Vinylpalladium complexes

In analogy to the mechanism of the palladium-catalyzed enyne cyclization, it is postulated that exposure of palladium(O) to acetic acid promotes in situ generation of hydridopalladium acetate LnPd"(H)(OAc). Alkyne hydrometallation affords the vinylpalladium complex A-10, which upon r-carbopalladation of the appendant alkyne provides intermediate B-7. Silane-mediated cleavage of carbon-palladium bond liberates the cyclized product along palladium(O), which reacts with acetic acid to regenerate hydridopalladium acetate to close the cycle (Scheme 33). 

On the other hand, the reaction of bisallenes with Bu3SnSnBu3 may generate cis-bisallene Pd(SnBu3)2 (C) and/or a chelated cj-allylpalladium complex D reversibly. The fast carbocydization of these intermediates would give rise to a vinylpalladium complex E, which then yields a ds-compound by reductive elimination and/or a cis-bicyclodiene through (T-bond metathesis as a kinetically controlled product (Scheme 16.66). 

Abstraction of chloride from a vinylpalladium complex by silver acetate has been reported,45 as well as halide abstraction by silver triflate from arylpalladium halides.46 More interestingly in the context of creating C-C bonds, silver perchlorate was able to promote the reaction of (r -aryl)palladium bromide with dienes. Silver-promoted bromide abstraction led to the formation of (r 1-r 2-enyl)palladium complexes, which evolved further through the regioselective formation of a C-C bond between the aryl group and the former diene. Reactions with nonconjugated dienes suggested that the reaction proceeds via carbometallation of the less crowded double bond. Isomerization and (3 elimination led to a (ri3-allyl)palladium complex (Scheme 10.26).47... 

Yamamura, M., Moritani, I., Murahashi, S.-l. Reaction of s-vinylpalladium complexes with alkyllithiums. Stereospecific synthesis of olefins from vinyl halides and alkyllithiums. J. Organomet. Chem. 1975, 91, C39-C42. 

Olefins from vinyl halides and alkyllithiums. Vinyl halides react with this paUadium(O) complex to give p-vinylpalladium complexes (2) with retention of configuration. These complexes react with alkyllithium or Grignard reagents to... 

In contrast to the diverse insertion chemistry of vinylpalladium intermediates discussed in Sects. IV.3 and IV.5, the reactions of vinylpalladium complexes with electrophiles had not been reported until recently. Although a single report on the annulation of the o-mer-curio benzaldehyde with diphenylacetylene into the corresponding indenols and inde-nones catalytic in palladium and stoichiometric in copper had been communicated in 1992, the more synthetically useful protocol for the catalytic version of this type of transformation remained unknown until 1999. In this section the intermolecular carbopalladation of alkynes with aryl halides followed by the intramolecular trapping of the formed vinylpalladium species with ketones, aldehydes, and nitriles will be discussed. 

TT-Allylpalladium complexes have long been recognized as an allyl cation equivalent and widely utilized for the allylation of a variety of hard and stabilized carbon nucleophiles. Both allenylpalladium and vinylpalladium complexes are chemically neutral, or very feebly nucleophilic, if any. Under conditions described below, however, these rr-allyl-, vinyl-, and allenylpalladium complexes alter their reactivity in mode and behave as nucleophilic species. "... 

It is known that the oxidative addition of aryl or vinyl halides to a low-valent palladium complex produces an aryl- or vinylpalladium complex, which reacts with carbon monoxide to afford an acylpalladium complex. If alcohol and amine are added to this reaction system, we can obtain ester or amide. " Intramolecular reactions of aryl or... 

StiiieP l and Ortar independently reported that enol triflate could be oxidatively added to a low-valent palladium complex to give vinylpalladium complex. Since enol triflate is easily prepared from a keto-carbonyl group by treatment with Tf20 or PhNTf2 in the presence of a base, this means that a new carbon-carbon bond formation can be achieved on the carbonyl carbon using a palladium catalyst. 

A combination of palladium-ene cyclization with carbonylation reactions is particularly attractive since it permits the stereoselective formation of four carbon-carbon bonds in a single process. In this reaction, 7r-allylpalladium complex 47 (Scheme 11) is formed from allyl carbonate 46 and Pd(0), and it is then converted into vinylpalladium complex 48 via palladium-ene cyclization. Carbonylation of 48 gives bicyclic compound 49, which is further reacted with carbon monoxide to give carboxylic acid 50. 

Yamamura, M. Moritani, L Murahashi, S. 1975. The reaction of o-vinylpalladium complexes with atkyUithiums. Stereospecific syntheses of olefins from vinyl halides and aUcyllithiums. J. Orgamomet. Chem. 91 C39-C42. 

Heck-type addition of the vinylpalladium complex to the Vilsmeier iminium species 43, generated from DMF and POCI3 (Scheme 9.15) [53]. 

After investigation of the palladium-catalyzed cascade cyclization/Suzuki coupling reactions of 1,6-enynes, Zhu and Zhang turned their attention to the analog 1,2,7-triene 333 for the synthesis of various functionalized five-membered cycles 334/335 [115] (Scheme 6.88). Oxidative addition of allyl hahides or carbonates to Pd(0) generates 7i-allylpalladium species 336, followed by intramolecular cycUzation to form vinylpalladium complex 337. Subsequent transmetallation with aryl boronic acids... 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

In the presence of in, V -generated palladium(O) species, an electrophilic 7t-allyl complex 228 is formed, which is readily engaged in an intramolecular carbometallation (Scheme 57). The resulting vinylpalladium species then undergoes a Stille-type cross-coupling to provide a triene.232... 

The palladium(II)-catalyzed oxidation of allenes with chloride was studied by Hege-dus et al. [3], In this reaction the dimeric products 4 and 6 as shown in Scheme 17.4 were obtained. The (allene)palladium(II) complex formed can react with chloride ions in two different ways (Scheme 17.4) [4]. Attack at the terminal carbon gives a vinylpalladium intermediate 2 whereas attack at the middle carbon produces a 2-chloro(jt-allyl)palladium complex 3. The former complex is the kinetic intermediate (k2 > kj) and is in equilibrium with the (allene)palladium complex. The 2-chloro(jt-allyl)palladium complex is formed more slowly but is more stable and has been isolated [2]. The vinyl complex can undergo further reaction with excess allene to give a new (jt-allyl)palladium complex, which undergoes attack with chloride to give the observed dimer 6 [3]. The dichloride from attack on the 2-chloro-(jT-allyl)palladium complex 3 was not observed. 

The reaction of an allene with an aryl- or vinylpalladium(II) species is a widely used way of forming a Jt-allyl complex. Subsequent nucleophilic attack on this intermediate gives the product and palladium(O) (Scheme 17.1). Oxidative addition of palladium ) to an aryl or vinyl halide closes the catalytic cycle that does not involve an overall oxidation. a-Allenyl acids 27, however, react with palladium(II) instead of with palladium(O) to afford cr-vinylpalladium(II) intermediates 28 (Scheme 17.12). These cr-complexes than react with either an allenyl ketone [11] or with another alle-nyl acid [12] to form 4-(3 -furanyl)butenolides 30 or -dibutenolides 32, respectively. 

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

The insertion of alkynes into arylpalladium complexes might also be accompanied by the insertion of carbon monoxide into the resulting vinylpalladium intermediate. The carbonylative annulation of TV-protected 2-iodoanilines and internal alkynes under an ambient pressure of carbon monoxide resulted in the formation of 2-quinolones (4.14.), The protection of the nitrogen atom in the aniline is crucial to the success of the reaction. 

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. 

In the absence of base, vinylmercury reagents and lithium tetrachloropalladate(II) react with alkenes to form ir-allylpalladium complexes arising from addition of the vinylpalladium chloride to the alkene followed by palladium hydride elimination, a reverse readdition and rr-allyl formation (equation 10).31... 

In the presence of triethylamine, decomposition of the ir-allylic complexes to conjugated dienes may occur, particularly when electron-withdrawing substituents are present on a methyl group in the 1- or 3-position of the iT-allyl system.31 Cyclic alkenes and vinylpalladium chlorides also yield iT-allylic complexes in the absence of an amine. If a tertiary amine is present, however, l,4-dienes are obtained (equation 1l).32... 

Vinylation of dienes in the presence of piperidine or morpholine yields aminodienes as major products. Sometimes trienes are minor products. The reaction is believed to proceed by way of a ir-allylpal-ladium complex formed by addition of the vinylpalladium halide to the least-substituted diene double bond. Nucleophilic attack of the amine upon the ir-allylic complex gives the aminodienes, while hydridopalladium halide elimination yields trienes (Scheme 6).97... 

Similarly, the vinylation of benzene was achieved on treatment with silver acetate and vinylpalladium(II) complexes (Scheme 10.48).45... 

Lu and Xie have reported a three-component coupling for the synthesis of o -alkylidenc-y-lactams 69 (Scheme 22) [62], Treatment of N-(2,4-dienyl)alkynamide 66 with an aryl iodide 67 affords a cr-vinylpalladium intermediate 70 through regioselective insertion of the active ArPdX species into the triple bond. Subsequent intramolecular carbopalladation of the diene affords 7r-allylpalladium complex 71, which undergoes nucleophilic attack by amines 68 at the less hindered terminus to afford the product 69. 

Mechanistically, this new insertion-CI-Diels-Alder hetero domino sequence can be rationalized as follows (Scheme 64) After the oxidative addition of the aryl halide 115 or 118 to the in situ generated Pd(0) species the arylpalladium halide 120 intramolecularly coordinates and inserts into the tethered triple bond via a syn-carbopaUadation to furnish cyclized vinylpalladium species 121 with a p-acceptor substitution in a stereospecific fashion. Transmetallation of the in situ generated copper acetylide 122 gives rise to the diorganylpalladium complex 123 that readily undergoes a reductive elimination and liberates the electron poor vinylpropargylallyl ether 124. The triethylamine catalyzed propargyl-allene isomerization furnishes the... 

Activation of sp2-C-0 bond in vinyl triflates has also been reported in the elec-trocarbonylation with C02 by using PdCl2(PPh3)2 as catalyst to afford a,(3-un-saturated carboxylic acids [92]. The reaction is proposed to involve the formation of a Pd(0) complex followed by an activation by electron transfer and the formation of vinylpalladium(II) intermediate in an oxidative addition. 

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