Rhodium (II) catalysed reactions of α-Diazo phosponates

The syntheses and reactions of some α-diazo phosphonates are reviewed. The work presented is based on the inter- and intra-molecular reactions of the previously proposed, transient rhodium carbenoid species with nucleophiles. In order to shed some light on the mechanism of the rhodium (11) catalysed insertion reactions of diazo compounds with nucleophiles and consequently to verify the presence of the rhodium carbenoid species or not, studies have been carried out to determine what factors affect the rate of insertion. Thus, the intermolecular rhodium (11) catalysed insertion reactions of a series of diazo compounds in the presence of various alcohols have been investigated. Some novel diazo compounds have been prepared and more efficient, more amenable routes to some known diazo compounds have been developed. It was found that rhodium (11) trifluoroacetamide is a significantly more active catalyst than rhodium (11) acetate, under the test conditions. Two new methods leading to the formation of functionalised cyclic ethers (five-, sixand seven-membered rings) have been developed. Both involve the insertion of an alcohol into an a-diazo phosphonate catalysed by rhodium (11) acetate as the initial step but whereas one method proceeds via an intermolecular Wadsworth-Emmons reaction, the other proceeds via the intramolecular variant of the same reaction. 7/6- and 717-Fused bicyclic ethers have been prepared from 3-oxooxepane 2- phosphonates via the Wadsworth-Emmons reaction. The oxepanes themselves were prepared in good yields by an intramolecular rhodium (11) catalysed O-H insertion reaction of a parent diazo alcohol. The same bicyclic systems could also be accessed by two different routes starting with t-butyl-7-hexyl-3-oxooxepane-2- carboxylate. Various novel synthetic routes leading to the formation of functionalised bicyclic, tricyclic and tetracyclic systems have been investigated. All involve the intramolecular rhodium (11) catalysed O-H insertion reaction of a diazo alcohol as a key cyclisation step.