Methylene organometallic reagents

Application of 7r-allylpalladium chemistry to organic synthesis has made remarkable progress[l]. As deseribed in Chapter 3, Seetion 3, Tt-allylpalladium complexes react with soft carbon nucleophiles such as maionates, /3-keto esters, and enamines in DMSO to form earbon-carbon bonds[2, 3], The characteristie feature of this reaction is that whereas organometallic reagents are eonsidered to be nucleophilic and react with electrophiles, typieally earbonyl eompounds, Tt-allylpalladium complexes are electrophilie and reaet with nucleophiles such as active methylene compounds, and Pd(0) is formed after the reaction. 

The addition of organometallic reagents to the carbonyl group of conveniently substituted aldonolactones constitutes a viable chain-extension method. The reaction leads to the formation of hemiacetals of glyculoses, 1-methylene sugars, and C-glycosyl compounds, which are precursors of, or occur as subunits of, a variety of natural products. 

Methylene homologations of organometallic reagents using organozincs... 

The stable organometallic reagent (T]s-C,H5)(CO)2FeCH2S (Me)2BF4-readily prepared, and commercially available (from Aldrich) but expensive, was shown [471] to be an efficient methylene transfer reagent for the direct cyclopropanation of alkenes. 

It is, therefore, to be expected that a compound such as trimethyl-methylene stiborane, (CHs SbCtE, should be relatively unstable and attempts to synthesize this species would have to be carried out at low temperature. Another difficulty arises from the experimental fact (104) that all conventional methods of synthesis for stibonium ylides lead to pentaalkylstiboranes instead of ylides (see Introduction). Thus mcthyl-ation of (CH3)4SbX or (CHY) sHbX2 halides by organometallic reagents of lithium, magnesium, aluminum, or zinc invariably yield (CHY) 5Sb as the sole product. 

The Michael-type addition reaction of a carbonucleophile with an activated olefin constitutes one of the most versatile methodologies for carbon-carbon bond formation [1]. Because of the usefulness of the reaction as well as the product, many approaches to the asymmetric Michael-type addition reactions have been reported, especially using chirally modified olefins [2-8]. However, the approach directed towards the enantioselective Michael-type addition reaction is a developing area. In this Chapter, the recent progress of the enantioselective Michael-type addition reaction of active methylene compounds and also organometallic reagents with achiral activated olefins under the control of an external chiral ligand or chiral catalysts will be summarized [9]. 

The most commonly employed strategy for the synthesis of five-membered heterocycles bearing methylene moiety in the a-position relies on the addition of organometallic reagents 23 derived from 2-bromomethylacrylic acid and its derivatives to carbonyl compounds 24a or imines 24b (Scheme 4.4). Cyclization of the 2-methylene-4-hydroxy or 4-aminoalkanoates 25 obtained, leading to the formation of the target a-methylene-y-lactones or lactams 26, occurs spontaneously under reaction conditions or is induced by employing acidic or basic conditions. Over the years, different metals, mainly zinc [28a, 33], tin [34], or indium [35], were... 

Adamantanethione was converted into a thiiran by addition of a methylene group, derived from methylenetriphenylphosphorane, across the carbon-sulphur double bond. The action of Grignard and other organometallic reagents on thiones gives thiirans together with other products, and thiirans also are obtained by treatment of 3-piperidino-2-phenylindene-l-thione with several diazoalkanes. ... 

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