Allenes dimerisation

The molecular structure of sila-heterocycles is reviewed <98MI649>. The Raman and infrared spectra of 1-chloro-l-methylsilabutane have been studied in detail <99MI399>. The first allenic coompoimds with both silicon and phosphorus doubly bonded (ArP C Si(Ph)Tip, Tip = 2,4,6-triisopropylphenyl) have been prepared and dimerised to give 38 and 39 in the ratio of 3 2 <99MI774>. 

Disubstituted furans were provided in high yield through a novel oxidative cyclisation-dimerisation reaction between two different allenes as shown below <02AG(E)1775>. 

Allenic ketones undergo a PPh3-catalysed dimerisation to give 2-alkylidene-277-pyrans, a formal [4+2] cycloadduct. The reaction is more successful with aromatic allenyl ketones and favours the (fs)-isomer (Scheme 3) <07JOC1051>. 

Thiopropenoylsilanes 42, readily accessible from silylated allenes, self-dimerise in a head-to-head fashion to give silylated 1,2-dithiins sometimes the corresponding 1,3-dithiin is also formed <03TL2831>. 

The 3,6-disilylated-3-vinyl-l,2-dithiins 38, obtained by the self-dimerisation of silylated allenes, undergo a Lewis aeid-promoted rearrangement to the bicyclic endodisulfide <05TL4711>. Tethered bilayer lipid membranes have been obtained using 4-hydroxy-l,2-dithianes as the anchor for coupling reactions with the lipid <05AJC738>. 

Conversion of the initial cycloadducts through to the desired indoles was achieved using DDQ or activated Mn02. Whilst the eight-membered ring allene 1,2-cyclo-octadiene dimerises at ambient temperature, the compound with a t-butyl group at the... 

Allenes appear to react in such a way that their central carbon atom is bound, in the most abundant product, to the central carbon atom of a second allene molecule, as in dimerisations -, to the terminal carbon of a vinyl derivative, or to the most fluorinated atom of a fluoro-halogenoethylene, as has been found in the reactions of propadiene with F2C=CFC1 and F2C=CCl2 . where 85% and 95% of the product, respectively, conformed to this orientation. 

An intramolecular secondary isotope effect has been measured by analysing the product of 1,2-cycloaddition of l,l-c(2-allene to acrylonitrile in benzene at 210-225°C. The result is (Ah/Ad)2s°c 1-18 per D atom it has the opposite direction of that found for a Diels-Alder reaction of M-r/j-allene, and is consistent with a two-step mechanism (see Section 4.1.5). A diradical intermediate seems probable. A similar result was obtained from the study of isotope effects for the dimerisation of allene. Since the reaction of / (-)-... 

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