Analysis of a hydrodesulfurization process .2. The reactions of 2- and 3-methylthiophenes with tris(triethylphosphine)platinum(0)

The oxidative insertion reactions of 2-methylthiophene (2-MeT) and 3-methylthiopene (3-MeT) with Pt(PEt3)(3) (1), were examined as models for the first step in a platinum catalysed hydrodesulfurization. 2-MeT gave a single isomer, 1-bis(triethylphosphine)platina-2-thia-3-methylcyclohexa-3,5- diene (2), characterized spectroscopically. 3-MeT gave two isomers (ratio 1:1.6), 1-bis(triethylphosphine)platina-2-thia 5-methylcyclohexa-3,5-diene (3b), identified spectroscopically, and 1-bis(triethylphosphine)platina-2-thia-4 methylcyclohexa-3,5-diene (3a), identified spectroscopically and by an X-ray crystal structure determination. The thiophenes in the thiametallacycles can be displaced by free thiophenes and measurements show that the equilibrium constants decrease in the order [Pt(C,S-3-MeT)(PEt3)(2)] + T[K-eqm(2/0) = 2.30 (after 7 h)]; [Pt(C,S-3-MeT)(PEt3)(2)] + 2-MeT [K-eqm(3/2) = 2.20 (after 12 h)] and [Pt(C,S-2-MeT) (PEt3)(3)] + T [K-eqm(2/0) = 1.45 (after 20 h)]. The order of stability in the complexes is T > 2-MeT > 3-MeT. Equilibrium is attained more rapidly in the 3-MeT complexes than in those of 2-MeT. Details of the X-ray structure of the previously prepared complex 5, [Pt(PEt3)(2){C,S-C12H8S}], derived from dibenzothiophene, are also given and these two structures are compared with that for complex 4, [Pt(PEt3)(2){C,S-C8H6S}], derived from benzothiophene. Complexes 3a, 4 and 5 share the common feature of a six-membered thiaplatinacycle, which is planar in 3a, nearly so in 4, but very severely distorted in 5, however, many of the bond lengths and angles are similar. (C) 1997 Elsevier Science Ltd.