Hydridopalladium species

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

As expected, the reaction of complexed III proceeds with high stereoselectivity. Oxypalladation followed by elimination of a hydride of palladium should yield HPd(OAc) ((CH3)8CC(OAc)=CH2). Oxidation of the hydridopalladium species to a hydroperoxide is an attractive possibility as the first step in catalyst regeneration. For n-hexenes which have... 

Yields are greater than 100% based on Pd(II) probably because of regeneration of the hydridopalladium species by reduction of acetylene. 

A variety of palladium-catalyzed organic reactions involve the oxidative addition process. A typical example is seen in the catalytic arylation and alkenylation of olefins (eq (60)) [85]. Aryl- and alkenylpalladium(ll) complexes (9) formed by oxidative addition undergo olefin insertion into the palladium-carbon bond to give an alkylpal-ladium species (10), which provides arylated and alkenylated olefins via p-hydrogen elimination. The hydridopalladium species 11 thus generated is reduced to a Pd(0) species upon its interaction with a base and carries the sequence of reactions... 

We reported that the palladium-catalyzed intramolecular cyclization of the A/L(o-alkynylphenyl)-imines 54 gave the 3-alkenylindoles 55 in good to high yields (Scheme 19).100 A hydridopalladium species, generated in situ through the reaction of Pd-(OAc)2, P(n-Bu)3, and H20, reacts with alkynes to produce 56, which undergoes the cycloisomerization via the carbopalladation— -elimination—olefin isomerization. 

Nagao et al. reported that the ring expansion of the hydroxy methoxyallenylisoindolinones 222 occurred in the presence of a Pd(0) catalyst to give the isoquinolones 223 in high yields (Scheme 72).146 Oxidative addition of an O—H bond of 222 to palladium ) gives the hydridopalladium species 224, and subsequent intramolecular hydropalladation of... 

The reaction of phenols having electron-donating groups with propiolic acids gives coumarin derivatives under similar conditions to those for Eq. 68 (Eq. 70) [159]. The same reaction has been found to occur using a Pd(0) species such as Pd2(dba)3 in formic acid [160]. In this case, different types of pallada-tion mechanism by hydridopalladium species have been proposed. 

The first catalytic transformation of SO2 into organic sulfur compounds was reported in 1968 [114], The reaction of ethylene and SO2 in the presence of PdCl2 provides a mixture of sulfones (Eq. 7.21). The result of a loss of the catalyst activity under dehydrative conditions is suggestive of the participation of a hydridopalladium species. A mixture of Pd(acac)2, PPhs, and AlEts is an active catalyst for synthesis of cyclic sulfones (2,5-dialkenylsulfolane) which can be obtained from 1,3-dienes up to 75% yield (Eq. 7.22) [115]. 

Although hydridopalladium species have often been postulated as important reactive intermediates in a number of reactions, their precise role in catalysis and/or organic chemistry remains a challenging research area. Due to their instability, identification of hydridopalladium species is rare and they have never been detected under true catalytic conditions. In this section, a brief account of the preparation and characterization of hydridopalladium complexes will be given followed by a discussion on their chemical behavior. 

To date only a limited number of hydridopalladium complexes have been successfully isolated and characterized unequivocally. The different synthetic routes, as well as structural, spectral, and theoretical studies of discrete hydridopalladium species, have been reviewed extensively in 1996. " With the notable exception of the unusual zerovalent complexes (such as [PdHJ ), almost all of the hydridopalladium complexes reported so far have been monohydride complexes coordinated by phosphine ligands. 

This class of hydridopalladium species are the best characterized, especially those containing bulky phosphine ligands. The coordination of these bulky phosphine ligands stabilizes the hydride species in two ways. First, the basicity of the metal center usually increases with steric bulk and second, the bulkier phosphines are more likely to adopt a trans configuration, thus reducing the chance of decomposition via the elimination of HX. 

The effort to identify the precise role of hydridopalladium species in catalytic reactions could not be underestimated. There is a need for a systematic study of the precise nature of these transient species. The task will be a very challenging one but will invariably lead to improvements in current protocols, as well as the design of novel catalytic... 

In the absence of phosphine ligands and amines, chelation of the allylic hydroxy group to Pd(II) may yield a flve-membered cyclic o--complex. This chelate ring makes it difficult to achieve the conformation required for elimination of H and preferentially directs the elimination of the hydridopalladium species to give the enol. 

The organopalladium complex inserts in a, 2-syn mode and a cr-complex is produced. After -elimination, the allylic compound might be liberated. Alternatively, readdition of the hydridopalladium species in the reverse direction eventually leads to the cyclic system with the double bond in conjugation with the heteroatom (or next to the CH2 group) [54,55]. The isomer with the double bond conjugated to the aromatic or vinylic substituent is not observed, since formation of the aforementioned isomer requires either anr -elimination or readdition of the hydridopalladium species at the opposite face of the ring system. As earlier mentioned, a correcfly positioned hydrogen accessible for iy -elimination is a prerequisite... 

The last, relatively often-cited, substructure leading to 6-cxo-cyclizations is vinyl halide H (Figure 6.4). Santos and Pilli [110] reported the synthesis of quinolizidinones 154 and 155 from lactam 152 (Scheme 6.44). The reaction probably proceeds via an exclusive 6-cxo-cyclization, giving exo-bismethylene 153, followed by either thermally promoted proton shift or through a sequence of readdition and elimination of the hydridopalladium species. 

One can also envision that the quinone could trap the hydridopalladium species resulting from the B-hydrogen elimination that releases the oxidized organic product (Equation 16.129). Insertion of quinone into the palladium hydride would form an enolate that would tautomerize to the phenoxide complex. Protonation with the reagent containing an 0-H or N-H bond would generate the free hydroquinone and Pd(II). As noted in Chapter 17 on carbonylation, quinone has been used as an additive with this mechanism in mind to prevent tire Pd(II) hydroesterification catalysts from undergoing reduction to palladium(O). ... 

The impact of silver(l) and thaUium(I) salts [221] on Pd-catalyzed reactions extends beyond just increasing regioselectivities and enhancing reaction rates [202]. Without these additives, the arylation of aUyl alcohols 27 afforded aldehydes and ketones 30, rather than the fS-arylaUyl alcohols 33 (Scheme 8.7). Apparently, the P-hydride ehmination in the intermediate 28 is faster in the direction leading to enol 31. Alternatively, p-hydride ehmination to give the allyl alcohol 33 followed by readdition of the hydridopalladium species to the double bond and subsequent P-hydride ehmination to give the more stable enol 31 would explain the facts. This... 

The broad reactivity pattern displayed by transition metal hydrides is of special relevance in catalytic reactions " and has led to a great deal of theoretical studies. " The organometallic chemistry of hydridopalladium complexes has been reviewed. The involvement of hydridopalladium species is prevalent in Pd-catalyzed reactions. Then-precise roles in catalytic hydrogenation (Sect. VII), Tsuji-Trost (Sect. V.2.1), and other related reactions are covered in detail in later sections. Only a brief introduction to their general reactivity of mechanistic relevance will be presented here. 

Concerning the mechanism, model reactions suggested that a cationic hydridopalladium species is formed prior to dimerization of the methyl acrylate. 

A similar compound with a bipyridyl central core was sought according to Scheme 3.42. In this manner, a greater degree of planarity could be achieved due to reduced interactions in the absence of 2- and 2 -steric interactions. To that end, 2-chloro-3-nitropyridine was homocoupled in the presence of copper/bronze and dimethylformamide. The bipyridine ring system was brominated at the 5-and 5 -position under harsh conditions (due to its electrophilicity) to afford intermediate 98 that was then coupled with two equivalents of TMSA. These coupling conditions unfortunately afforded the hydroxyamine and a very small amount of the dinitro-coupled product. The electron deficient 98 presumably underwent nitro loss and Pd-catalyzed reduction by the hydridopalladium species that are present in the coupling catalytic cycle to afford the undesired 99 (Scheme 3.42). ... 

Besides the generation of palladaorganyls from aryl/alkenyl halides, they can also be formed by addition of palladium species to triple (and in some case double) bonds (see also 1). In particular, hydridopalladium species, generated in situ from palladium(O) and carboxylic acids, react with alkynes to form hydridopalladaorganyls that in turn react with... 

To a hydridopalladium species J generated from Pd(PPh3)4 and benzoic acid, hydropalladation of alkyne 20 and subsequent jS-hydride elimination occur to give allene 21 and to regenerate the active catalyst J (cycle I). Subsequent hydropalladation of 21 with J affords the q -allyl palladium K, and external nucleophilic attack gives hydroamination product 23 (cycle II). Intramolecular version is also documented in Sect. 4.1. 

---