Catalytic hexatriene

According to this scheme, the catalyst serves primarily to promote dehydrogenation. Cyclization of the hexatriene was shown years ago (JJ.) to occur thermally in the gas phase at temperatures well below these dehydrocyclization conditions. Thus, the overall reaction is projected to be the combination of several catalytic dehydrogenation steps and a non-catalytic cyclization step. This projection implies that the design of the catalytic reactor may be important in order to optimize the ratio of void space for cyclization and catalyst space for dehydrogenation. 

The first synthesis of thiepin 1,1-dioxide (131) was performed by Mock in 1967 75>. With exess bromine in chloroform 2,7-dihydrothiepin 1,1-dioxide (129), prepared by 1,6-addition of sulfur dioxide to m-hexatriene, gave a dibromide 130 which, on treatment with two equivalents of triethylamine, afforded 131. Upon catalytic reduction, 131 rapidly absorbed three molar equivalents of hydrogen to yield hexahydrothiepin 1,1-dioxide (132). The 1,1-dioxide 131 is a fairly stable compound ... 

Hexatriene has been prepared by many workers. The more successful methods are the catalytic pyrolyses (alumina, 260-325°) of l,3-hexadien-5-ol6 7 8 and 2,4-hexadien-1 -ol.9 Other methods which give hexatriene of questionable purity or involve less convenient laboratory methods are dehydration of 1,5-hexadien-3-ol by sodium bisulfate at 170°,10 or by phthalic anhydride at 160-200°,2 and by catalytic hydrogenation of di-vinylacetylene.11 Additional methods are listed in footnote 5. The present procedure is a practical laboratory method of preparing pure hexatriene in satisfactory yields. 

THF can also have an accelerating effect on reactivity, in cases where the weakly basic solvent can get directly involved in the reaction via a catalytic pathway and complexation with the free silene is weak. Such an effect has been observed for the reaction of alcohols with the aryldisilane-derived l,3,5-(l-sila)hexatriene derivatives 21a-c, as shown by the second-order rate constants for reaction of the three silenes by MeOH and TFE in isooctane, MeCN and THF solution (Table 11 note that the third-order rate constants for reaction of 21a-c with MeOH have been omitted)48. Table 11 also includes data for the reactions with acetone in the same three solvents, as an example of a reaction which has no catalytic component47. The rate constants for all three reactions decrease in the order isooctane > MeCN > THF for 21a, which complexes relatively strongly with the ether solvent, as demonstrated by the distinctive red shift in its UV absorption spectrum in THF (kmax = 460 nm) compared to isooctane and MeCN (kmax = 425 nm)48. Compound 21b exhibits a ca 10 nm shift of its absorption band in THF solution while none is detectable in the case of 21c, indicating that the equilibrium constant for THF complexation within this series of silenes decreases with increasing phenyl substitution at... 

A standard synthesis shall first be discussed. Dialkyldichlorosilanes and but-2-en-l-ylmagnesium (the latter having been obtained by reduction of 1,3-butadiene with Mg) yield silacyclopentenes e.g., 117 plus but-2-en-l-ylmagnesium afford 1,1-dimethyl-1-silacyclopent-3-en (-3-silolene) (332) (equation 151)175. In a related manner, 1,3,5-hexatriene (333) and 117 or 219—in the presence of Mg and catalytic amounts of CuCl, in... 

---