 gives the completely reduced 2-phenylethyl complex (100) as the exclusive product [Eq. (90)]. The reaction probably involves initial hydride attack at C , elimination of methoxide, and a second hydride attack to give the final product. 

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,... 

The photolysis or pyrolysis of diazoesters is the only source of carboalkoxy-carbenes and although numerous examples can be found in the literature concerning the reactivity of these carbenes, very little information is available on the kinetics of the decomposition. The photolysis of methyldiazoacetate yields carbo-methoxycarbene which adds stereospecifically to 2-butene. The quantum yield of the photolysis of ethyldiazoacetate has been determined in various solvents at different wavelengths (Table 12) . Thermal decomposition occurs above 150 °C although the presence of catalysts greatly accelerate the decomposition . Carboalkoxycarbenes are very selective with respect to insertion reactions, due to... 

The direct photolysis and benzophenone-sensitized decomposition of methyl diazomalonate has been shown to produce two different spin states of biscarbo-methoxycarbene. In the direct photolysis experiments stereospecific addition to olefins and a high yield of insertion products in the presence of alkanes demonstrate the singlet character of the carbene in the sensitized decomposition the yield of insertion products decreases, and addition to cis- or ra 5 -4-methyl-2-pentene yields about 88 % trnwj-cyclopropane. In this case, as with most carboalkoxy-carbenes, the addition reaction to olefins can proceed by two paths ... 

However, a methyl group in a-position to the carbonyl group in 14e leads to methoxycarbene 13e, where the hydrogen shift can compete with the silyl shift. Since the hydrogen shift is irreversible, vinyloxysilane 16 is formed as the major product. 

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). 

To a solution of methyl /rani-oct-2-en-7-ynoate (95 mg, 0.625 nnnol) in benzene (250 mL) was added pentacarbonyl[(methyl)methoxycarbene]chromium(0). After being heated at 70 C for 1.75 h, the mixture was concentrated in vacuo and purified by chromatography on Florisil (hexane/EtOAc 99 1) to give a mixture of E- and Z-enol ethers yield 43 mg (33%) ratio ( /Z) 3 1. 

Dipotassium cyclooctatetraenide was used for the generation of methoxycarbene (carbenoid) (from dichloromethyl methyl ether) which reacted further with this dianion. 

The reaction of methoxycarbene (generated from chloromethyl dichloromethyl ether using methyllithium as the base) with twelve-membered cycloalkenes, dienes and trienes reveals that the carbene undergoes faster addition across E than Z double bonds, and that Z-configurated double bonds react stereoselectively (see also the example in Houben-Weyl Vol. El 9 b, p 1636), e.g. formation of 4 and 5. 

In a similar reaction, /er/-butylmethyl chloromethyl ether was converted into ter/-butyl-methoxycarbene which underwent addition to hex-3-yne giving cyclopropene... 

Methoxycarbene.1 The reagent (1) decomposes when heated to give methoxy-carbene. Thus thermolysis in the presence of tetramethylethylene gives an essentially quantitative yield of l-methoxy-2,2,3,3-tetramethylcyclopropane (2). 

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