Methylene tricarbonyl chromium

The reactivity of phenylacetic esters with electron-deficient alkenes is generally fairly poor, even under phase-transfer catalytic conditions. The reaction with cinnamic esters is often accompanied by hydrolysis and the yield of the adduct with chalcone is generally <60% [10]. The activity of the methylene group towards alkylation has been enhanced by the initial complexation of the phenyl ring with chromium tricarbonyl (see Section 6.2), but this procedure has not been applied to the Michael reaction. 

Pentacarbonyl(methoxymethyl-methylene)chromium, 83 Tri-p,-carbonylhexacarbonyldiiron, 320 Tricarbonyl(methyl benzoate)chromium, 19... 

Apart from the di- and oligoolefm iron tricarbonyl complexes, which nowadays are frequently used in organic synthesis [71, 72], the chemistry of the readily accessible cyclohepatriene chromium and molybdenum tricarbonyls 2 and 3 was the focus of intense research efforts as well. Only a few months after the synthesis of 2 and 3 was published [58,59], both Hyp Dauben and Peter Pauson reported that these compounds react with triphenylmethyl tetrafluoro-borate in methylene chloride to give the tropylium complexes 4 and 5 in excellent yield (Scheme 7.1) [73, 74]. Later this method of hydride abstraction was also used for the preparation of the tropylium cation itself and subsequently led to the generation of several cationic rc-complexes of iron, manganese and cobalt [71, 72], The reactions of the cations of 4 and 5 with nucleophilic... 

Asymmetric reaction of the sulfides bearing a tricarbonyl(ri -arene)chromi-um complex was shown to be successful by Gibson and Simpkins [48,49]. The benzyhc methylene groups in tricarbonyl(ri -phenylmethyl alkyl sulfide)chro-mium(O) and tricarbonyl(ri -l,3-dihydroisobenzothiophene)chromium(0) were highly asymmetrically functionahzed by deprotonation with a chiral bis-Uthium amide and subsequent electrophihc reactions (Tables 4 and 5). 

The reactions of transition metal fulvene complexes are dominated by a nucleophilic attack at the exocyclic methylene group with formation of cyclopen-tadienyl complexes. As an example, Behrens et al. treated tricarbonyl(6,6-dimethylfulvene)chromium(O) (75) with potassium diphenylphosphide resulting in the formation of the ionic complex 76 in 77% yield (Scheme 10.26). There are many similar reactions with fulvene complexes known [52, 77, 78]. 

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