1.4- Hexadiene, 2-isopropyl

Other aliphatic acyclic dienes such as isoprene (2)102,111, 2-isopropyl-1,3-butadiene (24)102, and , -2,4-hexadiene (5)78 also yield the corresponding cyclobutene, all via excitation of the s-cis conformer. The latter yields the disrotatory ring closure product, c -3,4-dimethylcyclobutene (54), stereospecilically78. 

The formulas of 37-39 show the substitution of three carbonyls by a conjugated diene. Due to the fact that dienes are four-electron donors, the complexes are electron-deficient species. The electron deficiency is reduced by formation of C—H—Cr three-center, two-electron bonds, previously found for a number of other complexes (43). When 2-methyl-5-isopropyl-l,3-cyclohexadiene is used as a potential ligand, the expected complexes 37r and 39r are not obtained, but instead complexes with the isomeric l-methyl-4-isopropyl-l,3-cyclohexadiene group (37r and 39r ) are formed [Eq. (19)]. Similarly, lk and 11 form not only the corresponding complexes 39k and 391, but also the isomeric complexes with (Z)-l,3-hexadiene (39k ) and with (Z)-2-methyl-1,3-pentadiene (391 ). 

The reaction of 6b with H B thf yields two products depending on the amount of borane reagent used. An excess of BH3 leads to a dihydrogenation of the hexadiene part of 6b to give the 2,3-dihydro-1,2,5-azadiborole 7b with two isopropyl substituents as the main product and only traces of the 2,4,5-azadicarbahexaborane 8b. When a 1 1 reaction is allowed to proceed the new azacarborane 8b is obtained5. 

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... 

In some quite special cases decarbonylation of ketones assumes real preparative importance e.g., ultraviolet irradiation converts thujone into carbon monoxide and 2-isopropyl- 1,4-hexadiene readily and unusually rapidly 72... 

Alpha-cleavage may occur at both sides of carbonyl group resulting in the loss of a carbon monoxide molecule. This reaction is demonstrated in Scheme 2. Thujone, which possesses a minty-camphoraceous odor, decomposes into 2-isopropyl-1,4-hexadiene after losing a carbonyl group during irradiation (Eastman et al., 1963). 

Giles, N. F. Wilson, G. M. Vapor-liquid equilibria on seven binary systems ethylene oxide + 2-methylpropane acetophenone + phenol eis-l,3-diehloropropene + 1,2-diehloropropane 1,5-hexadiene + allyl ehloride isopropyl acetate + acetonitrile vinyl chloride + methyl chloride and 1,4-butanediol +. gamma.-butyrolactone. J. Chem. Eng. Data 2006, 51, 1954-1962. 

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