Cycloheptatriene hydride donor

In addition to their reactions with alkenes and carbanions as nucleophiles benzhydryl cations react with hydride donors.282 284 These hydride transfer reactions show the same linear dependence of log k upon E as the reactions with alkenes and the same constant relative selectivity, that is with slopes of plots close to 1.0, for structures ranging from cycloheptatriene to the... 

Some representative examples of hydride donor couples are triarylmethanes (3a,b), cycloheptatrienes (4a,b), a-heterosubstituted methanes (5a,b), and heterocycles such as 1,4-dihydropyridines (6a,b). 

It is possible to selectively reduce an unsaturated side-chain catalytically without reducing the azulene rings (66MI1). Recently, an effective metal-free method has been published for the selective reduction of the C = C double bond even in azulenic enones, for instance, thiophenes 54 (Scheme 81) or 55a and 55b, using the hydride donor cycloheptatriene and protic acid (06OL3137). 

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. 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

---