2,4-Dimethyl-2,3,5-hexatrien

The (R) allene 121 was obtained with high anti stereoselectivity in the reaction of (i )-(—)-l-trifluoroacetoxy-l-phenyl-2-propyne (120) with PhZnCl. 2-Alkynyloxiranes react smoothly with alkynyl, alkenyl and arylzinc reagents. Reaction of 2-methyl-2-(l-propynyl)oxirane (122) with vinylzinc chloride (123) yields 2,4-dimethyl-2,3,5-hexatrien-l-ol (124) [31],... 

Dimethyl-hexatrien-(l,3,5) l,3-Dimelhyl-3-vinyl-cydobuten + l,3-Dimetfiyl-bicydo[3.1.0]hexen-(2) — 4... 

Agreement is somewhat poorer for substituted hexatrienes. Consider now the ( )-isomer of 2,5-dimethyl-l,3,5-hexatriene (81) for which Roth gives an enthalpy of formation of 95.8 kJ mol-1. Simple olefin additivity, as done above for the parent hexatriene, results in a value of 103 kJ mol-1. Modifying the above 5 to 3.5 kJ mol-1 as found for the relatedly branched conjugated diene (isoprene, 82) gives a new value of ca 100 kJmol-1 for the enthalpy of formation of 81. The discrepancy has shrunk to ca 4 kJmol-1. 

Consider now the 1,1,6,6-tetramethylated derivative of (Z)-l,3,5-hexatriene (83), a species more properly named (Z)-2,6-dimethyl-2,4,6-octatriene and occasionally and trivially called ds-allo-ocimene . To estimate its enthalpy of formation, let us use simple olefin additivity along with ... 

Photochemical d.s jraw.v-confonnational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K108. Irradiation of the -isomer gives rise to various retainers, while irradiation of the Z-isomer results only in ,Z-isomerization. Photochemical translcis conformer interconversion has also been observed for , -1,3,5,7-octatricnc in matrices at temperatures below 10 K32. 

Stable germylene (97) with 2,5-dimethyl-2,3,4-hexatriene and A -phenylmaleimide provided the stable germirane derivatives, bis(alkylidene)germirane (98) and bicy-clogermirane (99, Scheme 14.47). ... 

Problem 12.6 Draw conclusions about the relationship of to the structure of the absorbing molecule from the following A , values (in nm) ethylene (170), 1,3-butadiene (217), 2,3-dimethyl-l,3-butadiene (226), 1,3-cyclohexadiene (256), and 1,3,5-hexatriene (274). 

Many conjugated dienes react twice with methyl 3-bromo-2-propenoate and triethylamine, making this procedure convenient for preparing a variety of dimethyl 2,4,6,8-decatetraenedioates.98 1,3,5-Hexatriene and its substituted derivatives undergo vinylation also, to form tetraenes and pentaenes in low yields.83... 

A. M. Brouwer, J. Cornelisse, and H. J. C. Jacobs, /. Photochem. Photobiol. A Chem., 42, 313 (1988). Photochemistry of 2,5-Dimethyl and 2,5-di-tert-Butyl-l,3,5-hexatrienes— Conformation and Reactivity—A Quantitative Study. 

Based on the above observations, we have focused on a dimer with inversion symmetry as a first simple model for our investigation. The molecule considered is 1-1-dimethyl-ammine-6,6-dicyano-hexatriene (hereafter referred as DMADC3), whose structure is sketched in Figure 4.17. 

Direct photolysis of l,l-dicyano-3,3-dimethyl-l,4,5-hexatriene 29 gives an allenyl-cyclopropane 30 via DPM rearrangement in 57% yield, while acetone-sensitization yields a hausan 31 via an intramolecular [2 + 2] cycloaddition (Scheme 4.15) [20]. 

In Problem 14.46, we concluded that one alkyl group increases A.max of a conjugated diene by approximately 5 nm. Since 2,3-dimethyl-l,3,5-hexatriene has two methyl substituents, its UV kmax should be about 10 nm longer than the A.max of 1,3,5-hexatriene. 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

The photocyclization of (Z)-1,3,5-hexatriene represents a well-studied example of the influence of ground-state conformational equilibrium on the product composition (Havinga, 1973). The unsubstituted triene exists mainly in the s-trans-s-trans conformation and therefore cannot undergo photocyclization only Z-E isomerization has been observed. With the 2,5-dimethyl derivative the s-cis-s-cis conformation predominates and photocyclization yields dime-thylcyclohexadiene and some dimethylvinylcyclobutene. Finally, with the 2-methyl derivative the s-cis-s-trans conformation is preferred, and a relatively good yield of methylbicyclo 3.l.0]hexene as well as methylvinylcyclobutene and methyIhexatriene-1,2,4 is found. These results are collected in Scheme I. 

A similar reaction with 1,3,5-hexatriene and dimethyldichlorosilane yields the 1,1-dimethyl-2-vinyl-l-silacyclopent-3-ene when THE and CU2CI2 are employed [110],... 

Hexatricne has = 258 nm. In light of your answer to Problem 14.43, approximately where would you expect 2,3-dimethyl-1,3,5-hexatriene to absorb Explain. 

The proposal that the wavelength dependence of the product yields from irradiation of Z-14 is due largely to selective excitation of specific conformers is supported by the observation that the UV spectrum of the compound can be simulated accurately by a 9 1 combination of the spectra of Z-2-rcrr-butyl- (Z-131) and Z-2,5-di-fert-butyl-l,3,5-hexatriene (Z-15), which serve as model compounds for the cZi and cZc conformers of Z-14, respectively . Both Z-15 and the perfluorinated ,Z, -4,5-dimethyl-2,4,6-octatri-ene derivative (135) are thought to adopt preferred helical cZc conformations, and undergo highly selective electrocyclic ring closure to the corresponding cyclohexadienes 132 and 136, respectively upon irradiation (equations 49 the reaction of Z-15 the... 

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