Dithiol catalysts

Acetylenes are reduced to cis-olefins by NaBHi, and Fe(II)/lipoamide and Fe(II)/dithiol catalysts in ethanol, and further to alkanes by NaBH, /-polyethyleneglycol/PdCl2 Allylic tosylates are selectively desulfonylated with LiBHEt3 in the presence of PdCl2(dppp) without isomerisation. RhCl(PPh3)3 changes the direction of the hydroboration of hexenone from C=0 to C C addition (Scheme 4). ... 

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Selectivity in formation of protective groups may also be achieved by a proper choice of reaction conditions and catalyst. Thus formation of the 3-monothioketal from 3,6-diketones is achieved by dilution of the ethane-dithiol-boron trifluoride reaction mixture with acetic acid. 3-Monocyanohydrins are obtained in good yield from 3,20-diketo-(5a)-pregnanes by diluting the exchange reaction with ethanol. Similarly, dilution of the... 

Thioketals are readily prepared by reaction of saturated 3-ketones with thiols or dithiols in the presence of boron trifluoride or hydrogen chloride catalysts. Selective protection of the 3-ketone in the presence of a 6-ketone is possible by carrying out the reaction in diluted medium. Similarly, 3-ketones react selectively with monothiols " " or with bulky dithiols in the presence of 6-, 7-, 11- and 12-ketones. 

Magnesium triflate and zinc triflate are outstanding catalysts for the intro duction of the thioketal group for the protection of the ketone function [126] The reaction of a variety of ketones with ethane 1,2-dithiol in the presence of these triflates proceeds under mild conditions to form the corresponding thioketals in high yield (equation 62)... 

For the use of diacetyl dithiol as a coagulant, catalyst and precipitant for sulphur and Group 11 sulphides, see Ref. 2. 

Fig. 10 Diiron complex having a 3,6-dichlorobenzene- 1,2-dithiolate ligand as a catalyst for the hydrogen generation...

Thiolate-Pt11 complexes, in particular dinuclear thiolate and dithiolate bridged complexes, have been prepared and characterized a catalyst precursor in hydroformylation reactions for use as.127,128... 

The palladium-catalyzed formation of sulfides can generate polyphenylene sulfide from a dithiol and a dibromoarene, or from 4-bromobenzenethiol (Equation (38)).17 In 1984 Asahi Glass obtained patents for the formation of this polymer in the presence of palladium and nickel catalysts.125,126 In addition, Gingras reported palladium-catalyzed couplings of aryl halides and thiols to form discrete phenylene sulfide oligomers.127,128 A number of polyphenylene sulfide wires, ranging from dimeric to pentameric structures, were prepared by the palladium coupling, albeit in modest yields ... 

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

Later, polythioesters were prepared by the polycondensation of dicarboxylic acid diesters with a dithiol [43] (Fig. 12). Initially, various lipases were screened in the polycondensation between diethyl adipate and hexane-1,6-dithiol at 120 °C in the presence of molecular sieves. The use of 70wt% immobilized CALB as catalyst proved to be efficient in these polycondensation reactions with dicarboxylic acid... 

It has been suggested that the use of zinc dithiolate complexes as vulcanization catalysts may be closely related to the ability of these complexes to form sulfur-rich complexes that have very labile disulfide sulfur atoms (230). 

A polymer-supported lipoamide-ferrous chelate system was used as catalyst for the reduction of diphenylacetylene to cis-stilbene with sodium borohydride the dithiol-iron(II) (1 1) complex formed was suggested to be the active species. The chitosanlipoamide system has the highest activity among various insoluble polymers investigated 95,96). 

A variety of enantiopure or enantiomerically enriched phosphines, diphosphines, phosphites, diphosphites, phosphinephosphites, thiols, dithiols, P,N-ligands, and P,S-ligands have been developed as chiral modifiers of rhodium and platinum catalysts [1-7], Representative chiral ligands discussed in this chapter are shown in Figure 7.1. 

To a freshly prepared catalyst (1 g, 0.2 mmol of iodine) under stilling, a mixture of benzaldehyde la (2 mmol) and ethane-1,2-dithiol (2.2 mmol) was added and stirring continued for 10 min or till the reaction was complete. For TLC monitoring, a small amount of the solid reaction mixture was taken out with a spatula and washed with a little amount of ethyl acetate to get a solution. On completion the reaction mixture was loaded on a short column of silica gel (60 to 120 mesh) and eluted with ethyl acetate. The organic layer was washed with a dilute solution of sodium thiosulfate followed by water and dried over anhydrous sodium sulfate. Evaporation of the solvent under reduced pressure and purification of the residue by column chromatography yielded the pure product. 

Thioacetals and -ketals. A1C13 is an effective catalyst for the condensation of aldehydes and ketones with thiols or dithiols. The reaction of thiols with carbonyl groups with an a-proton results in low yields but the reaction with dithiols proceeds in high yield.8... 

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