Cycloheptatriene, isomerization substituted

Recently, a new reactivity index has been proposed (80H(14)1717> which predicts accurately the site selectivity of photocyclization of substituted cycloheptatrienes to their bicyclic valence tautomers. Unfortunately, application of the method to substituted lH-azepines is far less successful. For example, for 2-methyl-l-methoxycarbonyl-lH-azepine (37 R = 2-Me) AGrs values for C-2—C-5 and C-4—C-7 cyclization are calculated as 0.093 and 0.040 kJ mol-1, respectively, i.e. predicting the 1-methyl isomer (39) as the major product. Experimentally, however, the reverse is true, the yields being 93.5% for 3-methyl (38 R = Me) and 6.5% for 1-methyl (39 R = Me). The corresponding photoinduced valence isomerizations of 1-benzazepines to 3,4-benz-2-azabicyclo[3.2.0]hepta-3,6-dienes (38a) have been recorded (80JOC462). These isomerizations have also been achieved thermally in the presence of silver ion (80TL3403). 

A study of photochemical 1,7-hydrogen migrations within the cycloheptatriene derivatives (127) has been reported. Hansen and his coworkers have demonstrated that irradiation of the heptalenes (128) brings about reversible isomerization into a mixture of isomers composed of (129) and starting material (128). To some extent, the ratio of photo-product starting material is dependent upon the substitution as indicated by the yields shown under the appropriate structure. ... 

The photochemistry of other cyclic conjugated trienes (e.g., 1,3,5-cyclooctatriene and substituted benzenes) will be discussed and their valence isomerization reactions will be compared with those of substituted cycloheptatrienes. The photochemistry of acyclic conjugated trienes is necessarily more complex than that of cyclic trienes due to the possibility of cis-trans isomerization. The presence of several geometric isomers in a reaction mixture opens up the possibility of other modes of reaction. These transformations will be briefly summarized. Nonconjugated acyclic and cyclic trienes will not be considered in this article. 

A study of the photoequilibrium involving 1,5-hydrogen transfer between the isomeric cycloheptatrienes (205a) and (205b) has been reported. The influence of substitution in the aryl ring, e.g. (206), has also been studied." ... 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

Sol 1. Isomerization of substituted cycloheptatrienes takes place through sequences of [1,5] sigmatropic rearrangements and electrocyclic reactions. 

Norcaradienes are intermediates in several types of reaction, including intramolecular and intermolecular additions of carbenes to aromatic rings, the chromic acid oxidation of cycloheptatriene to benzaldehyde, the thermal isomerization of 7-substituted norbornadienes to cycloheptatrienes, °° and the reaction of pyridine iV-oxides with benzynes. An interesting feature of the latter reaction is the transfer of oxygen from the pyridine to the benzene nucleus (Scheme 61). 

Valence Isomerism ° Norcaradienes. The dicyano-substituted compound (U5) (see p. 38) exists only as the norcaradiene, and at 110°C it undergoes thermolysis to a benzylfulvene rather than valence isomerization. Protonation of the cycloheptatriene (330) at — 60°C gives an equilibrium in which only the norcaradiene (331) is detectable by n.m.r. spectroscopy. The n.m.r. 

Addition of an T -allyl-Fp complex to this compound affords an T -aIlyl-Fp-substituted cycloheptatriene system. Two double bonds are involved in an (T -diene)iron complex. The remaining free double bond of the silyl enol ether attacks as a nucleophile at the cationic r -alkene-Fp moiety to form an (Tj -diene)iron complexed cyclopentane annulated cycloheptadienone. Treatment with CAN in methanol under carbon monoxide atmosphere releases the methoxycarbonyl-substituted free ligand (Scheme 4-25). Reaction of the Ti -dienyliumiron intermediate of Scheme 4-25 with an ( , Z)-isomeric mixture of ri -crotyl-Fp proceeds with high diastereoselectivity. Four new stereogenic centers are formed in the course of this formal [3+2] cycloaddition. A hetero [3+2] cycloaddition is also feasible between T -ailyl-Fp complexes and aromatic aldehydes in the presence of zinc chloride or titanium(IV) chloride to provide tetrahydrofuran derivatives (Scheme 4-26). A 1,2-shift of the iron complex fragment occurs in the course of this reaction. Employment of imines affords the corresponding pyrrolidines. ... 

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