Nickel complexes desulfurizations

Thiocarbonyl derivatives of 1,3-dioxolanes and 1,3-oxathiolanes are readily isomerized to the 2-carbonyl compounds as shown in Scheme 20. Alkylation of the sulfur atom with alkyl halides usually leads to ring-opened products (Scheme 21) (69JOC3011). Most of the other chemistry of the sulfur derivatives has focused on desulfurization and subsequent generation of alkenes. The reaction is shown in equation (20) and proceeds with cis elimination via carbene intermediate (see Section 4.30.2.2.5) and is usually carried out with a phosphine (73JA7161) or a zero-valent nickel complex (73TL2667). 

In sulfur extrusion reactions mediated by Raney nickel or nickel complexes (90MI1) or in reactions carried out under photolytic conditions (69JOC1233), biradical intermediates such as 44 and 47 are presumed to be present (Scheme 9). However, there is evidence that lithium-assisted desulfurization of phenothiazines 49 proceeds via open-chain intermediates 50 (58JA380) (Scheme 10). 

Even as stoichiometric reagents, such nickel complexes as bipy-(COD)Ni have already demonstrated their value in desulfurizing and ring-contracting processes, in hydrodesulfurization, in selective deoxygenation of epoxides, and in the controlled cleavage of allyl ethers. Their ability to form metallocyclopentadienes from biphenylenes or from thiophenes can develop into a valuable alternative route to such reactive metal heterocycles. 

Radius and co-workers also described the first example of C-S bond cleavage of alkyl- and aryl-sulfoxides induced by the same complex [(IiPr)4Ni2-(p-COD)]. For example, the trans-addition was obtained from DMSO as substrate in toluene (76% yield). The chemical behavior of such compounds is of interest because they have important roles in bioinorganic chemistry, and nickel complexes are common desulfurization agents. ... 

Desulfuration.1 This complex as such or in combination with 2,2 -bipyridyl (bpy) or triphenylphosphine (a NiCRAL) can effect desulfuration of heteroarenes. aryl thioethers, dithioketals, sulfoxides, or sulfones in DME or THF at 63° in 1.5-30 hours. NiCRA is sufficient for aryl thioethers, dithioketals, but NiCRALs are more efficient for desulfuration of heteroarenes. Yields can be comparable with those obtained with Raney nickel. 

The reaction sequence outlined in Scheme 12 illustrates how macrocyclic polyether-thiono diesters such as RR)-lfi6 can be prepared (184) from 0,0-dimethyl 2,6-pyridinedicaibothiolate and (RR)-S4, Potassium thiocyanate forms a 1 1 crystalline complex with (RR)-1S6 and presumably the potassium ion serves as a template for the (1 -I-1) cyclization. Raney nickel desulfurization of (/ R)-186 yields the chiral pyridino-18-crown-6 derivative RR)-191. 

Other reactive forms of nickel including nickel boride and nickel alkoxide complexes can also be used for desulfurization. Tri-w-butyltin hydride is an alternative reagent for desulfurization.204... 

Only a limited number of examples are known of applications of thietanes in organic synthesis. Prominent among these examples would be electrophilic ring opening reactions leading to polyfunctional sulfur compounds (33)-(37), utilization of 3-thietanones (55) and metal complexes (87) derived therefrom as oxyallyl zwitterion equivalents in cycloaddition reactions, synthesis of dipeptide (63) with a /3-thiolactone, Raney nickel desulfurization of thietanes (e.g. 120 cf. Table 7) as a route to gem-dimethyl compounds, and desulfurization of thietanes (e.g. 17) in the synthesis of cyclopropanes (also see Table 7). 

Both of the above approaches employed a metal ion as a template about which the corrin cyclization was performed, but the nickel or cobalt ions could not subsequently be removed. In order to obtain metal-free corrins, a new route was therefore devised (67AG865) which employed the novel principle of sulfide contraction (Scheme 22). Thus the sodium salt of the precorrin (284) (Scheme 23) was transformed into the thiolactam (285), and loose complexation with zinc(II) ions caused cyclization to give (286), which was treated with benzoyl peroxide and acid to give the ring-expanded compound (287). Contraction with TFA/DMF gave the corrins (288) and (289), and the major of these (289) was desulfurized with triphenylphosphine and acid to give (288). Finally, demetallation with TFA gave the required metal-free corrin (290), a source for a whole variety of metal derivatives. 

Thiane oxides have been shown to be reduced cleanly back to the thiane with phosphorus pentasulfide under conditions to which sulfones, sulfinates, ketones, esters and amides are inert (78CJC1423) the potential of this reaction, though not yet applied, is obviously considerable, especially when coupled with the old-established Raney nickel desulfurization technique. Thiane itself is desulfurized to pentane (with traces of cyclopentane) but the opportunity to construct alkyl chains of great complexity regio- and stereo-specifically is there. At the very least, the reduction to tetrahydrothiopyrans presents a very useful entree into a wide range of 2-substituted thianes (Scheme 2). 

The thiophene can be considered a four-carbon synthon via reductive desulfurization with Raney nickel. A strategy using the thiophene as a lynch-pin for the construction of complex building blocks was utilized in a total synthesis of the cytotoxic sponge metabolite haliclamine A <02JOC6474>. Thienyl cyclopropyl ketone (108) was elaborated into 109. Reduction of 109 with Raney Ni gave 110 which was taken on to haliclamine A. 

Tetrachloro-l, 3-dithietane forms a bis adduct with mercury (I) nitrate,whose structure was investigated by IR and Raman spectra. A cyclo-pentadienyl manganese(0) dicarbonyl complex of the dimer of 2,3-diphenylcyclo propenethione has been reported. Raney nickel desulfurizes the tetraester 538 and the desulfurization of the questionable dithietanes obtained by dimerization of thiofluorenone and thiobenzophenone give, respectively, bis-fluorenylidene and tetraphenylethylene. The electrochemical oxidation of 1,3-dithietane has been investigated. 2,2 -[Oxybis(methylene)]bis-l,3-dithietane is inert to boron trifluoride etherate after 30 days at room temperature. ... 

Hernandez-Maldonado, A.J. and Yang, R.T. Desulfurization of diesel fuels via pi-complexation with nickel(II)-exchanged X- and Y-zeolites. Industrial Engineering Chemistry Research, 2004, 43 (4), 1081. 

They then turned to the less hindered reagent trimethyloxonium fluoroborate in nitromethane and obtained the desired salt (3). The product is rearranged to (4) on treatment with n-butyllithium, and reduction with sodium in liquid ammonia gives the squalenelike hydrocarbon (5). Desulfurization with Raney nickel gives complex mixtures in the case of allylic sulfides. 

The presence of a chiral sulfoxide group in the P-position allows the highly selective reduction of imines by Li 5-BU3BH or LiBEt3H [482, 1120] (Figure 6.20). Reduction of P-sulfonylimines with DIBAH/ZnCl2 at -78°C is also very efficient, and produces nonracemic amines after Raney nickel desulfurization [1121, 1122] (Figure 6.20). A chelated complex that is similar to the one involved in the reduction of p-ketosulfoxides ( 6.1.2.2) is proposed as a reaction intermediate. 

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