Hoffmann-Noyori reaction

Formally the process is comparable to an electrochemical reduction on mercury drops (p. 290), but lower yields (30-75%) are obtained sonochemically than electro-chemically (>90%). With unsymmetrical dibromoketones, mixtures of two isomeric a-acetoxy ketones are obtained, but the selectivity is enhanced by using bulkier acids, e.g., trimethyl- or triethyl acetic acid. Oxyallyl cations from a,a -dibromoketones add to olefins or dienes in a [3 + 2] or [3 + 4] mode, the Noyori or Hoffmann-Noyori reaction. 21,322... 

The reaction of a diene with a three-carbon, two-electron species in a [4+3] cycloaddition (Figure 2) has been studied extensively for the synthesis of seven-membered rings. Noyori, ° Hoffmann, White,Mann, Harmata, " Trost, Giguere, and others have reported spectacular examples of these processes, a few of which are illustrated here. ... 

Preparation of bicyclic systems via the reductive coupling of 1,3-dienes with a,a -dibromoketones to give bicyclic systems has been known for some time. Hoffmann discovered that moderate yields of coupled product were available using a zinc/copper couple. The reaction took between 12-80 h [158] however, yields could be significantly improved by use of a trimethyl-silylchloride mediator [159]. Complementary work by Noyori [160] had already shown that use of diiron-nonacarbonyl led to significant increases in yield despite the obvious disadvantages of its expense and toxicity. Hoffmann has recently examined the effects of ultrasound on his original reaction [161]. 

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