Alkenylpalladium complex

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

Formation of carboxylic acids ami their derivatives. Aryl and alkenyl halides undergo Pd-catalyzed carbonylation under mild conditions, offering useful synthetic methods for carbonyl compounds. The facile CO insertion into aryl- or alkenylpalladium complexes, followed by the nucleophilic attack of alcohol or water affords esters or carboxylic acids. Aromatic and a,/ -unsaturated carboxylic acids or esters are prepared by the carbonylation of aryl and alkenyl halides in water or alcohols[30l-305]. 

The palladium-promoted conversion of 1,3-dienes to pyrroles proceeds via 4-acetoxy-2-alkenylpalladium complexes (Scheme 50g) (81CC59), and a similar pathway may be involved in the palladium mediated reaction of but-2-ene-l,4-diol with primary amines to give A-substituted pyrroles (74CC931). 

The regiochemical reversal induced by the acidic compounds led them to propose the mechanisms illustrated in Scheme 11. In both catalytic cycles, the alkyne linkage inserts into the H-Pd bond the left cycle (no acidic additive) forms linear alkenylpalladium complex, while the right cycle (with HX additive) generates branched alkenyl species. The provenance of the difference in the insertion regioselectivity has not been clearly addressed. 

Aryl- or alkenylpalladium complexes can be generated in situ by the trans-metallation of the aryl- or alkenylmercury compounds 386 or 389 with Pd(II) (see Section 6). These species react with 1,3-cyclohexadiene via the formation of the 7r-allylpalladium intermediate 387, which is attacked intramolecularly by the amide or carboxylate group, and the 1,2-difunctionalization takes place to give 388 and 390[322], Similarly, the ort/w-thallation of benzoic acid followed by transmetallation with Pd(II) forms the arylpalladium complex, which reacts with butadiene to afford the isoeoumarin 391, achieving the 1,2-difunctionalization of butadiene[323]. 

A mechanistic rationale for the Pd-catalyzed addition of a C-H bond at nitriles to allenes is outlined in Scheme 3. The oxidative insertion of Pd(0) into the C-H bond of nitrile 1 produces the Pd(II) hydride species 16 (or alternatively a tautomeric structure E E2C=C=N PdH Ln may be more suitable, where E = H, alkyl, aryl and/or EWG). Carbopalladation of the allene 2 would afford the alkenylpalladium complex 17 (carbopalladation mechanism), which would undergo reductive coupling to give the addition product 3 and regenerates Pd(0) species. As an alternative mechanism, it may be considered that the hydropalladation of allenes with the Pd(II) intermediate 16 gives the jr-allylpalladium complex 18 which undergoes reductive coupling to afford the adduct 3 and a Pd(0) species (hydropalladation mechanism). 

The most popular Pd-catalyzed method for the production of furans and benzofurans involves reactions of alkynols. Acyclic alkynols are converted into furans, while benzene substituted alkynols are transformed into benzofurans. The use of this strategy is widespread for the synthesis of benzofurans however, it is occasionally used for the syntheses of furans. For example, intramolecular alkoxylation of alkyne 189 proceeds via an alkenylpalladium complex and subsequent carbonylation to form furan 190 [156, 157]. In addition, 3 -hydroxyalkyl-benzo[/)J furans were prepared by Bishop et al. via a Pd-catalyzed heteroannulation of silyl-protected alkynols with 2-iodophenol in a fashion akin to the Larock indole synthesis [158]. In a related series of experiments, Qing demonstrated that alkynes 191 could be efficiently converted into furans 192 [159]. 

Intramolecular oxypalladation of the allenyl aldehyde 249 generates the intermediary alkenylpalladium complex 250, and subsequent carbonylation affords the unsaturated ester 251 in 88 % yield. Propylene oxide is added as a scavenger of HCl [107]. 

Insertions of alkynes into palladium-hydride complexes have been reported to take place as part of catalytic cycles, resulting in circumstantial yet credible evidence for the occurrence of alkenylpalladium complexes. The first example of palladium-catalyzed hydroarylation of alkynes with organoboronic acids B(OH)2R has been given. The mechanistic interpretation based on labeling studies involves hydropalladation of the alkyne to give an intermediate alkenylpalladium(ii) species that reacts with the organoboronic acid (Scheme 11). Similar direct coupling reactions... 

The arylation and vinylation of intemd alkynes has not been developed as much as the reactions of alkenes and 1-alkynes. An example of this type of reaction is depicted in equation 28 (78), The alkenylpalladium complexes formed from the reaction of Pd and internal alkynes are usually stable. Internal alkynes have been vinylated by intramolecular cascade reactions. In these reactions the product of insertion into an alkyne bond is able to subsequently insert into an alkene bond. Elimination from this complex gives the final reaction product. Equations 29 (82) and 30 (82) provide examples of this type of reaction. 

The proposed mechanism hypothesized the nucleophilic attack of the oxygen to the Pd-complexed C-C triple bond, through the enol amide form, producing the oxazole skeleton by formation of the c-alkenylpalladium complex. The intervention of water provided, through its enol form, the 4,5-dihydrooxazole-5-carbaldehyde. The oxidizing system also promoted the dehydrogenation step (Scheme 54). 

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