Formation Mechanism of Furfuryl Sulfides from O-Furfuryl Dithiocarbonates: Density Functional Theory Study for Aromatic [3,3]-Sigmatropic Rearrangement

Abstract

Recently, we proposed a new reaction mechanism for the formation of allylic trithiocarbonate from sodium O-(2-alkenyl) dithiocarbonate, 1) in which a suspension of sodium hydride in benzene, containing 2-alkenol and an excess of carbon disulfide is heated to obtain the corresponding di(2-alkenyl)trithiocarbonates in good yields.2,3) The trithiocarbonates are convenient precursors of the thermodynamically stable allylic thiols (Chart 1).Based on data obtained from density functional theory (DFT) calculations at the B3LYP/6-31G(d) level, we demonstrated that sodium O-(3-phenylallyl) dithiocarbonate undergoes [3,3]-sigmatropic rearrangement to form sodium S-(1-phenylallyl) dithiocarbonate, which then allylically isomerizes to the more thermodynamically stable sodium S-(3-phenylallyl) dithiocarbonate.Furfuryl and benzyl alcohols were subjected to xanthation in dimethyl sulfoxide (DMSO) containing excess CS 2 in the presence of NaOH followed by alkylation to give the mixtures of trithiocarbonates. 3)On the other hand, it was observed that heating of O-furfuryl S-alkyl dithiocarbonates (xanthates) (1) afforded furfuryl alkyl sulfides (3) together with S-furfuryl S-alkyl dithiocarbonates (2). It was initially assumed that the sulfide (3) had formed via an intramolecular SNi-type fragmentation reaction mechanisms (Chart 2). 4)These considerations prompted us to reinvestigate the key step of the conversion of the furfuryloxy (ArCH 2 O-) moiety to the furfurylthio (ArCH 2 S-) moiety in the formation of the trithiocarbonate from sodium O-furfuryl dithiocarbonates as well as in the formation of S-furfuryl S-alkyl dithiocarbonates (2) and furfuryl alkyl sulfides (3) from O-furfuryl Salkyl xanthates (1) with the assumption that aromatic [3,3]-sigmatropic rearrangement was operative. This paper presents a detailed discussion of the reaction mechanisms in the formation of O-furfuryl dithiocarbonates on the basis of the DFT calculations 5) at the B3LYP/6-31G(d) and B3LYP/6-31Gϩ(d) levels. Results and DiscussionRearrangement of O-Furfuryl Dithiocarbonate Anion (1-Anion) On the basis of the reaction behavior of the Oallyl dithiocarbonate anion, 1) two possible reaction pathways were proposed for the formation of the S-furfuryl dithiocar- ; 4-22-1 Ikeda, Kumamoto 869-1404, Japan: and b Liberal Arts Education Center, Aso Campus, Tokai University; 5435 Kawayo, Minami-Aso, Kumamoto 869-1404, Japan. Received October 27, 2010 accepted February 28, 2011; published online March 7, 2011 Density functional theory (DFT) calculations at the B3LYP/6-31G(d) and B3LYP/6-31G؉(d) levels demonstrated that O-furfuryl S-alkyl dithiocarbonate (1) undergoes aromatic [3,3]-sigmatropic rearrangement to the energetically unfavorable S-(2-methylene-2,3-dihydrofuran-3-yl) S-alkyl dithiocarbonate (2), which then rearranges to furfuryl alkyl sulfide (3) with COS extrusion to regain the aromaticity lost in the first step. Formation Mechanism of Furfuryl Sulfides from O-Furfuryl