Hydrides desulfurizations

The preparation of enamines by reduction of aromatic heterocyclic bases and their quaternary salts or of lactams is not the most useful approach (97). The lithium aluminum hydride reduction of N-acyl enamines has been used with both fruitful and unsuccessful results. A series of 3-N-acetyl -d -cholestenes (104) has been prepared by desulfurization of the appropriate thiazolidine (105) (98,99). Lithium aluminum hydride reduction of the... 

While enamines can usually be obtained directly from ketones and secondary amines their formation by an indirect route may bo advantageous. The previously mentioned condensation of rnethyl ketones during azeotropic enamine formation has prompted the alklyation (J) or acylation and reduction (59) of Schiff s bases. A parallel method uses the formation and desulfurization of N-acylthiazolines followed by hydride reduetion (60,61). 

Corey and Chaykovsky had discovered that dimethyl sulfoxide is converted to methyl-sulfmyl carbanion upon treatment with sodium hydride " and that this conjugate base of DMSO reacts with various electrophiles This finding has opened up various reactions with a-sulfinyl carbanions derived from sulfoxides, since the sulfinyl function can be removed either by thermolysis or by subjecting the compound to reductive desulfurization. Thus a-sulfinyl carbanions have become versatile synthetically useful reagents. 

Other reactive forms of nickel including nickel boride283 and nickel alkoxide complexes284 can also be used for desulfurization. Tri-n-butyltin hydride is an alternative reagent for desulfurization.285... 

A novel route to synthesize l,3 -triazine-2,4(l//,3//)-diones through the desulfurization of thiocarboamides, such as 1,3-disubstituted 2-thioureas, trisubstituted thioureas and N-substituted thioamides by silver cyanate has been reported <00H(53)929>. Treatment of urazole 23 with one equivalent of sodium hydride under anhydrous conditions, followed by addition of dimethyl sulfate, leads to l,3,5-triazine-2,4-dione 24 in 80% yield . 

The carbene complexes generated by desulfurization of thioacetals with the titanoce-ne(II) species react with internal alkynes to produce the conjugated dienes 79 with high stereoselectivity (Scheme 14.34) [77]. The process appears to involve syn-elimination of P-hydride from the alkyl substituent that originates from the carbene complex after the formation of titanacyclobutene 80. 

The recent discovery that acetylated glycopyranosyl bromides may be reduced in high yields with lithium aluminum hydride to the corresponding 1,5-anhydroglycitols661 represents an improvement over the reductive desulfurization process and will doubtless make this class of compound even more readily available in the future. 

Prins64 has recently discovered that lithium aluminum hydride hydrogenates 1,2-epoxides smoothly in high yield this single-step reaction would appear to outmode the Raney nickel desulfurization process for this purpose. 

Aliphatic and aromatic sulfides undergo desulfurization with Raney nickel [673], with nickel boride [673], with lithium aluminum hydride in the presence of cupric chloride [675], with titanium dichloride [676], and with triethyl phosphite [677]. In saccharides benzylthioethers were not desulfurized but reduced to toluene and mercaptodeoxysugars using sodium in liquid ammonia [678]. This reduction has general application and replaces catalytic hydrogenolysis, which cannot be used [637]. 

Disulfides can be either reduced to two thiols or desulfurized. The former reaction was achieved in high yields using lithium aluminium hydride [680, 681], lithium triethylborohydride [100] and sodium borohydride [682]. 

Thioamides were converted to aldehydes by cautious desulfurization with Raney nickel [1137, 1138] or by treatment with iron and acetic acid [172]. More intensive desulfurization with Raney nickel [1139], electroreduction [172], and reduction with lithium aluminum hydride [1138], with sodium borohydride [1140] or with sodium cyanoborohydride [1140] gave amines in good to excellent yields. 

Desulfurization of 1,3-dithiolanes.9 Selective hydrogcnolysis of dithioketals and dilhioacetals is possible with 4 equivalent of this hydride in the presence of A1BN. Yields are 75-95%. The other products are ethane and (Bu3Sn)2S. 

DESULFURATION Molybdenum carbonyl. Sodium tricthylborohydride-Iron(II) chloride. Tri-n-Butyltin hydride. 

Many of the intermediate metallathiabenzenes react with hydrogen gas at low temperatures to yield the fully saturated desulfurized hydrocarbon is high yield (4, 89, 98). Another important feature exhibited by these compounds is that many of them will add dihydrogen to the metal center and produce a stable hydride-containing product. This stable product can be destabilized by the addition of an external neutral ligand that forces the hydride to transfer to the carbon bonded to the metal, thus breaking the... 

Nickel boride (prepared by adding NaBH4 to nickel chloride in methanol - THF) is an efficient, non-pyrophoric reagent for the desulfurization of benzo[ Jthiophene and dibenzothiophene (90CC819, 92JOC1986). The reaction proceeds under very mild conditions and is probably mediated by nickel hydride. 

Nicolaou and his group [139, 354,355] used this nucleophilic addition to thionolactones as a route to cyclic ethers, via an efficient desulfurization step with tin hydrides (Ph3SnH or n-Bu3SnH). ( )-Lauthisan and a brevetoxin A fragment were prepared according to this scheme. 

Desulfonylation Lithium-Ammonia, 158 Lithium triethylborohydride, 168 Magnesium-Methanol, 170 Sodium dithionite, 281 Sodium naphthalenide, 294 Desulfurization Lithium aluminum hydride-Bis(cyclopentadienyl)nickel, 158 Lithium l-(dimethylamino)naph-thalenide, 165 Lithium-Ethylamine, 158 Dethioacetalization (see Hydrolysis of thioacetals and -ketals)... 

A tandem 1,4-addition-Meerwein-Ponndorf-Verley (MPV) reduction allows the reduction of a, /i-unsaturated ketones with excellent ee and in good yield using a camphor-based thiol as reductant.274 The 1,4-addition is reversible and the high ee stems from the subsequent 1,7-hydride shift the overall process is thus one of dynamic kinetic resolution. A crossover experiment demonstrated that the shift is intramolecular. Subsequent reductive desulfurization yielded fiilly saturated compounds in an impressive overall asymmetric reductive technique with apparently wide general applicability. 

The exocyclic C(2)-S bond of a 2-thiopenem 50 can undergo radical reactions. As depicted in Scheme 28, its desulfurative stannylation reaction with azobis(isobutyronitrile) (AIBN) and tributyltin hydride leads to a penem stannane 51 <1995TL775>. The latter allows subsequent palladium(0)-mediated cross-coupling reactions with... 

---