Isoprene formation, isomerization

Bromination of isoprene using Br2 at —5 ° C in chloroform yields only /n j -l,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with one-third excess of isoprene at —15 ° C to form the 1,2-addition product, 2-chloro-2-methyl-3-butene (60). When an equimolar amount of HCl is used, the principal product is the 1,4-addition product, l-chloro-3-methyl-2-butene (61). The mechanism of addition is essentially all 1,2 with a subsequent isomerization step which is catalyzed by HCl and is responsible for the formation of the 1,4-product (60). The 3,4-product, 3-bromo-2-methyl-1-butene, is obtained by the reaction of isoprene with 50% HBr in the presence of cuprous bromide (59). Isoprene reacts with the reactive halogen of 3-chlorocyclopentene (62). 

In accord with the proposed mechanism, copyrolyses of la or lb with 2,3-dimethyl-1,3-butadiene (DMB) or isoprene lead to silacyclopentene derivatives via a formal [4+1] cycloaddition of the silylenes (Scheme 2). The simultaneous existence of the silaethenes 2a/2b and the resulting silylenes 4a/4b in the gas phase is proven by the formation of the corresponding 1,3-disilacyclobutanes (5) and - in case of isoprene as the quenching partner - of the two isomeric silacyclohexenes 7 (Scheme 2) [2]. 

Addition of diarylcarbenes to buta-1,3-dienes results in 1,2-adduct formation and generation of the corresponding 2-alkenyl-l,l-diarylcyclopropanes in reasonable to excellent yield. When the C-C double bonds are nonidentical, mixtures of isomeric monoadducts are formed. Thus, irradiation of a mixture of diphenyldiazomethane and isoprene yielded 2-methyl-l,l-diphenyl-2-vinylcyclopropane (4, 38%) and 2-isopropenyl-l,l-diphenylcyclopropane (5, 35%). ... 

As mentioned earlier, conformational isomerization about the formal single bonds of polyene systems is facile in the ground state, where it occurs with activation barriers on the order of 2-4 kcalmol in acyclic systems . The process also occurs in acyclic dienes upon direct excitation, as was shown by SquiUacote and coworkers using low temperature matrix isolation techniques, at temperatures where thermal conformational reequilibration is suppressed (10-20 K) . Thus, direct irradiation of trans-1,3-butadiene in an argon matrix at 15 K results in the efficient formation of the c/s-conformer, distinguishable from the trans-conformer by its distinct UV absorption and infrared spectra . The process is quite general, at least for aliphatic dienes such as isoprene (2), 2-isopropyl-1,3-butadiene (24), 2,4-hexadiene (5) and 2,3-dimethylbutadiene... 

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