Reductions tetrakis palladium

Because the Sonogashira coupling process outlined in Scheme 18 is initiated by the in situ reduction of palladium(n) to palladium(o), it would be expected that palladium(o) catalysts could be utilized directly. Indeed, a catalytic amount of tetrakis(triphenylphosphine)-... 

These cycloadditions are more sensitive to the quality of the catalyst, the major side reaction being protodesilylation of the allylsilane subunit. Since this can not be measured readily either in the case of the tetrakis(triphenylphosphane)palladium(0) or the palladium acetate/triisopropyl phosphite methods, an improved method for generating the palladium(O) species has been developed22. This involves in situ preparation of tetrakis(triisopropyl phos-phite)palladium(O) by direct reduction of palladium acetate with butyl lithium. This method is illustrated by the addition of the methyl-substituted TMM-Pd complex to eyelopentenone. 

In another reduction, the propargylic phosphate 64 is reduced with samarium(II) iodide in the presence of tetrakis(triphenylphosphine)palladium and tert-butanol as a proton source the allene 65 is produced almost exclusively, <1% of the isomeric alkyne 66 being present in the product mixture [19]. 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

Conjugate reduction of a,/l-enals and -enones. Tri-n-butyltin hydride in the presence of tetrakis(triphenylphosphine)palladium effects conjugate reduction of a, /J-unsaturated aldehydes and ketones in the presence of a proton source (water, acetic acid). Yields are improved by addition of a radical scavenger.15 Double bonds bearing... 

Kuroboshi, M. Waki, Y. Tanaka, H. Palladium-catalyzed tetrakis(dimethylamino)ethy-lene-promoted reductive coupling of aryl halides./. Org. Chem. 2003, 68, 3938-3942. Luo, F.-T. Jeevanandam, A. Basu, M. K. Efficient and high-turnover homocoupling reaction of aryl iodides by the use of palladacycle catalysts. A convenient way to prepare poly-p-phenylene. Tetrahedron Lett. 1998, 39, 7939-7942. 

Reduction of allylic acetates. This three-component system is effective for reduction of simple allylic acetates and even of 3-acetoxyglycals, particularly if Pd[P(C6H5)3]4 is replaced by tetrakis(tri-p-tolylphosphine)palladium(0). 

More recently, methods based on the use of mild reductants, able to transfer a single electron to the polyhaloalkyl halide, have been described. Various metals or their derivatives have been employed ruthenium, platinum and their complexes in low oxidation state, iron" and its carbonyl complexes, or tetrakis(triphenylphosphane)palladium. Sodium arcncsul-finate, sodium dithionite" and various oxidants have also been used. Other examples of polyhaloalkyl halide additions to simple alkenes are summarized in Table 1. Typical examples are the formation of diiodide 6, chloro iodide 7, and iodo steroid 8. ... 

Reduction of acid chlorides to aldehydes One of the most useful synthetic transformations in organic synthesis is the conversion of an acid chloride to the corresponding aldehyde without over-reduction to the alcohol. Until recently, this type of selective reduction was difficult to accomplish and was most frequently effected by catalytic hydrogenation (the Rosenmund reduction section 6.4.1). However, in the past few years, several novel reducing agents have been developed to accomplish the desired transformation. Among the reagents that are available for the partial reduction of acyl chlorides to aldehydes are bis(triphenylphosphine)cuprous borohydride , sodium or lithium tri-terf-butoxyaluminium hydride, complex copper cyanotrihydridoborate salts °, anionic iron carbonyl complexes and tri-n-butyltin hydride in the presence of tetrakis(triphenylphosphine)palladium(0). ... 

In catalytic hydrogenation, chlorine is replaced by hydrogen in chlorinated aromatic hydrocarbons (equations 41 and 42), phenols (equation 43), amines (equation 44), carboxylic acids (equation 45), and nitro compounds (equation 46). - Hydrogenolysis of chlorine in chloronitro compounds takes precedence over reduction of nitro groups, provided that contact with the halogen-free product is not too long. The reaction is achieved using palladium on carbon or tetrakis(triphenylphos-... 

This methodology has also been applied to the conversion of alkyl esters into vinyl ethers with high stereoselectivity favoring the (Z)-isomer (Scheme 30). While standard Li/amine reduction conditions were not applicable, the enol phosphates could be reduced using triethylaluminum and tetrakis(tri-phenylphosphine)palladium. 

Reaction of bis(disilanyl)dithiane 32 with the corresponding palladium(O)-isonitrile complex affords a four-membered cyclic bis(silyl)palladium(II) complex 34 quantitatively together with the formation of a disilane (Eq. 15) [30]. The formal intramolecular metathesis of the two Si-Si bonds of 32 may proceed through initial formation of tetrakis(silyl)Pd(IV) complex, corresponding to the platinum complex 33. The double oxidative addition of the two Si-Si bonds may be followed by reductive elimination of the disilane with accompanying formation of four-membered bis(silyl)palladium complex 34, due to difficulty in reductive elimination leading to formation of a three-membered cyclic disilane. 

The fully aromatic species are usually quite resistant to hydride reduction. Attempted reduction of l,2-dimethyl-5-nitroimidazole with tributyltin hydride failed to yield 1,2-dimethylimidazole <90JCS(Pl)9l9>, but the apparent reduction of 5-iodo-l-methylimidazole to l-methylimidazole by phenylsulfonylacetonitrile and sodium hydride with tetrakis(triphenylphosphine)palladium(0) as catalyst may be an example <92S552>. Lithium aluminum hydride (but not diborane) is able to convert 2-(but not 4-)fluoroimidazoles into the hydrogen species <84JOCi95i>. 

Reduction of aryl bromides. In a new method for this reaction, sodium formate serves as the hydride donor and tetrakis(triphenylphosphine)palladium(0) as catalyst. Sodium formate is generally superior to sodium methoxide because of ease of handling and compatibility with functional groups. 

Oxidative addition of palladium(O) species into unsaturated halides or triflates provides a popular method for the formation of the a-bound organopalladium(II) species. It is important to use an unsaturated (e.g. aryl or alkenyl) halide or tri-flate, as (3-hydride elimination of alkyl palladium species can take place readily. Oxidative addition of palladium(0) into alkenyl halides (or triflates) occurs stere-ospecifically with retention of configuration. The palladium is typically derived from tetrakis(triphenylphosphine)palladium(0), [Pd(PPh3)4], or tris(dibenzylidene-acetone)dipalladium(O), [Pd2(dba)3], or by in situ reduction of a palladium(II) species such as [Pd(OAc)2] or pd(PPh3)2Cl2]. 

Method E is based on the tetrakis(dimethylamino)ethylene (TDAE) as a soluble and very mild organic reductant with Pd(PhCN)2Cl2 as the catalyst in the coupling reactions of aryl iodides and bromides [18]. Excellent yields of biaryls were obtained under mild reaction conditions, at 50 C in DMF as the solvent. The latter method is the most efficient homogeneous palladium-based catalytic system for the coupling of aryl iodides and bromides to biaryls. 

---