Palladium catalysts configuration

Hydrogenolysis of the acetylene-cumulenes 19 can be achieved by means of a partially poisoned palladium catalyst to yield the regular [22]porphyrin(2.2,2.2) 20 with cisjrans.cis,-trans configuration of the two-carbon bridges. 

A mild approach that avoids the use of BuLi has been developed for enantioenriched chiral allenylzinc reagents. Configurationally predictable reagents can be prepared through reaction of a chiral mesylate with Et2Zn in the presence of a palladium catalyst, usually Pd(OAc)2 and PPh3 [110-112], The reagent reacts in situ with an alde-... 

A double bond in the 7,8-(VI) or 8,9-(VII) position of a steroid which has the trans A/B ring configuration isomerizes to the 8,14 position (VIII) when treated with hydrogen and a palladium catalyst or platinum in the presence of acetic acid (69) (Fig. 12). Once the double bond reaches the 8,14 position it cannot be hydrogenated however it may be isomerized by treatment with HCl in chloroform to yield a isomer which can be reduced catalytically. 

Stereoselective allylic alkylations have been carried out with the aid of palladium catalysts. The 17-(Z)-ethylidene groups of steroids (obtained from the ketones by Wittig olefination) form n-allyl palladium complexes in the presence of copper(n) salts (B.M. Trost, 1974, 1976). Their alkylation with dimethyl malonate anions in the presence of 1,2-ethane-diylbis[diphenylphosphine] (— diphos) gives a reaction exclusively at the side chain and only the (20S) products. If one starts with the endocyclic 16,17 double bond and replaces an (S)-20-acetoxy group by using tetrakis(triphenylphospbine)palladium,the substitution occurs with complete retention of configuration, resulting from two complete inversions (B.M. Trost, 1976). 

Catalytic hydrogenation of 1 9, over palladium catalyst gave a practically quantitative yield of the crystalline disaccharide derivative 2 2, the constitution, and configurations at all chiral centers, of which have been verified by x-ray crystal structure analysis ( 1 5). De- 0-silylation of the 5,8-diol 2 2. 

Automobile and Hydrocarbon Emissions. The oxidation of carbon monoxide and hydrocarbons is catalyzed by platinum/palladium/rhodium on alumina. If catalyst poisons such as lead and phosphorus are not present, the major problems become initiation of oxidation at low temperature, thermal stability at high temperature, resistance to thermal schock, and a high external surface area catalyst configuration. 

AUylic alkylations. This complex in combination with 2,2 -bipyridyl (bpy) catalyzes nucleophilic alkylation of allylic acetates and carbonates, but is less active than molybdenum or palladium catalysts. The displacement occurs with retention of configuration, as with Mo and Pd catalysts. However, alkylation occurs almost entirely at the more substituted end of the allylic group, regardless of the nucleophile. 

Partial catalytic hydrogenation of polyynes to give polyenes with cw configuration has been accomplished with a variety of substrates. Palladium catalysts, particularly the Lindlar catalyst ", are generally the most satisfactory. Side-reactions include over-hydrogenation, isomerization of cis to Irons isomers and double-bond migration. 

In the presence of a fluoride ion generating reagent such as tris(diethylamino)sulfonium difluorotri-methylsilicate (TASF, 99) and a palladium catalyst, vinyltrimethylsilane couples with vinyl and aryl iodides." For substituted vinylsilanes, the fluorodimethylsilyl group is particularly effective in the coupling reaction, which takes place with retention of configuration of both substrates (Scheme 17). Silicon-based C—C bond formation is as useful as that which employs organoborons " for the synthesis of stereo-defined conjugated dienes. 

Vinylic halides can be coupled to give 1,3-butadienes (134) by treatment with activated copper powder in a reaction analogous to the Ullmann reaction (13-11). " This reaction is stereospecific, with retention of configuration at both carbons. Vinyhc halides can also be coupled with Zn—NiCl2, " " and with -BuLi in ether in the presence of MnCl2. " The coupling reaction with vinyltin reagents and vinyl halides occurs with a palladium catalyst. " ... 

The available literature data support the assertion that the outcome of the methylene cycloadditions depends to a large extent on the ability of the olefin to be coordinated to the palladium center. In that respect, the mechanism of palladium-catalyzed cyclopropanation appears to differ significantly from that of rho-dium(ll)-catalyzed cyclopropanations. One advantage of using palladium catalysts with diazomethane is associated with the possibility of synthesizing polycyclopropane adducts, a topic of current interest (vide infra) which has no general satisfactory solution with other diazo compound/catalyst combinations. This point is exemplified below for the cyclopropanation of the esters of trans-polyunsaturated acids. Moreover, the reactivity of the double bonds depends both on their position in the linear hydrocarbon chain and on their configuration (eq. (f)). 

Terminal 2-bromo-l-alkenylboronates are readily obtained by bromoboration of terminal alkynes with tribromoborane. The subsequent displacement of the bromine atom with organozinc reagents proceeds w ith strict retention of configuration in the presence of a palladium catalyst [27] (eq (16)). 

The reactions of allyl, alkenyl, and arylstannanes with allyl, alkenyl, and aryl halides or triflates proceed in the presence of palladium catalysts to give the cross-coupling products (eq (107)) [102]. In the reaction of alkenylstannanes, their configuration is generally retained. If these reactions are carried out in an atmosphere of carbon monoxide, the corresponding ketones are obtained (eq (108)) [103]. 

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