New Chemistry of Multiynes under Thermal and Metal-Catalyzed Conditions

This thesis consists of two main parts, Part I and Part II. The five chapters in Part I deal with the thermal reaction of multiynes focusing on the new reactivity of arynes generated via the hexadehydro Diels-Alder (HDDA) reactions. The last three chapters in Part II deal with metal-catalyzed reactions of multiynes focusing on the synthesis of novel ruthenium alkylidene complexes and investigating their catalytic activities. Chapter 1 provides a brief overview of the thermal reactions of multiynes focusing on the history and development of dehydro Diels-Alder reactions. This chapter covers the discovery and development of HDDA reaction as a tool for aryne generation. This chapter also includes a systematic overview of the application of HDDA reaction for the synthesis of highly substituted aromatic compounds including several natural products. In Chapter 2, an Alder-ene reaction of arynes generated via HDDA reaction with functionalized alkenes is described. This study led to the development of a formal allylic C–H functionalization reaction under mild reagent-free conditions. The reaction proceeded showed excellent functional group tolerance and high regioselectivity. New insight into the stereoselectivity of this reaction was obtained which was supported by DFT calculations. In Chapter 3, the competition between Alder-ene reaction and addition reaction to arynes were investigated with functionalized alkenes having highly polar functional groups like amines, carboxylic acids and unsaturated aldehydes. Systematic investigation revealed the various factors affecting the selectivity between these two manifolds of reactions. The reaction of unsaturated aldehydes with arynes predominantly underwent an addition reaction to form 2H-chromene. The mechanism for this transformation was also established through DFT calculations. Chapter 4 describes a novel intramolecular Alder-ene reaction to form benzocyclobutene. The high kinetic barrier to form a 4-membered ring was overcome by installing a maximum gearing effect through an internal hydrogen bonding. The reaction is cis-diastereoselective and the origin of this selectivity was studied using DFT calculations. In Chapter 5, a diversity-oriented approach for the synthesis of several bioactive natural products of the Selaginella family is reported. The strategy relies on HDDA mediated synthesis of the arene core followed by various standard synthetic manipulations that lead to a unique approach to access the different natural products of this family. This strategy accomplished the first synthesis of selaginellin H and made significant progress in the synthesis of selaginpulvilin G. In Chapter 6, a brief overview of the metal-catalyzed reactions of multiynes is described. It describes the origin and development of Pt- and Au-carbenoid mediated transformations of multiynes and Ru-alkylidene carbene catalyzed enyne metathesis of multiynes. This chapter also describes the application of these transformations in natural product synthesis, organometallics, and polymer chemistry. Chapter 7 describes a unified approach for the synthesis of various alkene-chelated ruthenium alkylidene complexes. Both steric and electronic controls were used for the stabilization of these complexes. The structural properties of these complexes were studied through X-ray crystallography and the catalytic activities for these complexes were studied for various standard metathesis processes. Careful tuning leads to the development of catalysts with enhanced catalytic activities compared to standard metathesis catalysts. Chapter 8 describes a novel approach for the synthesis of ruthenabenzenes relying on metathesis and metallotropic cascades of multiynes. This study develops these ruthenabenzene complexes as an aromatic equivalent of the Grubbs catalyst. These complexes show robust catalytic activities towards both metathetic and non-metathetic reactions and have high catalyst recoverability. DFT calculations reveal the mechanism for the formation of these complexes and also provides insight into how they show robust catalytic activities.