Abstract
The thesis discusses new ruthenium and molebdenum-mediated organic transformations. For convenience and better understanding, the thesis is divided into four chapters. The first chapter deals with carbon-carbon triple bond cleavage. All previous reports involving catalytic cleavage of alkyne require additional promoters such as 2-aminopyridine or 2-aminophenol. In this chapter we have described a new pathway for the cleavage of alkyne in the absence of organic promoters. We have found that ethynyl alcohol in presence of 10 mol % TpRu(PPh3)(CH3CN)2PF6 and 20 mol% LiOTf in toluene at 100 oC produces alkene and carbon monoxide. Isotope labeling experiment shows that the reaction proceeds via ruthenium-allenylidene intermediate. The second chapter deals with cyclization of propargylic alcohols. During our investigation on the ruthenium-catalyzed cleavage of triple bond, we observed that increase of carbon chain by one unit in the propargylic substrate and introducing electron donating substituent at the proper position of benzene ring gives monoruthenium complex-catalyzed cyclization in presence of catalytic amount of Lewis acid. Treatment of 6-aryl hex-1- yn-3-ols with 10 mol % of TpRu(PPh3)(CH3CN)2PF6 in presence of catalylitic amount of Zn(OTf)2 (20 mol %) at 100 oC in toluene afforded 1-ethynyl-1,2,3,4-tetrahydro-napthalene derivatives with alkyne functionality. In chapter 3, we describe molebdenum-mediated cyclocarbonylation of 1-ethynyl-2 allenylbenzenes to 1H-cyclopenta[a]inden-2-one derivatives at ambient condition. 1,2,3,3a,8,8a Hexahydrocyclopenta[a]indene is often encountered in naturally occurring polyphenol species such as pallidol and gneafricanin which shows interesting biological activities. Although reaction of 1-ethynyl-2 allenylbenzenes with metal specious are expected to give competitive Myers–Saito or Schmittel cyclization which also occur under ambient conditions, we report a clean and efficient Pouson-Khand type cyclocarbonylation of 1-ethynyl-2-allenylbenzene derivatives with stoichiometric amount of Mo(CO)3(CH3CN)3 in dichloromethane at 25 oC. The final and fourth chapter establishes ruthenium-catalyzed cycloisomerization of cis-3-en-1-ynes to cyclopentadiene and related derivatives through a 1,5-sigmatropic hydrogen shift of ruthenium-vinylidene intermediates. [1,5] sigmatropic hydrogen shift of cis-3-ene-1-ynes proceeds sluggishly even at elevated temperatures. One possible approach is to mimic the thermal rearrangement of cis-1-allen-4-enes using a suitable metal species to generate metal-vinylidene intermediates. We observed that C(3) and C(5) substituted 1-ethynyl-3-ols in presence of 10 mol % TpRuPPh3(CH3CN)2 in benzene at 80 oC produced cyclopentadiene derivatives which are appealing building blocks to construct the skeletons of complex molecules via the formation of intermediate cis-enyne. The role of the ruthenium catalyst is twofold in the reaction: (1) dehydration of 1-ethynyl-3-ol and (2) cyclization of cis-enyne via 1,5-sigmatropic hydrogen shift.