Tandem Cyclization of Enynes Containing a Thioether or Ether Linkage via Ruthenium Allenylidene and Vinylidene Complexes

Abstract

The two aromatic S-enynes HCCCH­(OH)­(C₆H₄)­SCH₂C­(R)CH₂ (<b>1a</b>, R = Me; <b>1b</b>, R = H) containing olefinic groups with and without an internal methyl substitutent and the O-enyne HCCCH­(OH)­CMe₂CH₂OCH₂C­(Me)CH₂ (<b>1c</b>) also with an internal methyl substituent on the olefinic group but with no aromatic group have been prepared. In the [Ru]­Cl-induced ([Ru] = Cp­(PPh)₃Ru) reactions of <b>1a</b>,<b>c</b>, the presence of the methyl group promotes cyclization reactions and their tandem cyclizations are further induced by MeOH. The reaction of <b>1a</b> in CH₂Cl₂ gives the three products <b>2a</b>–<b>4a</b>. Complex <b>2a</b>, with a seven-membered thio ring bonded at C_β of the vinylidene ligand, is formed via a C–C bond formation between two unsaturated groups in moderate yield. Complex <b>3a</b> is formed via migration of PPh₃ from the metal onto the terminal carbon of the alkynyl group followed by coordination of the S atom. The carbene complex <b>4a</b> is formed by S addition to the internal carbon of the alkynyl group accompanied by migration of the allylic group from sulfur to the newly formed thiophene ring. Tandem cyclization of <b>1a</b> in MeOH generates the organic product <b>8a</b> via <b>2a</b>. In the reaction, the vinylidene complex <b>7a</b>, a formal methanol addition product of <b>2a</b>, is also formed as a side product. Deprotonation of <b>7a</b> gives the acetylide complex <b>9a</b>. The reaction of <b>1c</b> affords the vinylidene complex <b>2c</b> in CH₂Cl₂ via a similar cyclization process with no other side product. Deprotonation of <b>2c</b> followed by allylation gave the disubstituted vinylidene complex <b>10c</b>. Tandem cyclization of <b>1c</b> in MeOH also gives the organic product <b>8c</b>. In the reaction of [Ru]Cl with <b>1b</b> containing no methyl group in the olefinic part, no C–C bond formation was observed. The reactions of [Ru]­NCCH₃⁺ with <b>1a</b>,<b>b</b> each gave only <b>4a</b>,<b>b</b>, respectively, with no side product. All of these reaction products are characterized by spectroscopic methods as well as elemental analysis. In addition, the structures of three complexes <b>5a</b>, <b>9a</b>, and <b>10c</b> have been confirmed by X-ray diffraction analysis