Abstract
Chapter I This chapter deals with ruthenium-catalyzed synthesis of furan epoxyalkyne functionality. Ruthenium catalyst TpRuPPh3(CH3CN)Cl effected the cyclization of epoxyalkynes to furans in the presence of Et3N. The reactions worked well for various epoxyalkynes with suitable oxygen and nitrogen functionalities with low loading of catalyst. It failed with disubstituted epoxyalkynes. The mechanism was elucidated by a deuterium labelling experiment and involved a ruthenium-vinylidenium intermediate. Chapter II Chapter II describes ruthenium catalyzed cis-trans isomerization of epoxide. Ruthenium catalyst TpRuPy2Cl was effective for cis-trans isomerization of various functionalized epoxides. Enantiospecific isomerization of chiral epoxides is achieved without loss of enantiopurity, and epimerization occurs only at the epoxide carbon of the activating group. The mechanism of isomerization involves cleavage of the C-O bond at the epoxide carbon of the activating group. SN2 attack of the epoxide by ruthenium is proposed as the key step. This isomerization enhances the usefulness of epoxides in organic synthesis. Chapter III This chapter discusses ruthenium catalyzed cycloisomerization of (o-ethynyl)styrenes. Treatment of a series of 2’,2’-disubstituted (o-ethynyl)styrenes with TpRu(CH3CN)2PPh3PF6 ( 10 mol %) in benzene (80 °C, 12-18h) efficiently gave 2-alkenyl-1H-indene derivatives. This catalytic reaction represents an atypical enyne cycloisomerization with skeletal rearrangement of starting enyne, where the C=C bond is completely cleaved and inserted by the terminal alkynyl carbon. The reaction mechanism was elucidated by a series of deuterium and 13C labeling experiments, as well as by changing the substituents at the phenyl moieties. The mechanism is proposed to involve the following key steps: 5-endo-dig cyclization of ruthenium-vinylidene intermediate, a nonclassical ion formation, and the “methylenecyclopropane-trimethylenemethane” rearrangement. Chapter IV Bergman type of cyclization of 1,2-bis(ethynyl)benzene via hydrohalogenation is described in this chapter. Treatment of 1,2-bis(ethynyl)benzene (91) with aqueous HX (X = Br, I) in hot 3-pentanone (100-105 °C, 2h) afforded 1,2-bis(1’-haloethenyl)benzene species 99-Br and 99-I in 98% and 95% yields, respectively. The hydrochlorination of endiyne 91 failed to proceed at elevated temperature but was implemented efficiently by PtCl2 (5 mol%) in hot 3-pentanone (100 °C, 2h) to give 1,2-bis(1’-chloroethenyl)benzene 99-Cl in 80% yield. In the presence of PtCl2 (5 mol %), these halides 99-Cl,99-Br and 99-I were subsequently converted to 1-halonaphthalenes 100-Cl,100-Br and 100-I in the mothersolution via sequential 6-p electrocyclization and dehalogenation reactions. PtCl2 (5 mol%) also effected direct haloaromatization of endiyne 91 with HX (X = Cl, Br, I) and gave 1-halonaphthalenes 100-Cl,100-Br and 100-I in 64-71% yields. Scope and regioselectivity of haloaromatization of various enediynes catalyzed by PtCl2 has investigated. Chapter V Last chapter describes PtCl2 (5 mol%) catalyze cycloisomerization of cis-2,4-dien-1-al. Chemselective cycloisomerization of cis-2,4-dien-1-als to 3- cyclopentenones and 4-alkylidene-3,4-dihydro-2H-pyran was achieved using PtCl2 and PdCl2(PhCN)2 respectively. In the presence of p-TSA catalyst, PtCl2 led to formation of conjugated 2-cyclopentenones. These new metal-catalyzed reaction highlights the synthetic utility of cis-2,4-dien-1-als with the availability of various carbocyclic and oxygen heterocyclic compounds. A plausible mechanism is proposed on the basis of reaction observation and isotope-labeled experiment.