Formic acid diene dimerization

Dimerization of alkylallene 160 proceeded regioseleetively at each C-2 carbon, and the cross-conjugated triene 161 was obtained in high yield using P(< -Tol)3 as a ligand and p-nitrophenol as an additive [53]. In the presenee of formic acid, reductive dimerization at C-2 carbons occurred to give the conjugated dienes 164 and 165. The dimerization is explained by the formation of the palladacycle 163, formed by oxidative cyclization, as an intermediate [54]. 

The 2,3-alkadienyl acetate 851 reacts with terminal alkynes to give the 2-alkynyl-1,3-diene derivative 852 without using Cul and a base. In the absence of other reactants, the terminal alkyne 853 is formed by an unusual elimination as an intermediate, which reacts further with 851 to give the dimer 854. Hydrogenolysis of 851 with formic acid affords the 2, 4-diene 855[524]. 

Hydrogenation of dienes with up to 20 1.0 diastereoselectivity and 99% ee is mediated by carbene complexes. The scope and limitations of these reactions were investigated.288 Asymmetric transfer hydrogenation to prochiral ketones, catalysed by a Ru(II) complex (10) or its dimer, with formic acid-triethylamine has been reported, (0 The protocol leads to high yields and enantioselectivity up to 96%. It has been suggested that 16-electron Ru(II) and the Ru-H intermediates are involved in this reaction.289... 

Enantioselective cyclooligomerizations of butadiene can also be achieved when a chiral diaza-diene-iron(II) chloride complex and a fourfold excess of ethylmagnesium iodide are used. Optical yields of 9 to 16% are obtained with two chiral diazadienes of the type R N = CRCR = NR, prepared by formic acid catalyzed condensation of aqueous glyoxal (R1 = H) or 2,3-butadione (R1 = CH3) with (R)-l-phenylethylaminc or (1S,2S,3S,5R)-3-(aminomethyl)pinane. The (di-azadienc)iron(0)-catalyzed dimerization of butadiene also gives 4-vinylcyclohexene with up to 16% ee60. 

The linear dimerization of isoprene often affords mixtures of head/tail dimers. For example, [Pd(OAc)2, P(CH2CH2CN)3l catalyzes dimerization of isoprene with triethylammo-nium formate in dimethylacetamide (55 °C, 3 h), affording dimers in 91% yield as a 10 49 37 98/99/100 mixture of head/tail coupling regioisomers (Scheme 30). It is interesting to note that, as in the case of butadiene and formic acid in the presence of phosphine-modified Pd(OAc)2, only 1,7-dienes are formed in this reaction of isoprene with formic add. 

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