Cyclopen tenes selectivity

The cycloadditions of 1-substituted 1,2-cyclohexadienes and among them their dimerization are of interest because of the position selectivity. Does the reaction occur at the substituted or the unsubstituted ethylene subunit For that question to be answered, 1-methyl- (74), 1-phenyl- (75), 1-cyclopropyl- (76), l-(3-phenylpropyl)-(77) and l-trimethylsilyl-l,2-cyclohexadiene (79) were generated from the corresponding 1-substituted 6,6-dibromobicyclo[3.1.0]hexanes with methyllithium. Several of these dibromides are thermolabile, which particularly applies to the phenyl (93) [76] and the cydopropyl derivative [70], In those cases, it is advisable or necessary to prepare the dibromide in situ, that is, the dibromocarbene is liberated from tetrabro-momethane with methyllithium at -60 °C in the presence of the respective cyclopen-tene. Without workup, from the thus formed 6,6-dibromobicyclo[3.1.0]hexane, the 1,2-cyclohexadiene is then generated by addition of methyllithium at -30°C. 

The cyclohexadiene derivative 130 was obtained by the co-cyclization of DMAD with strained alkenes such as norbornene catalyzed by 75[63], However, the linear 2 I adduct 131 of an alkene and DMAD was obtained selectively using bis(maleic anhydride)(norbornene)palladium (124)[64] as a catalyst ]. A similar reaction of allyl alcohol with DMAD is catalyzed by the catalyst 123 to give the linear adducts 132 and 133[66], Reaction of a vinyl ether with DMAD gives the cyclopen tene derivatives 134 and 135 as 2 1 adducts, and a cyclooctadiene derivative, although the selectivity is not high[67]. 

The nature of the oxidation products is traceable to the nature of the rhodium-alkene interaction. Terminal alkenes and internal ones (e.g. cycloheptene), which form 77-complexes of rhodium(I), e.g. [RhCl(alkene)2]2, are selectively converted into methyl ketones, whereas alkenes which form 7r-allylic complexes of rhodium(III) (e.g. cyclopen-tene) give alkenyl ethers via oxidative substitution of the alkene by the solvent alcohol.204... 

The reaction of allenylsilanes with electron deficient ir-systems constitutes a powerful and general method for the tegio- and stereo-selective preparation of substituted cyclopentenes. As first reported, (trimethylsilyl)allenes function as three-carbon nucleophiles in TiCU-promoted (trimethylsilyl)cyclopen-tene annelations (Scheme 31). The annelation process involves the polarization of an a,p-unsaturated carbonyl compound by TUTU to generate an alkoxyallylic cation. Regiospecific electnqrhilic substitution of this cation at C-3 of the allenylsilane produces a silicon-stablizied vinyl cation. A 1,2-shift of the tri-... 

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