Homogeneous Reactions of Thiophenes with Transition Metals: A Modeling Approach for Elucidation of the Hydrodesulfurization Mechanism and an Effective Method for the Synthesis of Unusual Organosulfur Compounds

The fragment [(triphos)RhH], generated by thermolysis of (triohos)RhH₃(1) in refluxing THF, reacts with thiophene (T) or benzo[b]thiophene (BT) to yield (triphos)Rh(η³-SCH═CH—CH═CH₂) (2) and (triphos)Rh-{η³-S(C₆H₄)CH═CH₂} (3), respectively [triphos = MeC(CH₂PPh₂)₃]. Compound 2 is selectively protonated at the terminal metal-bonded carbon atom (C₂) by HBF₄•OEt₂to give, after anion exchange, the η⁴-C,C,C,S-thiocrotonaldehyde complex anti-[(triphos)Rh{η⁴-SCHCHCH(CH₃)}]BPh₄(4), which reacts with CO to yield [(triphos)Rh(CO)-{η²-S═CH-CH═CH(CH₃)}]BPh₄(5) and thermally isomerizes to syn-[(triphos)Rh{η⁴-SCHCHCH(CH₃)}]BPh₄(6) in solution. Complex 4 also reacts with MeI by selective delivery of Me⁺to the sulfur atom to give, after anion exchange, [(triphos)Rh(η³-MeSCH═CH—CH═CH₂)]BPh₄(7). On the other hand, Ph₃C⁺selectively attacks the C₂carbon atom to yield [(triphos)Rh{η⁴-SCHCHCH(CH₂CPh₃)}]PF₆(8), whose structure has been determined by X-ray diffraction. Complex 8 crystallizes in orthorhombic space group P2₁2₁2₁(no. 19) with a = 10.834(6) Å, b = 15.012(6) Å, c = 39.902(9) Å, Z = 4, and V = 6489.66 ų. The cation [(triphos)Rh{η⁴-SCHCHCH(CH₂CPh₃)}]⁺presents a distorted square pyramidal structure with one P atom occupying the apical position, while the remaining two P atoms plus the C6-S and the C7-C8 bonds occupy the basal sites; the C8 atom bears the trityl substituent. The vinylthiophenolate complex 3 is also selectively protonated at C₂with HBF₄•OEt₂to yield [(triphos)Rh{η⁴-S(C₆H₄)-CH(CH₃)}]BPh₄(9), which undergoes an intramolecular hydrogen shift from carbon to sulfur slowly at room temperature and rapidly in refluxing THF to produce [(triphos)Rh{η³-HS(C₆H₄)CH═CH₂}]BPh₄(10); complex (10) is deprotonated by t-BuOK to reform 3. As in the case of 2, MeI and Ph₃CPF₆react with 3 by selective attack of S and C, yielding [(triphos)Rh{η³-MeS(C₆H₄)CH═CH₂}]BPh₄(11) and [(triphos)Rh{η⁴-S(C₆H₄)CH(CH₂CPh₃)}]PF₆(12), respectively. All the rhodium complexes obtained by addition of electrophiles to 2 or 3 upon treatment with CO quantitatively transform into [(triphos)Rh(CO)₂]Y (Y = BPh₄, PF₆), liberating the thio ligands in solution. In this manner we have prepared the new organosulfur compounds 2-n-propenyl-4-methyl-4H-1,3-dithiine, 5,5,5-triphenyl-trans-2-pentenethial, 2-ethylidenecyclohexadienethione, and 2-(3,3,3-triphenylpropylidene)cyclohexadienethione and provided a convenient synthetic method for cis-1-(methylthio)butadiene, 2-vinylthiophenol, and o-(methylthio)styrene, which have been previously made by more complicated multistep syntheses. The chemistry herein described provides useful information on the fundamental aspects of hydrodesulfurization catalysis as well as a novel entry into the synthesis of organosulfur compounds.