Abstract
In this thesis, several new nickel-catalyzed cyclization and coupling reactions and their application in organic synthesis are discussed in detail. For convenience and better clarity, the thesis is divided in to six chapters. The first two chapters deal with the synthesis of coumarin derivatives, the third chapter depicts the synthesis of indenols via carbocyclization, fourth chapter describes about the synthesis of medium-size ring lactones and synthesis of 1,2-dihydroarenes is discussed in last two chapters. The nickel-catalyzed cyclization of 7-oxabenzonorbornadienes with alkyl propiolates is investigated in the chapter 1. The reaction of 7-oxabenzonorbornadienes with alkyl propiolates catalyzed by nickel complexes gave benzocoumarin derivatives in high yields with high regio- and stereoselectivity. The catalytic mechanism for the benzocoumarin formation is of interest, involving nickalocyclopentene and oxanickalocycle as key intermediates. A methodology for the synthesis of dibenzocoumarins has been discussed in chapter 2. The reaction of 7-oxabenzonorbornadienes with methyl 2-iodobenzoates in the presence of nickel catalyst gave dibenzocoumarins in moderate to good yields. The nickel-catalyzed one-pot reaction also affords highly conjugated dibenzocoumarins, albeit in moderate yields. Nickel-catalyzed carbocyclization reaction is described in chapter 3. Various indenol derivatives have been synthesized from the reaction of ortho-iodophenyl ketones with alkyl propiolates catalyzed by nickel complexes. This carbocyclization reaction is highly regioselective providing indenols in moderate to excellent yields. The proposed mechanism involves vinyl-nickel as key intermediate. A nickel-catalyzed cyclization reaction for the synthesis of medium-size ring lactones has been investigated in chapter 4. The reaction of 2-iodobenzyl alcohols with alkyl propiolates catalyzed by nickel complexes affords seven-member lactones in moderate to good yields with high regio and stereoselectivity. The important aspect of the mechanism is the Lewis-acid promoted cis-trans isomerization step. Reductive coupling reaction of oxa- and azabicyclic alkenes with alkyl propiolates to give ring opening addition products catalyzed by nickel complexes has been described in chapter 5. The ring opening reaction affords 2-alkenyl-1,2-dihydroarenes derivatives with remarkable regio- and diastereoselectivity and under very mild conditions. In addition, the reductive coupling reaction is atom economical. The methodology is also applicable for the synthesis of functionalized cyclohexenols and bicyclic-□-lactones in good yields with excellent selectivity. Chapter 6 describes about the synthesis of 2-alkynyl-1,2-dihydroarenes derivatives from the reaction of oxa- and azabicyclic alkenes with terminal acetylenes in the presence of nickel complexes. The ring-opening addition reaction offers a convenient method for the construction of the dihydronaphthalene framework in one pot from easily accessible starting material. Significant feature of this nickel-catalyzed reaction is the mechanism. The active catalyst for the present reaction is Ni(II) unlike in other reactions, where Ni(0) was the active species.