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(q3-SCH=CH-CH=CHz()2 ) and (triphos)Rh- { v3-S(C6&)CH=CH2} (3), respectively [triphos = MeC(CH2PPh&]. Compound 2 is selectively protonated at the terminal metal-bonded carbon atom (CZ) by HBF4.OEtz to give, after anion exchange, the v4-C,C,C,S-thiocrotonaldehyde complex anti-[(triphos)Rh{v4-SCHCHCH(CH3)}]BP(4b) , which reacts with CO to yield [(triphos)Rh(CO)- {T,~-S=CH-CH=CH(CH~)}](B5P) a~n d thermally isomerizes to syn-[(triphos)Rh{v4-SCHCHCH(CH3)}]BP(b6 ) in solution. Complex 4 also reacts with Me1 by selective delivery of Me+ to the sulfur atom to give, after anion exchange, [(triphos)Rh(y3-MeSCH=CH-CH=CHz)]BPb (7). On the other hand, Ph$+ selectively attacks the CZ carbon atom to yield [(triphos)Rh{v4-SCHCHCH(CH2CPh3)}]PF6(S ), 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, 2 = 4, and V = 6489.66 A3.T he cation [(triphos)Rh(q4-SCHCHCH(CHzCPh3)}]+p resents 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 CZ with HBF4.OEt2 to yield [(triphOS)Rh{v4-s(c61&)- CH(CH3)}]BPb (9), which undergoes an intramolecular hydrogen shift from carbon to sulfw slowly at room temperature and rapidly in refluxing THF to produce [(triphos)Rh( q3-HS(C&)CH=CH2}]BPb (10); complex (10) is deprotonated by t-BuOK to reform 3. As in the case of 2, Me1 and Ph3CPF6 react with 3 by selective attack of S and C, yielding [(tripho~)Rh(~~-MeS(C6b)CH==CH~}(]1B1P) h&4d [(triphos)Rh{v4-S(C61&)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(C0)2]Y (Y = BPb, PFs), liberating the thio ligands in solution. In this manner we have prepared the new organosulfur compounds 2-n-propenyl-4-methy1-41H,-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.