Alkylidenation reactions

Following the same strategy as used for methylenation, general alkylidenation of carbonyl compounds would be realized by preparation of the corresponding gem-dizinc species. The preparation of these gem-dizinc species, however, is con- 

Using these dizinc species in conjunction with jS-TiCls, the alkylidenation of 

Zn in these equations denotes pyrometallurgy grade zinc, containing 0.04-0.07% lead. When pure, lead-free zinc is used, a catalytic amount of PbCl2 should be added. 

Diastereoselectivity was observed in this reaction, the Z-isomers being the major products. 

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The exact nature of the alkylidenes formed on various oxide surfaces is still uncertain, as is the nature of the alkylidenes responsible for the often observed metathesis activity. Mo(N)(CH2CMe3)3 also has been employed as a precursor to a surface-bound species believed to be of the type Mo(NH)(CHCMe3)(CH2CMe3) (Osurf) [115]. Although the alkylidene carbon atom could not be observed in solid state NMR spectra, which is typical of surface supported alkylidenes, reaction with acetone to give 2,4,4-trimethylpent-2-ene quantitatively confirmed the presence of the reactive neopentylidene complex. Such species would initiate various metathesis reactions when prepared on partially dehydroxylated silica. 

A dimetal species, obtained by treatment of vinylzinc with Schwartz s reagent, leads to a very effective alkylidenation reaction as shown in Scheme 1843. 

The mechanism snfficiently explains how prodnctive condensation metathesis occms since terminal alkenes are contained in every step of the productive cycle (Scheme 5). In this kinetically controlled regime (high concentration of terminal alkene present), alkylidene reactions with unsubstituted (terminal) alkenes are favored over interchain redistribution reactions or other degenerate eqnilibria. However, when terminal alkene concentration decreases near the end of the polycondensation reaction, competing... 

AUcene metathesis see Alkene Metathesis) reactions have also been used to give isolable alkylidenes. Reactions of Ta(=CHBu0[NCN](OBu0X (X = Cl, 24a OBfr 24b) with Me3SiCH=CH2 afford Ta(=CHSiMe3)[NCN](OBfr)X stabilized by ortAo-chelating arylamine ligands. ... 

Insertion into metal carbon bonds for alkylidene [reaction (j)] and alkylidyne [reaction (k)] complexes that yield metallacycles will also be considered. 

The disproportionation of olefins first reported by Banks and Bailey ) and - as a special case - the ring-opening polymerization of cycloalkenes 3) comprise a re-arrangement of the carbon skeleton of the olefins by opening and closure of double bonds. Thus the reaction - generally referred to as inetathosls reaction - is a trans-alkylidenation reaction. 

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