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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 (triphos)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·OEt2 to 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 C2 carbon atom to yield [(triphos)Rh{η4-SCHCHCH(CH2CPh3)}]PF 6 (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) A?, b = 15.012-(6) A?, c = 39.902(9) A?, Z = 4, and V = 6489.66 A?3. 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 C2 with HBF4·OEt2 to yield [(triphos)Rh{η4-S(C6H4)-CH(CH 3)}]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=CH 2}]BPh4 (10); complex (10) is deprotonated by t-BuOK to reform 3. As in the case of 2, MeI and Ph3CPF6 react with 3 by selective attack of S and C, yielding [(triphos)Rh{η3-MeS(C6H4)CH=CH 2}]BPh4 (11) and [(triphos)Rh{η4-S(C6H4)CH(CH 2CPh3)}]-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.