Olefins methylene homologations

The reaction of olefins with dihalogenomethanes to form homologs with one additional carbon atom has been included in the list (no. 37) of reactions in Table I because CH2X2 should form a carbenoid methylene-nickel bond able to give a metallacycle with the olefin, followed by hydrogen transfer. 

Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with NzO at room temperature, but isolated cuprous ions are entirely inactive for this reaction 60). More interesting selectivity may be found in the reaction of olefins with methylene complexes the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in /i-CH2Co2(CO)2(Cp)2 61), /<-CH2Fe2(CO)8 (62), and /<-CH2-/i-ClTi(Cp)2Al(Me)2 (65) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). 

Examples of alkylation, dealkylation, homologation, isomerization, and transposition are found in Sections 1, 17, 33, and so on, lying close to a diagonal of the index. These sections correspond to such topics as the preparation of acetylenes from acetylenes carboxylic acids from carboxylic acids and alcohols, thiols, and phenols from alcohols, thiols, and phenols. Alkylations that involve conjugate additions across a double bond are found in Section 74 (alkyls, methylenes, and aryls from olefins). 

The catalysts derived from supported iron clusters exhibit in Fischer-Tropsch synthesis a high selectivity for propylene. Those catalysts are also selective for the stoechiometric homologation of ethylene to propylene and of propylene to n and iso butenes. The results are explained on the basis of a new mode of C-C bond formation which implies < - olefin coordination to surface methylene fragments or methylene insertion into a metal alkyl bond. 

Complexes that possess a methylene bridge between two transition-metal atoms react with olefins similarly to the Ti—CHj—A1 system. Thus treatment of octacarbonyl-/<-methylenediiron at 55°C with ethylene or propene produces homologated olefins. This reaction proceeds via the formation of a dimetallacyclopentane, then -hydrogen transfer s ... 

Olefin homologation. The methyl groups of 1 exchange with some aluminum alkyls and halides, but the methylene group is unreactive. However, 1 reacts with ethylene (toluene, 20", 18 hours) to form propylene in 327. yield. Reactions of this type are generally improved by addition of trimethylamine. This olefin homologation may involve the transient species a formed by CH2—Al dissociation (equation II). 

The B-alkyl-9-BBN undergoes an interesting reverse reaction to afford the parent alkene when treated with benzaldehyde. Consequently, the reaction is uniquely employed for the synthesis of exocyclic olefins (Chart 24.3). The hy-droboration of cyclic olefins with an internal double bond, followed by homologation with carbon monoxide in the presence of lithium trimethoxyaluminum hydride afford B-(cycloalkylmethyl)-9-BBN. This intermediate on treatment with benzaldehyde leads to an exocyclic methylene compound (Chart 24.3) [16]. Since the synthesis proceeds from the cycloalkene, thus it provides a valuable alternative to the customary methylenation of carbonyl compounds by Wittig and related procedures. The method also provides a clean synthesis of deuterium-labeled compounds (Eq. 24.10) [16], without positional scrambling or loss of label. Consequently, methylmethylene-d -cyclopentane in 52% isolated yield is obtained. 

The compounds [Cp2TiCHaAlR2] have been prepared, these are versatile methylene transfer reagents, which can be used for the conversion of ketones into terminal olefins and for the homologation of olefins. Scheme 18. ... 

Homologation of the aldehyde 13 and subsequent oxidation were straightforward, but subsequent methylenation of the hindered carbonyl was not. At last, it was found that Peterson olefination worked well. Metathesis then delivered the cyclopentene 2. The last carbons of the skeleton were added by intramolecular aldol cyclization of the thioester 16. 

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