Palladium, stereochemistry

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

Alkenyl zirconium complexes derived from alkynes form C—C bonds when added to aHyUc palladium complexes. The stereochemistry differs from that found in reactions of corresponding carbanions with aHyl—Pd in a way that suggests the Cp2ZrRCl alkylates first at Pd, rather than by direct attack on the aUyl group (259). 

Product stereochemistry is a function of the specific catalyst used for hydro-genation. For example, palladium generally gives more of the thermodynamically stable product than other catalysts. This effect has been attributed to an increased rate of equilibration of the steps in the hydrogenation process. Consequently, palladium should not be used to hydrogenate readily isomerizable olefins such as A - and A -steroids. ... 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

Table 27.2 Oxidation states and stereochemistries of compounds of nickel, palladium and platinum...

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

Fischer s original method for conversion of the nitrile into an aldehyde involved hydrolysis to a carboxylic acid, ring closure to a cyclic ester (lactone), and subsequent reduction. A modern improvement is to reduce the nitrile over a palladium catalyst, yielding an imine intermediate that is hydrolyzed to an aldehyde. Note that the cyanohydrin is formed as a mixture of stereoisomers at the new chirality center, so two new aldoses, differing only in their stereochemistry at C2, Tesult from Kiliani-Fischer synthesis. Chain extension of D-arabinose, for example, yields a mixture of D-glucose and o-mannose. 

Recent advances in the stereochemistry of nickel, palladium and platinum. J. R. Miller, Adv. Inorg. Chem. Radiochem., 1962,4,133-195 (270). 

Palladium, (diammine)bis(thiocyanato)-isomerism, 1, 185 Palladium, dichlorobis(amine)-substitution reactions stereochemistry, 1, 318 Palladium, dichlorobis(pyridine)-substitution reactions, 1, 314 Palladium, dinitritobis(triisopropylphosphine)-substitution reactions, I, 314 Palladium, ethylene-synthesis... 

Recent Advances in the Stereochemistry of Nickel. Palladium, and Platinum J R. Miller... 

The synthesis of 1-alkenylboronic acids from l-alkenylmagnesiums or -lithiums suffers from difficulty in retaining the stereochemistry of 1-aikenyl halides, but the palladium-catalyzed coupling reaction of diboron 82 with 1-aikenyl halides or tri-flates directly provides 1-alkenylboronic esters (Scheme 1-43) [157, 158]. Although the reaction conditions applied to the aryl coupling resulted in the formation of an... 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

For unsymmetrical allylic systems both the regiochemistry and stereochemistry of the substitution are critical issues. The palladium normally bonds anti to the acetate leaving group. The same products are obtained from 2-acetoxy-4-phenyl-3-butene and 1 -acetoxy-l-phenyl-2-butene, indicating a common intermediate. The same product mixture is also obtained from the Z-reactants, indicating rapid ,Z-equilibration in the allylpalladium intermediate.118... 

The role of the ligands is both to stabilize the Pd(0) state and to tune the reactivity of the palladium. The outline mechanism above does not specify many detailed aspects of the reaction that are important to understanding the effect of ligands, added salts, and solvents. Moreover, it does not address the stereochemistry, either in terms of the Pd center (tetracoordinate pentacoordinate , cisl, transl) or of the reacting carbon groups (inversion , retention ). Some of these issues are addressed by a more detailed mechanism.190... 

In recent years, cross-coupling methodology has emerged as a viable tool for enamide synthesis, and, indeed, there are a number of published protocols which employ palladium- or copper-catalyzed stereospecific amidations of vinyl halides [17]. For example, Buchwald and coworkers had recently shown that a copper-catalyzed cross-coupling of vinyl bromides or iodides proceeded with retention of stereochemistry (Scheme 9.16), though the only example using a tetrasubstituted vinyl halide, 23, lacked the need for any stereochemical control in the halide portion [18]. Based on this it seemed feasible that the desired enamide 22 could potentially be assembled via a comparable coupling between amide 24 and a stere-odefined vinyl halide such as 25. 

An asymmetric variant of this reaction was developed using chiral Pd complex 111 with either silanes or disiloxanes [66-68]. Both relative and absolute stereochemistries were controlled in this system and good yields (60-85%) were obtained after oxidation (Eq. 18). Formation of the silane-containing product was inhibited by the presence of water due to competitive formation of the palladium hydrides and silanols [68]. The use of disiloxanes as reductants, however, provided expedient oxidation to the alcohol products without decreasing the isolated yields enantioselectivity was 5-15% lower in this more robust system [66]. Benzhydryldimethylsilane proved to be a good compromise between high yield and facile oxidation [66]. Palladium com-... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

Oximes Amines can be prepared by the catalytic hydrogenation— hydrogenolysis of oximes over nickel or noble metal catalysts. Nickel is used usually in the presence of ammonia. Noble metals are used under mild conditions. The stereochemistry of the reaction depends on the circumstances. On Ra-Ni the trans-2-alkylcyclohexylamine (41) was the main product,529 whereas on palladium the cis product (42) was produced (Scheme 4.137).530... 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

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