Chloropalladation

When butadiene is treated with PdCU the l-chloromethyl-7r-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed(296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-I,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the tr-allyl complex 339 after the cyclization[288]. 

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

Pallado-. palladous, palladium (II). -chlorid, n. palladous chloride, palladium(II) chloride, -chlorwasserstoffsaure, /. chloropalladous acid, chloropalladic(II) acid, -hydro d, n. palladous hydroxide, palladium(II) hydroxide. 

An example of this reaction in an acyclic case is given in equation 21. Dienyne 55 afforded compound 56 in a highly selective 1,4-addition. In this case the relative amount of the trans chloropalladation adduct was higher than in the reaction of 53 and the chlorovinyl group was 90% E54. 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

Styryl tellurides are also detellurated on treatment with lithium chloropalladate (2 equiv), giving stereoisomeric mixtures of the homocoupled 1,3-butadienes. 

These studies, plus analogy with other Pd(II) chemistry (3), leave little doubt that the general mechanism for these exchanges involves addition of acetate (acetoxypalladation) or chloride (chloropalladation) followed by elimination of acetate (deacetoxypalladation) or chloride... 

Chloropalladation is somewhat different. Acetate does not attack cis because it is believed that chloride is much more strongly complexed to Pd(II) than acetate, and thus, acetate cannot get in the coordination sphere to attack cis. Chloride is already in the coordination sphere and thus in position for cis attack. Chloride might also be expected to attack trans from outside the coordination sphere as does acetate. For these reasons, chloropalladation may not be as stereospecific a process as acetoxypalladation in fact, this is the case. For example 1-chlorocyclo-... 

From a mechanistic point of view (see below) it is important to note that the chloropalladation of labeled 2,2-diphenyl-l-methylenecyclopropane-3,3-d2 gave only two isomeric complexes, with absence of the isomer in which both the phenyl and deuterium reside on the allylic moiety (equation 323). This allows the exclusion of a symmetrically bound >/4-trimethylenemethane (TMM) intermediate or rapidly equilibrating >/3-TMM species,... 

The true stereochemistry of 1, 3-chloropalladation is revealed in the reactions of cis-9-methylenebicyclo[6.1.0]nonane to give a single allylic complex (equation 324), and of trans-9-methylenebicyclo[6.1.0]nonane to give selectively a 4 1 mixture of syn and anti stereoisomeric allylic complexes (equation 325)389 393. In contrast to the noncyclic allylic complexes described above which interconvert under the reaction conditions, these mono-cyclic allylpalladium complexes are configurationally stable even under reflux in benzene for 8 h in the presence of 5 mol% PPh3. 

Interestingly, chloropalladation reaction of the more constrained cu-7-methylenebicy-clo[4.1.0]heptane did not afford the expected dis-in kinetic product but rather the rearranged to >/ to rf ) thermodynamic isomer whose structure (as the acac mononuclear complex) was confirmed by X-ray crystallographic analysis (equation 327)394. More recently, 1-aryl-substituted derivatives of this bicyclic methylenecyclopropane (equation... 

Unlike the alkyl and aryl-substituted methylenecyclopropanes discussed above, both cis- and franj-Feist s esters undergo chloropalladation with proximal 1,2-ring opening, to give isomeric n3-[3-chloro-l,2-bis(methoxycarbonyl)but-3-enyl]palladium complexes (equation 331)397. Formation of the but-3-enyl complexes is rationalized by sequential... 

As with the chloropalladation reaction (vide supra)m the rearrangement of >/2-methyl-enecyclopropane to >/4-TMM was shown experimentally to proceed stereoselectively by disrotatory ring cleavage of the distal frontier molecular orbital considerations, which predict that the out-of-phase interaction between the metal orbital in the distal ring-opening of -methylenecyclopropane complexes can be minimized by bending the bond up away from the metal (equation 348)410 ... 

Palladous chloride (1 48 g, 0 0083 mole) (Note 1) is placed in a 10-ml. Erlenmeyer flask, and 3.6 ml. (0 043 mole) of 37% hydrochloric acid is added. The flask is shaken at about 30° until the palladous chloride is dissolved. The chloropalladous acid solution is transferred to a 150-ml. beaker with 45 ml of... 

Dienes underwent facile chloropalladation producing an unstable cr-vinylpalladium(II) intermediate, which inserted another mole of allene to give an isolable 2-substituted r)3-allylpalladium complex. 

Another useful feature of the organopalladium reaction is that it can produce fairly hindered olefins from ortho-substituted aromatic mercurials. For example, "2,4,6-triisopropylphenylpalladium chloride , prepared in situ, from the mercurial and lithium chloropalladate in acetonitrile solution, reacts with styrene to produce a 40% yield of fra s-2,4,6-triisopropylstilbene. This is about the same yield of product that is obtained with the unsubstituted "phenylpalladium chloride under the same conditions 24>. 

In principle, the same methods used for the preparation of platinum catalysts may be applied for palladium catalysts. When palladium chloride is used as a starting material, it is usually dissolved into an aqueous solution as chloropalladic acid by adding hydrochloric acid prior to reduction or formation of precipitates. Unsupported and supported palladium catalysts have been prepared by reduction of palladium salts with alkaline formaldehyde,139 168 sodium formate,169 hydrazine,150 hydrogen,170,171 sodium borohydride,146,172 or sodium hydride-t-AmOH,173 or by reduction of palladium hydroxide174,175 or palladium oxide176 with hydrogen. 

Lead-Poisoned Pd-CaCQs (Lindlar Catalyst))82 Palladium chloride (1.48 g, 0.0083 mol) is placed in a 10-ml Erlenmeyer flask, and 3.6 ml (0.043 mol) of 37% hydrochloric acid is added. The flask is shaken at about 30°C until the palladium chloride is dissolved. The chloropalladic acid solution is transferred to a 150-ml beaker with 45 ml of distilled water. The pH of the solution is brought to 4.0-4.5 by slow addition of aqueous 3M sodium hydroxide. The solution is diluted to approximately 100 ml and placed in a 200- or 250-ml three-necked, round-bottomed flask equipped with a mechanical stirrer and a thermometer. Precipitated calcium carbonate (18 g) is added. The well-stirred suspension is heated to 75-85°C and held... 

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