2.5- Diphenyl-l ,5-hexadienes

In an electrochemical transformation of 111, 112 was obtained as product (equation 30)181 and 9,10-dicyanoanthracene (DCA) was used to photochemically initiate the reaction of 2,5-diphenyl-l,5-hexadiene (113) to 114 (equation 31)182. 

The fluorescence of DCA is also quenched efficiently by 2,5-diphenyl-l,5-hexadiene with a nearly diffusion-limited rate constant in MeCN (1.1 x 10lodm3 mol-1 s ), since the photoinduced electron transfer from the diene ( ° = 1.70 V vs. SCE) to DCA (E ed = 1.91 V vs. SCE) [170] is exergonic [184], The photoinduced electron transfer induces Cope rearrangement of the diene via the cyclohexane-1,4-radical cation intermediate. In... 

Figure 1. Arrhenius plot of the cyclization ratios of the thermal polymerization of 2,5-diphenyl-l, 5-hexadiene.

Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,-diphenyl-l,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene. Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl. These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5-hexadiene. 

Cl bHi bC12Pc12, Bis(2-phenyl-7r-allylpalladium chloride), 34B, 444 ClbHiBCl2Pt, Dichloro(2,5-diphenyl-l,5-hexadiene)platinum(II), 46B, 846... 

The first example of the rearrangement of a cyclopropyl radical to an allyl radical in solution was observed in the thermal decomposition of l-methyl-2,2-diphenylcyclo-propanecarbonyl peroxide The radical reacted by abstracting hydrogen from solvent or by rearranging to the l,l-diphenyl-2-methylpropenyl radical which dimerized to yield l,l,6,6-tetraphenyl-2,5-dimethyl-l,5-hexadiene (89). The proportion of dimeric product to that of cyclopropane is dependent on the solvent. If a good radical scavenger is used, such as chloroform, carbon tetrachloride or thiophenol, then only the unrearranged cyclopropane derivative is obtained. This is also the case when a radical trap such as iodine is added to a benzene solution. 

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. 

Keto esters are obtained by the carbonylation of alkadienes via insertion of the aikene into an acylpalladium intermediate. The five-membered ring keto ester 22 is formed from l,5-hexadiene[24]. Carbonylation of 1,5-COD in alcohols affords the mono- and diesters 23 and 24[25], On the other hand, bicy-clo[3.3.1]-2-nonen-9-one (25) is formed in 40% yield in THF[26], 1,5-Diphenyl-3-oxopentane (26) and 1,5-diphenylpent-l-en-3-one (27) are obtained by the carbonylation of styrene. A cationic Pd-diphosphine complex is used as the catalyst[27]. 

Photolysis of l,6-diazido-2,5-diphenyl-3,4-diaza-2,4-hexadiene (679) afforded 3,6-diphenyl-l,2,4-triazine (680) in 21% yield (72CL1185). 

The alkyne complex (68) reacts with two equivalents of ethene to give the 5-cfr-3,4-( )-diphenyl-l,3-hexadiene complex (69) as shown in Scheme 39. Use of 2-butyne leads to the metallacycle (70) via bis(alkyne) complex. " The bicyclic vanadacyclopentatriene complex (71) is formed from 1,7-nonadiyne. 

Ozonolysis as used below is the oxidation process involving addition of ozone to an alkene to form an ozonide intermediate which eventually leads to the final product. Beyond the initial reaction of ozone to form ozonides and other subsequent intermediates, it is important to recall that the reaction can be carried out under reductive and oxidative conditions. In a general sense, early use of ozonolysis in the oxidation of dienes and polyenes was as an aid for structural determination wherein partial oxidation was avoided. In further work both oxidative and reductive conditions have been applied . The use of such methods will be reviewed elsewhere in this book. Based on this analytical use it was often assumed that partial ozonolysis could only be carried out in conjugated dienes such as 1,3-cyclohexadiene, where the formation of the first ozonide inhibited reaction at the second double bond. Indeed, much of the more recent work in the ozonolysis of dienes has been on conjugated dienes such as 2,3-di-r-butyl-l,3-butadiene, 2,3-diphenyl-l,3-butadiene, cyclopentadiene and others. Polyethylene could be used as a support to allow ozonolysis for substrates that ordinarily failed, such as 2,3,4,5-tetramethyl-2,4-hexadiene, and allowed in addition isolation of the ozonide. Oxidation of nonconjugated substrates, such as 1,4-cyclohexadiene and 1,5,9-cyclododecatriene, gave only low yields of unsaturated dicarboxylic acids. In a recent specific example... 

On addition of a second pair of phenyl substituents at C4 and C6, the interallylic distance further lengthens so that the contribution of structure C to the TS wave function is further enhanced. The ability of the second pair of phenyl substituents to increase the optimal value of R from 7 = 2.218 A to 7 = 2.649 A allows each of the phenyl substituents in l,3,4,6-tetraphenyl-l,5-hexadiene to provide more stabilization for the Cope TS than the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. This cooperative effect of the four phenyl groups in 1,3,4,6-tetraphenyl-1,5-hexadiene lowers AT/ for Cope rearrangement by, not twice, but by four times as much as the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. 

Diphenylcarbodiaxone [called C-Hydroxyr diphenyltetrazolium Betaine or 1,2-Diphenyl-5-oxa-l,2,3,6-tetrazabicyclo [2.1.1]-1(6),3-hexadiene in CA Coll Formula Index 14-40 (1920-46), p718 [(called Diphenylcarbodiazon or Anhydrid of 2.3-Diphenyl-5-oxytetrazolium-hydroxid in Ger)... 

Diphenyl-cyelo- hexadien-(l,3) Benzophenon S.endo-6-Diphenyl-bicyclo [3.1.0] hex-en-(2) 85 59 1... 

TriplienyI-2-benzyl-2H-thiopyran gekt entsprcchendin6-Thiono-l,2,4,6-tetraphenyl-hexadien-(2,4) iiber5. 2H-Chromen wird analog in 6-Oxo-5-prope,n-(2)-yliden-cyclohcxa-dien-(l,3) uberfiihrt3 und 2,2-Diphenyl-2H-l-benzothiopyran in 6-Thiono-5-[3,3-diphenyl-propen-(2)-yKden]-cyclohexadien-(l, 3)5. 

According to121), 2,6-diphenyl-4-tert-butylperoxy-4-methoxy-2,5-cyclo-hexadien-l-one is decomposed under formation of methyl alcohol and 2,6-di-phenyl-l,4-benzoquinone. 4-tert-Butylperoxy-2,4,6-tri-tert-butyl-2,5-cyclohexadiene-1-one XXXVI (R = R1 = tert-Bu) is thermolyzed already at 65 °C to the mixture of compounds, where 2,6-di-tert-butyl-l,4-benzoquinone XXII was identified as the main component31,13°) the mixture contained, according to GPC, also 3,5-di-tert-butyI-4-hydroxybenzaldehyde XXXII and stilbenequinone XXIX. 

Another important observation was provided even before the Dewar papers, namely that the secondary deuterium kinetic isotope effect (SDKIE), at C4 of a 3,3-dicyano substituted-1,5-hexadiene was a larger normal effect in an absolute sense than the inverse SDKIE at C6. This must represent more bond breaking at C4, i.e. rehybridization from sp to sp, than bond formation at C6 involving rehybrization from sp to sp, a result inconsistent with a diyl-like transition state whose SDKIEs would be opposite in magnitude. Subsequent SDKIE determination by Conrad on 3-aIkyl substituted -1,5-hexadiene and 2-phenyl-l,5-hexadiene and on carbon-13 labeled 2,5-diphenyl-1,5-hexadiene revealed a change in transition state structure from a species with more bond making than breaking to one with a nearly a fully formed 1,4-cyclohexane diyl in that order. 

Ultraviolet irradiation of 2,3-dihydro-l,2-diphenyl-6-methyl-4-pyridone (MI-723) in methanol gives 2,5-diphenyl-6-methyl- and 2,3-dihydro-2,6-di-phenyld-methyl-4-pyridone and 5-aniline 1-phenyl-l, 4-hexadiene-3-one. ... 

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