Alkynes reductions, sodium borohydride

Hydrogenation using Raney nickel is carried out under mild conditions and gives cis alkenes from internal alkynes in yields ranging from 50 to 100% [356, 357, 358, 359, 360]. Half hydrogenation of alkynes was also achieved over nickel prepared by reduction of nickel acetate with sodium borohydride (P-2 nickel, nickel boride) [349,361,362] or by reduction with sodium hydride [49], or by reduction of nickel bromide with potassium-graphite [363]. Other catalysts are palladium on charcoal [364], on barium sulfate [365, 366], on... 

Reduction of cyclic tellurium oxychloride 22 with stoichiometric amounts of sodium borohydride or excess sodium bisulfite yields the telluride 56, whereas treatment with a great excess of sodium borohydride quantitatively leads to di-/>-methoxyphenyl ditelluride and alkyne 57 (Scheme 6) <1999OM803>. 

Hydrogenation of alkynes to cis-alkenes. This reduction can be carried out with sodium borohydride and a catalytic amount of PdCI, which is more effective than Pd/C, in polyethylene glycol and CHiCh. This system is superior to PdCI, and H, in polyethylene glycol. 

Vinyl halides add to allylic amines in the presence of Ni(cod)2 where cod=l, 5-cyclooctodine, followed by reduction with sodium borohydride. Aryl iodides add to alkynes using a platinum complex in conjunction with a palladium catalyst. A palladium catalyst has been used alone for the same purpose, and the intramolecular addition of a arene to an aUcene was accomplished with a palladium or a GaCl3 catalyst, " AUcyl iodides add intramolecularly to aUcenes with a titanium catalyst, or to alkynes using indium metal and additives. The latter cyclization of aryl iodides to alkenes was accomplished with indium and iodine or with Sml2. " ... 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Borides of palladium and of several base metals, " prepared by reduction of metal salts with sodium borohydride, have been successfiiUy used for the selective hydrogenation of alkynes. 

Nickel boride (P — 2) generated from nickel acetate and NaBH4 in ethanol exhibits notable selectivity in reduction depending on the substitution pattern of the alkene " ". Sodium borohydride with Co(II) has also proved to be of general utility for the reduction of various alkenes and alkynes (equation... 

The total synthesis started with a Birch reduction of p-methoxytoluene (382) to obtain the dihydro compound 383, which was treated with p-toluenesulfonic acid to obtain acetal 384. CyclopropanatiOTi with ethyl diazoacetate and transaceta-lization led to compound 385, which reacted to the unsaturated keto ester 386 on treatment with base. In the next step, the keto ester 386 was methylated with methylmagnesium chloride, and it reacted selectively at the 2-positon to yield 387. Lactonization with further methylation with methyl iodide afforded homo-lactone 389, which reacted with lithium salt 390 to alkyne 391 and was reduced with sodium borohydride to diol 392. Partial reductiOTi of the triple bond to the double bond was obtained with sodium in ammonia and further treatment with acid led to hydrolysis of the acetal, which subsequently cychzed to 394 (Scheme 8.1). 

A heterogeneous catalyst that permits hydrogenation of an alkyne to an alkene is the nickel boride compound called P-2 catalyst. The P-2 catalyst can be prepared by the reduction of nickel acetate with sodium borohydride ... 

Li has reported that cationic gold(III) complexes also catalyze the intermolecular hydroamination of terminal alkynes with aniline derivatives [17]. For example, reaction of a neat 1 1.5 mixture of phenylacetylene and aniline catalyzed by AUCI3 at room temperature followed by reduction with sodium borohydride led to isolation... 

Triazoles are obtained via 1,3-dipolar cycloadditions between azides and al-kynes, and they are probably the most common nitrogen heterocycles prepared on solid supports via 1,3-dipolar cycloaddition. A wide variety of 1,2,3-triazoles have been prepared on solid supports, with an alkyne or azide attached to the resin. In addition, various linkers have been studied. Alkyne-functionalized alcohol was attached to the MeOPEG resin with an oxalyl chloride linkage (Scheme 11.23). Cycloaddition of alkynes with carbohydrate-derived azides gave resin-bound 1,2,3-triazoles, and reductive cleavage with sodium borohydride released the products as primary alcohols. 

Partial reduction of alkynes to Z-alkenes is another important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that is most successful is Lindlar s catalyst, which is a lead-modified palladium-CaCOa catalyst.A nickel-boride catalyst prepared by reduction of nickel salts with sodium borohydride is also useful.A homogeneous rhodium catalyst has also been reported to show good selectivity. 

The aldehyde was then converted to a carboxylic acid via a Pinnick oxidation. Further elaboration to generate acyl telluride 185 was achieved by initial activation of the acid as a mixed anhydride using isobutyl chloro-formate under basic conditions. Nucleophilic displacement with sodium phenyl telluride, generated in situ from borohydride reduction of diphenyl ditelluride, completed the transformation. With acyl telluride 185 in hand, nitrogen deprotection employing TFA, followed by a reductive amination with aldehyde 186 furnished alkyne 187, which was envisioned to be a substrate for radical cyclization (Scheme 19). 

The reaction of tosylhydrazones of a -unsaturated enones with sodium boro-hydride in alcoholic solution leads not to reduction, but instead to elimination according to Scheme 35, to provide a synthesis of allyl ethers in good yields. Potassium carbonate or sodium alkoxides can replace borohydride as the base in this sequence. trans-A y ethyl ethers (74) may be synthesized stereoselectively by the addition of aluminium hydrides to 1-alkynes and subsequent reaction of the vinyl alane formed (Scheme 36). This route complements the same authors ... 

---