1,3,5-Cycloheptatrienes 1,7 -hydrogen migration

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. ... 

Hydrogen migrations in chromium complexes of cycloheptatriene are discussed. ... 

Cycloheptatriene and its derivatives readily undergo thermal [1,5] hydrogen migrations which are unimolecular and first-order in the cycloheptatriene complexes. Further studies show that the overall process involves a series of sequential [1,5] hydrogen migrations (Scheme S). The transition state is formulated as (45) rather than a discrete metal-hydride intermediate (46). 

The reactions of cycloheptatriene with transition metal atoms are similar to those of cyclopentadiene. In both cases, the reactions can involve extensive migration of hydrogen in the formation of the final product (9, 10, 133, 136) ... 

The two pathways, electrocyclic closure to the bicyclo[3.2.0]heptadiene and the [l,7]-shift of hydrogen, are in competition. This makes the experimental work more difficult, because the four possible isomers of the cycloheptatriene interconvert during the photolysis. Nevertheless the pattern of reactivity is clear, and the observation is that the better the donor substituent, the more it favours the formation of the bicyclo[3.2.0]heptadiene at the expense of the [l,7]-shift. This too is reasonable, since the hydride migration towards the donor-substituted atom 8.80 leads to a less stable cation, and ought to be slow. 

The migration of hydrogen in (cycloheptatriene)Mo(CO)3 has been studied (509) by use of the labeled compound (1-di-cycloheptatri-ene)Mo(CO)3 prepared by treatment of (THF)3Mo(CO)3 with the triene in hexane at room temperature. Between 80° and 120°C, the rate of migration, determined by NMR and mass spectra, is more than one thousand times greater than in the free olefin. The activation parameters are JH = 31.4 kcal/mole and = 7.7 cal/degree-mole. 

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