Triethylsilane-Palladium

Alkyl iodides, benzyl chlorides, benzyl bromides, and adamantyl bromides and iodides undergo reduction with triethylsilane/palladium chloride.195 The reduction of a /3-chloro ether occurs in excellent yield with this system (Eq. 56).195... 

Triethylsilane [1, 1218, before references]. Pettit et at. report a case where cleavage of a carbobenzoxy group with triethylsilane—palladium chloride in the presence of triethylamine proceeded in 70% yield. Hydrogenolysis was effected in somewhat lower yield (60%), and the yield obtained by solvolysis with CF3C02H was only... 

Vinyl and Aryl Halides and Triflates. The organosilane reduction of aryl halides is possible in high yields with triethylsilane and palladium chloride.195 The reaction is equally successful with aryl chlorides, bromides, and iodides. Aryl bromides and iodides, but not chlorides, are reduced with PMHS/Pd(PPh3)4 in moderate to excellent yields.199 This system also reduces vinyl bromides.199 p-Chlorobenzophenone is reduced to benzophenone with yym-tetramethyldisilo-xane and Ni/C in excellent yield (Eq. 59).200 There is a report of the organosilane reduction of aryl and vinyl triflates in very high yields with the combination of Et3SiH/Pd(OAc)2/dppp (l,3-bis(diphenylphosphino)propane) (Eq. 60).201... 

Few other examples of such reaction sequences have been described to date. Oh has reported the palladium-catalyzed reductive cyclizations of 1,6-enynes in the presence of formic acid or triethylsilane via an alkylpalladium intermediate and its application to organic synthesis. Palladium complexes also catalyze the conversion of a range of enynes to cyclic 6,7-unsaturated carboxylic acids in the presence of CO.260... 

A variation of the Rosenmund reduction is heating of an acyl chloride at 50° with an equivalent of triethylsilane in the presence of 10% palladium on charcoal. Yields of aldehydes obtained by this method ranged from 45% to 75% [80]. 

A highly efficient reduction of Fmoc-a-amino 5-ethyl or 5-benzyl 7 thioesters to aldehydes with triethylsilane and a catalytic amount of palladium on charcoal has been described 14,22 (Scheme 6). 

A palladium-catalysed reduction of ethyl thiolesters to aldehydes with triethylsilane has been proposed by Fukuyama [2611. The reaction, coupled with a mild conversion of carboxylic acids to thiolesters (see [261], footnote 2), affords a versatile method for the overall transformation of... 

Aryl halides can be dehalogenated with triethylsilane in the presence of a palladium catalyst. The method is versatile and can also be used for the reduction of acyl chlorides to aldehydes, or benzylicbromides to the corresponding hydrocarbons. If different types of halides are present in the molecule, selective dehalogenation takes place. Thus, an aryl iodide can be reduced in the presence of a chloride, and benzylic bromide is reduced more easily than an aryl bromide. Finally, the method is even able to distinguish between two aryl bromides in the same molecule (Scheme 4.41)67. 

Villemin, D. and Nechab, B., Rapid and efficient palladium catalysed reduction of aryl halides by triethylsilane under microwave irradiation, /. Chem. Res. (S), 2000, 432-434. 

Moreover, a Rosenmund reduction can be carried out too. Triethylsilane (120) in presence of palladium/carbon converts acetylchloride into acetaldehyde (133)92 ... 

It is also worthwhile to note that the relatively slow catalytic hydrogenolysis of A/ -Z-protected peptide derivatives pemnits some interesting chemical transformations to be performed in situ. For example, direct conversion of Z protection to Boc protection is possible when the hydrogenation is conducted in the presence of di-tert-butyl dicarbonate under neutral conditions.h Alternatively, the same transformation is achieved by the use of triethylsilane and di-tert-butyl dicarbonate in ethanol with catalytic amounts of palladium(II) acetate.h 1 More efficiently this one-pot transformation is achieved by catalytic transfer hydrogenation in the presence of di-terf-butyl dicarbonate (cf. Section 2.1.1.1.3.1.1.6).h l Similarly, peptide cyclization reactions have been performed in situ over Pd/C and the high yields of cyclic monomers are attributed to the high dilution effect as well as to catalysis of the charcoal surface.h l... 

Hydrogenolysis of aryl chlorides can also be achieved by treatment with triethylsilane in the presence of palladium, and by reduction with isopropyl alcohol under UV irradiation (equation 53). ... 

A selective reducing agent for replacement of halogens by hydrogen, triphenylstannane (triphenyltin hydride), reduces aryl bromides to arenes in high yields (Table 4). Triethylsilane (triethylsilicon hydride) converts bromobenzene to benzene in the presence of 10% palladium on carbon in 79% yield (Table 4). ... 

The mechanism for this palladium-catalyzed cross-coupling reaction comprises the initial oxidative addition of the electrophile 37 to the palladium(O) catalyst followed by transmetallation of triethylsilane to yield the corresponding bis(organo)palladium(II) complex 39, which then undergoes reductive elimination to form the alkene 40 and to regenerate the palladium(O) catalyst. 

Enones can be reduced to the saturated ketones with triethylsilane and Wilkinson s catalysis62 (equation 54). Interestingly, the same product was prepared via a Tiffeneau-Demjanov ring expansion wherein trimethylsilyl cyanide was used (equation 55). The selective reduction of the double bond of enones can also be carried out with diphenylsilane in the presence of palladium(O) or palladium(II) and zinc chloride63 (equation 56), or more effectively with phenylsilane and molybdenum hexacarbonyl64 (equation 57). This latter reagent was also used to reduce the double bond of a,/ -unsaturated esters, amides and nitriles. 

The hydrosilylation of isocyanates with triethylsilane catalyzed by PdCl2 gives either N-triethylsilylformamides (32) or C-triethylsilylamides (33) depending upon the structure of the isocyanates. Thus, the reaction of aryl isocyanates gives 32 as sole product via normal addition of the silane to the C=N bond of isocyanates, while alkyl isocyanates give 33 exclusively via reverse addition162,164 (equation 96). Palladium on carbon exhibits a similar catalytic activity, while RhCl(PPh3)3 does not promote the reaction at all. 

Benzyl esters, ethers and carbamates can be deprotected in the presence of other easily reducible groups by treatment of the substrate with palladium(ll) acetate and triethylamine in the presence of triethylsilane. Competing reduction of bromoarenes, cyclopropanes or alkenes is not observed [Scheme 643)... 

Reductive cyclization of 1,6-diynes in the presence of triethylsilane and a catalyst prepared in situ from palladium(O) and acetic acid leads to dialkylidenecyclopentanes. Thus the diester 505 gave compound 506. ... 

Reduction of Acyl Derivatives to Aldehydes. Aroyl chlorides and bromides give modest yields of aryl aldehydes when refluxed in diethyl ether with triethylsilane and Aluminum Chloride. Better yields of both alkyl and aryl aldehydes are obtained from mixtures of acyl chlorides or bromides and triethylsilane by using a small amount of 10% Palladium on Carbon catalyst (eq 7). This same combination of triethylsilane and catalyst can effect the reduction of ethyl thiol esters to aldehydes, even in sensitive polyfunctional compounds (eq 8). ... 

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