Palladium-catalyzed reactions alkene reduction

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

Complexes of internal alkynes of general formula Pd(7] -alkyne)(PR3)2 or Pd( 7 -alkyne)(diphos) have been reported, often prepared in the course of palladium-catalyzed reactions and other processes. Thus, most of them have been synthesized by decomposition of Pd(ii) complexes in the presence of the alkyne as shown in Equations (20) and (21). Insertion into a Pd-E bond and reductive elimination generates the silylated or stannylated alkene and Pd(0), which is trapped by the alkyne in excess. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

The mechanism for this palladium-catalyzed cross-coupling reaction comprises the initial oxidative addition of the electrophile 37 to the palladium(O) catalyst followed by transmetallation of triethylsilane to yield the corresponding bis(organo)palladium(II) complex 39, which then undergoes reductive elimination to form the alkene 40 and to regenerate the palladium(O) catalyst. 

In a direct comparison of the palladium-catalyzed and the hexabutyldistannane-mediated ene-halogcnocyclization of unsaturated a-iodo carbonyl compounds, identical mixtures of regio- and stereoisomers were obtained311. Thus, it was suggested that both reactions proceed via a radical mechanism, and that palladium does not initiate an organometallic cycle via oxidative addition, alkene insertion and reductive elimination. 

The present section summarizes reductive Mizoroki-Heck-type arylations-that is, palladium-catalyzed hydroarylation reactions of alkenes-which are essentially limited to (hetero-)norbomenes (Section 7.3.2). It should be noted however, that only a handful of remarkable examples are known that are not based on the bicyclo[2.2.1]heptane framework (see Section 7.3.3). 

In 2011, Lautens and co-workers reported a palladium-catalyzed intramolecular carbon-carbon bond-forming reaction between atyl iodides and alkenes. This new cross-coupling reaction forms two new bonds and all of the atoms in the starting materials are incorporated into the product. The use of a palladium catalyst with bulky phosphine ligands was found to be crucial for reactivity as the bullty phosphine ligands favour the reductive elimination of Pd(n) complexes. Good to excellent 5delds of the desired products were isolated (Scheme 2.38a). In the same year, they found that aiyl... 

Catellani and Lautens have independently reported unique palladium/ norbornene-catalyzed reactions of aryl halides, which mechanistically involve a reversible alkene insertion/p-carbon elimination process [11]. For example, iodobenzene reacted with 1-iodobutane and methyl acrylate to form the multiply-alkylated benzene 29 (Scheme 7.9) [12]. The following mechanism is proposed oxidative addition of phenyl iodide onto palladium generates phenylpalladium(ll) iodide. A double bond of norbornene inserts into the C-Pd bond to form an alkylpalladium species, which cleaves a C-H bond nearby to form the palladacycle 25. -Butyl iodide then reacts with 25 to form the Pd(IV) intermediate 26, which undergoes reductive elimination. Repetition of the cyclometalation/alkylation process leads to the formation of 27. Then, P-carbon elimination affords the arylpalladium species 28 together with norbornene. Subsequently, a Heck-type reaction takes place with methyl acrylate, giving rise to 29. 

This one-step transformation of an alkene to an allylic acetate compares well with other methods of preparation such as hydride reduction of a, 8-unsaturated carbonyl compounds followed by esterification. The scope and limitations of the reaction have been investigated. The allylic acetoxylation proceeds via a TT-allylpalladium intermediate, and as a result, substituted and linear alkenes generally give several isomeric allylic acetates. With oxygen nucleophiles the reaction is quite general, and reactants and products are stable towards the reaction conditions. This is normally not yet the case with nitrogen nucleophiles, although one intramolecular palladium-catalyzed allylic amination mechanistically related to allylic acetoxylation has been reported. ... 

Deoxygenation of Carbonyls. Carbonyl compounds can be deoxygenated to form alkenes in a palladium-catalyzed reduction of enol triflates (eq 72). The reaction is quite general, and has been applied to aryl as well as alkyl enol triflates. ... 

---