Abstract
The thesis describes nickel and cobalt-catalysed organic synthesis leading to couplings, carbocyclisations and formation of heterocyclic compounds. It is subdivided into three broad topics, spread out into a total of four chapters. The first chapter describes nickel-catalysed cyclisation of □-iodo-(z)-propenoates with oxabenzonorbornadiene derivatives resulting in the formation of benzocoumarins in good yields. An extension of the same methodology to reveal intramolecular cyclisation has also been described subsequently in the same chapter. The second and third chapters deal with reaction of saturated alkyl halides with reactive alkenes to form reductive coupling products in the presence of nickel and cobalt complexes. The fourth chapter consists of carbocyclisation of ortho-iodobenzaldehydes and ketones to give rise to polycyclic ketones in the presence of nickel complexes. The latter also includes an unusual □-alkyl elimination mechanism. The first chapter details a nickel-catalysed strategy to prepare coumarins which are well known drug precursors and excellent materials for OLED (organic light emitting devices). Treatment of o-iodoesters with oxabenzonorbornadiene derivatives in the presence of nickel complex, led to the synthesis of benzocoumarins in good to excellent yields. The methodology developed by us can be utilized for the preparation of a variety of substituted coumarin derivatives by simple and efficient utilization of nickel catalysis. The mechanism of this interesting reaction has also been discussed. The later part of the same chapter recounts the extension of the methodology to prepare lactams. When an ortho-iodoamide is subjected to the same nickel-catalysed reaction conditons, intramolecular Heck-type cyclisation occurred to form lactams in good yields. The lactam framework synthesized by this methodology is contained in many naturally occurring compounds e.g. Oxychelerythrine, Oxyavicine etc. The second chapter describes reaction of sp3 alkyl halides with electron withdrawing alkenes in the presence of nickel complex to give rise to reductively coupled products. The strategy encompasses a variety of primary, secondary and tertiary alkyl halides and bromides, which react with various activated olefins e.g. □□□-unsaturated esters, ketones, nitriles etc. to form saturated products in good to excellent yields. An extension of this reductive coupling methodology to oxabenzonorbornadienes has been depicted in a later part of the same chapter. Ring opening of oxabenzonorbornadienes to form substituted dihydronaphthol derivatives in the presence of nickel complexes is delineated in the final portions of the second chapter. The third chapter recounts cobalt-catalysed reductive coupling of saturated alkyl halides with electron withdrawing alkenes to form saturated products. The cobalt-mediated reductive couplings are the first reports of such utilization of cobalt complexes. The mechanism of this reaction has been extensively explained. Mechanistically, the reaction appears quite different from the previously reported similar couplings, as evinced by the various experiments that were performed. The reasons for why the reaction may be an oxidative addition driven one rather than a radical mediated one, have been put forward. The fourth chapter depicts the reaction of ortho-substituted iodobenzaldehydes and ketones with bicyclic alkene derivatives in the presence of nickel complexes to form annulated ketones. The first part of the chapter describes the reaction of o-iodobenzaldehydes with various bicyclic alkenes in the presence of nickel catalyst to form polycyclic ketones in good yields. In the second portion of this chapter, there is a description of the use of o-iodoketones with the same bicyclic alkenes to form similar polycyclic ketone products. It is surprising to note that when o-iodoaldehydes were replaced by o-iodoketones the products of the reaction were still the same i.e. ketones, although based on precedents, the expected products should have been cyclic tertiary alcohols. This bizarre behavior was investigated in detail by postulating unprecedented □-alkyl elimination during the reaction. Such □-carbon elimination is yet unknown in nickel-catalysed systems. Also, the fate of the moiety fractionated from the starting ketone compound was determined by isolating a key side product, leading to a better understanding of the mechanism of this reaction.