2-phenyl-1,5-hexadiene

Guided by Marks s report of the samarium-catalyzed hydroboration of alkenes, Molander has developed a samarium-catalyzed protocol for the cyclization/hydroboration of unfunctionalized 1,6-dienes. In an optimized procedure, reaction of 1,5-hexadiene and l,3-dimethyl-l,3-diaza-2-boracyclopentane catalyzed by Gp 2Sm(THF) in toluene at room temperature for 18 h followed by oxidation gave hydroxymethylcyclopentane in 86% yield (Equation (70) R = H, n — ). The transformation was stereoselective, and Sm-catalyzed cyclization/hydroboration of 2-phenyl-1,5-hexadiene followed by oxidation formed /ra/ i--l-hydroxymethyl-2-phenylcyclopentane in 64% yield (Equation (70) R = Ph, n = ). The samarium-catalyzed reactions was also applicable to the synthesis of hydroxymethylcyclohexanes (Equation (70), n=X) but tolerated neither polar functionality nor substitution on the alkenyl carbon atoms. 

When 1-phenyl-1,5-hexadiene was treated with complex 15, the chemoselective aziridination proceeded at the olefin portion attached to the phenyl group to give good enantioselectivity (Scheme 21). In other words, unconjugated olefins such as alkyl-substituted olefins were not aziridinated under these conditions. 

Scheme 21. Chemoselective aziridination of 1-phenyl-1,5-hexadiene with complex 15.

Methyl-1-phenyl-1,5-hexadiene was obtained in a similar manner as 3-(4-acetylphenyl)- -butene depicted in 2.4.1. 

Et(Ind)2ZrCl2/MAO gives copolymers of ethylene or propylene with nonconjugated dienes, such as 2-methyl-1,4-pentadiene, 7-methyl-1,6-octadiene and 1,7-octadiene, (Eq. 23) [103]. rac-Et(Ind)2ZrCl2/MAO also catalyzes copolymerizations of asymmetrically substituted linear dienes, 6-phenyl-1,5-hexadiene, 7-methyl-1,6-octadiene, and R-(+)-5,7-dimethyl- 1,6-octadiene. The copolymerization of R-(+)-5,7-dimethyl-l,6-octadiene with propylene to give the polymer with ca. 15% diene incorporation. The ratio of the diene-derived part is ca. 15% of the polymer [104]. 

The Cope rearrangement of 2-phenyl-l,5-hexadiene-l,l-d2 (2) to 2-phenyl-l,5-hexadiene-3,3-d2 (5) has no thermodynamic driving force whereas the Cope rearrangement of 3-phenyl-1,5-hexadiene (3) to 1-phenyl-1,5-hexadiene (6) does. However, the kinetic studies by Dewar and Wade [10] found the former reaction to be faster than the latter by a factor of four at 190 °C. The larger effect of a C2 than a C3 radical-stabilizing phenyl substituent on the rate of the Cope rearrangement indicates... 

Another observation, which can be made from the results in Table 30.2, is that at the optimal values of R for the intermediate in the Cope rearrangement of 2-phenyl-1,5-hexadiene and for the TSs in the Cope rearrangements of 1,3-diphenyl-, and 1,3,4,6-tetraphenyl-1,5-hexadiene, ASjubs, = — 2A (jis,. Therefore, since +... 

When a second phenyl group is added to C5 of 2-phenyl-1,5-hexadiene, the increase in the magnitude of AFsubst in the TS is about a factor of two but when a second pair of phenyls is added to C4 and C6 of 1,3-diphenyl-1,5-hexadiene, the increase is about a factor of six. The reason for this difference is that the intermediates in the Cope rearrangements of 2-phenyl- and 2,5-diphenyl-1,5-hexadiene have UB3LYP values of R... 

Table 30.2 shows that at/f = 1.599 A the C2 phenyl group in 2-phenyl-1,5-hexadiene provides a net stabilization of = —2.9 kcal/mol, and atR = 2.218 A the Cl and C3 phenyl groups in 1,3-diphenyl-1,5-pentadiene provide a net stabilization of = —2.0 kcal/mol. However, the TS for Cope rearrangement of 1,3,5-triphenyl-1,5-pentadiene occurs with interaUyhc bond lengths of about R = 2.110 A (Table 30.1). At this TS geometry, neither the phenyl groups at Cl and C3 nor the phenyl group C5 provides as much stabilization as these phenyl groups furnish in the TSs for the Cope rearrangements of, respectively, 1,3-diphenyl-1,5-pentadiene at / = 2.218 A and...

Therefore, the competitive substituent effect, both predicted and found for the Cope rearrangement of l,3,5-triphenyl-l,5-hexadiene, is a consequence of the fact the value of R in the TS is a compromise between the value of R= 1.599 A, at which the phenyl group at C5 can provide optimal stabihzation for the contribution of structure A, and the value of R = 2.218 A, at which the phenyl groups at Cl and C3 provide optimal stabilization for the contribution of structure C. At the compromise value of/ = 2.110A the three phenyl groups in 1,3,5-triphenyl-1,5-hexadiene are calculated to provide 4.5 kcal/mol less TS stabilization than the total amount they furnish in the Cope rearrangements of 2-phenyl-1,5-hexadiene at R= 1.599 A and in l,3-diphenyl-l,5-hexadiene at / = 2.218 A. 

Phenyl-l,5-hexadiene (25 g) is heated under nitrogen at 176-178° for 26 h. Distillation through a Widmer column then gives almost pure 1-phenyl-1,5-hexadiene (18 g, 72%), b.p. 102.5-103.5°/8 mm.157... 

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