Rhodium desulfurization

The synthesis of thiiranes with subsequent elimination of sulfur is an important procedure for the creation of C=C bonds, especially for sterically crowded systems (47,48), in analogy to the Eschenmoser-sulfide-contraction reaction (116). The spontaneous elimination of sulfur was observed in the rhodium-catalyzed reaction of diazo compound 62, which gave rise to the formation of cyclopentenone derivative 63 (117) (Scheme 5.24). A synthesis of indolizomycin was published by Danishefsky and co-workers (118) and involved a similar annulation step. In this case, however, the desulfurization reaction was achieved by treatment with Raney Ni. 

In the total synthesis of indolizomycin700, one of the key steps involved the cyclization of a thiolactam with an intramolecular a-di azoketone moiety, catalyzed by rhodium acetate (equation 195). The molecule is desulfurized by treatment with Raney nickel, giving a good yield of the required target. 

Removal of sulfur from these complexes remains challenging. Catalysts such as [M(COD)(PPh3)2]+ (M = Rh, Ir) only hydrogenate the C=C bond of benzothio-phene, whereas the hydrogenation with rhodium triphos complexes gives 2-ethyl-thiophenol.148 The addition of Co4(CO)i2 to (21-XXVIII, ML, = Cp Ir), however, leads to partial desulfurization, presumably with formation of cobalt sulfide.149... 

While metal complexes capable of desulfurizing or hydrodesulfurizing thiophenes are relatively numerous, homogeneous catalysts are very rare, being limited to rhodium and iridium TRIPHOS precursors that are unique in tolerating the harsh experimental conditions required for the second C-S bond cleavage of thiophenes [2 b]. 

Rhodium (I) catalyzed desulfurization reactions of a variety of thiiranes. These reactions give the corresponding alkenes using [Rh(CO)2Cl]2 (2 mol%) at room temperature under 28 bar of carbon monoxide (Scheme 36 and Table 34) <86TL3573>. 

Thioamides of two cyclopropanecarboxylic acids undergo coupling reactions with diazomethane derivatives in refluxing benzene containing rhodium(II) acetate. The reaction conceivably takes place via a thiirane intermediate which undergoes reductive elimination by reaction with excess diazo compound. An example of an intramolecular reaction is the formation of indolizine 2. If the diazo compound is not in excess a thiol is formed, this can be removed by Raney nickel desulfurization. ... 

More recent work on the hydrogenation of thiophenic molecules catalyzed by water-soluble metal complexes is pursuing the use of polyphosphine ligands (cf. Section 3.2.2). These studies follow the success obtained with the tridentate phosphine MeC (CH2PPh2)3 (TRIPHOS), which forms rhodium and iridium catalysts for the hydrogenation, hydrogenolysis, and desulfurization of various thiophenic... 

The alkaloid ( + )-retronecine (883, Scheme 129) is structurally similar to ( + )-heliotridine (850), with the exception that the stereocenter at C-7 is of opposite configuration. The basic approach to its synthesis involves a carbenoid displacement similar to that in the previous scheme. The acetyl protecting group of the common intermediate 875 (R=SPh) is changed to a TBS group, and the benzoate is converted to pivalate. Carbenoid displacement with dibenzyl a-diazomalonate in the presence of rhodium acetate gives 879. Reductive desulfurization... 

The aqueous biphasic hydrogenolysis of BT has been accomplished in either water/ n-decalin or water/naphtha mixtures with the rhodium(I) precursor [Rh(COD)-(SULPHOS)] (COD = cycloocta-1,5-diene) [17]. Rather harsh reaction conditions (160 °C, 3 MPa H2) and an equivalent amount of NaOH were required for high conversions of BT to 2-ethylthiophenolate. In these conditions, the thiolate product was totally recovered in the aqueous phase, leaving the hydrocarbon phase formally desulfurized (Scheme 5). 

HDS catalysts generally consist of (heterogeneous) Mo or W sulfides on alumina supports. However, Bianchini et al. described a two-step procedure for HDS of thiophenes by the hydrogenolysis of thiols, followed by the desulfurization of the thiols by applying their zwitterionic rhodium(I) complex, [Rh(sulphos((cod)] (see previous section) [17]. This complex is soluble in polar solvents, such as methanol and methanol-water mixtures, but not in hydrocarbons. Benzo[b]thiophene was chosen as substrate since it is one of the most difficult thiophene derivatives to degrade. Under the mild reaction conditions of the two-step process, the benzene rings of the (di)benzothiophenes were not affected. In the absence of a base, the double bond of benzo[b]thiophene was hydrogenated, while in the presence of a base (NaOH) 2-ethylthiophenolate was the major product (Scheme 1). 

The bulky phosphine ligands were synthesized via [2 + 2 + 2] cycloaddition of diynes with 1-alkynylphosphine sulfides using the cationic rhodium(I)/BINAP catalyst followed by desulfurization (Scheme 4.44) [47]. 

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