Rhodium(II) Catalyzed Cyclization of Diazo Thiocarbonyl Compounds for Heterocycloic Synthesis

Abstract

The mesoionic thioisomiinchnone system was prcpared fm the rhodium(I1) acetate catalyzed cyclization of a diamthioamide and was found to undergo smooth 1.3-dipolar cycloaddition with N-phenylmaleimide.In contrast to this system, the rhodium@) reaction of an a-diazc-bxo ester containing a thiocarbonyl p u p produd acyclic thiocarbonyl ylide whichextruded sulfur from a wllsient episulfide intermediate.Thiocarbonyl ylides have been the subject of much interest in recent years due to their potential role as intermediates in a variety of reactions including the formation of episulfidesl and novel heterocyclic ring systems.2Thefirst recognition oftheexistence ofthiocarbonyl ylidesappearsto have been made by Knott in 1955 in conjunction with considerations of the contributions of this segment to the electronic make-up of certain dyestuffs.3Thiocarbonyl ylide formation can be achieved by several different pathways.The most flexible and general mute involves the A3-1,2,4-thiadiazoline ring system (1) as the dipole p r e ~r s o r .~ Mild thermolysis of ( I ) 0~tNdeS nitrogen and produces the thiocarbonyl ylide (2) via a 1,s dipolar cycloreversion.5Two generalized routes to the precursor (1) are shown in Scheme 1.68 Thiocarbonyl ylide (2)generated as a reactive and short-lived intermediate has been trapped by various dipolarophiles, and kinetic analysis of the reaction has also unambiguously established the transitory existence of intermediate (2).The predicted conrotatory ring closure occurs to give episulfide (3) in the absence of trapping agents. 1 ,%Dipolar cycloaddition of 2 proceeds with retention of configuration as predicted.'Other methods for thiocarbonyl ylide generation involve the addiiion of thioketones to oxiranes,'O the photorearrangement of aryl vinyl s u l f i d e ~, ~I -~~ the desilylation of a-bmmo-bis(trimethylsilylmethyl) sulfide.13 and the release of disiloxane from bis(trimethylsilylmethyl) sulfoxide.l4The reaction of keto carbenoids with K-bonds which possess a lone pair of electrons is rapidly gaining prominence as an extremely efficient method for dipole generation.lsFrevious papers from these laboratories have outlined a