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)Rh], generated by thermolysis of (triphos)RhH3 (1) in refluxing THF, reacts with thiophene (T) or benzo[b]thiophene ( BT) to yield (triphos)Rh(eta(3)-SCH=CH-CH=CH2) (2) and (triphos)Rh{eta (3)-S(C6H4)CH=CH2} (3), respectively [triphos = MeC(CH(2)PPh(2))(3)]. Compound 2 is selectively protonated at the terminal metal-bonded carb on atom (C-2) by HBF4 . OEt(2) to give, after anion exchange, the eta( 4)-C,C,C,S-thiocrotonaldehyde complex anti-[(triphos)Rh{eta(4)-SCHCHCH (CH3)}]BPh(4) (4), which reacts with CO to yield [(triphos)Rh(CO){eta( 2)-S=CH-CH=CH(CH3)}]BPh(4) (5) and thermally isomerizes to syn-[(triph os)Rh{eta(4)-SCHCHCH(CH3)}]BPh(4) (6) in solution. Complex 4 also reac ts with MeI by selective delivery of Me(+) to the sulfur atom to give, after anion exchange, [(triphos)Rh(eta(3)-MeSCH=CH-CH=CH2)]BPh(4) (7) . On the other hand, Ph(3)C(+) selectively attacks the C-2 carbon atom to yield [(triphos)Rh{eta(4)-SCHCHCH(CH(2)CPh(3))}]PF6 (8), whose str ucture has been determined by X-ray diffraction. Complex 8 crystallize s in orthorhombic space group P2(1)2(1)2(1) (no. 19) With a=10.834(6) Angstrom, b=15.012(6) Angstrom, c=39.902(9) Angstrom, Z=4, and V=6489. 66 Angstrom(3). The cation [(triphos)Rh{eta(4)-SCHCHCH(CH(2)CPh(3))}]( +) presents a distorted square pyramidal structure with one P atom occ upying the apical position, while the remaining two P atoms plus the C 6-S and the C7-C8 bonds occupy the basal sites; the C8 atom bears the trityl substituent. The vinylthiophenolate complex 3 is also selective ly protonated at C-2 with HBF4 . OEt(2) to yield [(triphos)Rh{eta(4)-S (C6H4)CH(CH3)}]BPh(4) (9), which undergoes an intramolecular hydrogen shift from carbon to sulfur slowly at room temperature and rapidly in refluxing THF to produce [(triphos)Rh{eta(3)-HS(C6H4)CH=CH2}]BPh(4) (1 0); complex (10) is deprotonated by t-BuOK to reform 3. As in the case of 2, MeI and Ph(3)CPF(6) react with 3 by selective attack of S and C , yielding [(triphos)Rh{eta(3)-MeS(C6H4)CH=CH2}]BPh(4) (11) and [(trip hos)Rh{eta(4)-S(C6H4)CH(CH(2)CPh(3))}]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=BPh(4), PF6), liberating the thio ligands in solution. In this man ner we have prepared the new organosulfur compounds 2-n-propenyl-4-met hyl-4H-1,3-dithiine, 5,5,5-triphenyl-trans-2-pentenethial, 2-ethyliden ecyclohexadienethione, and 2-(3,3,3-triphenylpropylidene)cyclohexadien ethione and provided a convenient synthetic method for cis-1-(methylth io)butadiene, 2-vinylthiophenol, and o-(methylthio)styrene, which have been previously made by more complicated multistep syntheses. The che mistry herein described provides useful information on the fundamental aspects of hydrodesulfurization catalysis as well as a novel entry in to the synthesis of organosulfur compounds.