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

The two aromatic S-enynes HCCCH­(OH)­(C6H4)­SCH2C­(R)CH2 (1a, R = Me; 1b, R = H) containing olefinic groups
with
and without an internal methyl substitutent and the O-enyne HCCCH­(OH)­CMe2CH2OCH2C­(Me)CH2 (1c) 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 1a,c, the presence of the methyl group promotes cyclization reactions
and their tandem cyclizations are further induced by MeOH. The reaction
of 1a in CH2Cl2 gives the three
products 2a–4a. Complex 2a, with a seven-membered thio ring bonded at Cβ of
the vinylidene ligand, is formed via a C–C bond formation between
two unsaturated groups in moderate yield. Complex 3a 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 4a 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 1a in MeOH generates the organic product 8a via 2a. In the reaction, the vinylidene complex 7a, a formal methanol addition product of 2a,
is also formed as a side product. Deprotonation of 7a gives the acetylide complex 9a. The reaction of 1c affords the vinylidene complex 2c in CH2Cl2 via a similar cyclization process with no other
side product. Deprotonation of 2c followed by allylation
gave the disubstituted vinylidene complex 10c. Tandem
cyclization of 1c in MeOH also gives the organic product 8c. In the reaction of [Ru]Cl with 1b containing
no methyl group in the olefinic part, no C–C bond formation
was observed. The reactions of [Ru]­NCCH3+ with 1a,b each gave only 4a,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 5a, 9a, and 10c have been confirmed by X-ray diffraction
analysis.