Hydropalladation bonds

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

The reaction between aqueous K2PdCl4 and CMe2(OH)CsCCsCCMe2(OH) (L) has been reported to give the palladium(I) complex PdCl(L), which exchanges L for X with salts MX (M = alkali metal, X = SCN, Br, I) and adds py to give PdCl(L)(py). However, it must be said that the dark brown to black solids so formed are not fully characterized by contemporary standards. It was assumed that hydropalladation of one C C triple bond has occurred. ... 

Alike olefins, allenes also undergo palladium mediated addition in the presence of N-H or O-H bonds. Although these reactions show some similarity to Wacker-type processes, from the mechanistic point of view they are quite different. Allenes, such as the cr-aminoallene in 3.69., usually undergo addition with palladium complexes (e.g. carbopalladation in 3.69. and 3.70., or hydropalladation in 3.71.), which leads to the formation of a functionalized allylpalladium complex. Subsequent intramolecular nucleophilic attack by the amino group leads to the closure of the pyrroline ring.87... 

In a formally similar reaction an alkene and an alkyne moiety were coupled in the presence of a palladium catalyst in formic acid. The reaction cascade in this case starts by the hydropalladation of the triple bond, which is followed by a Heck-type carbon-carbon bond formation. Running the... 

The above-mentioned cyclization starts with the insertion of alkynes to the Pd—H bond to generate alkenylpalladium species (hydropalladation). Polycyclization of... 

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 Pd-catalyzed hydrocarbonations of methyleneaziridines 14 do not proceed through the formation of a Jt-allylpalladium intermediates, instead via a chelation effect. The hydropalladation of methyleneaziridines with the Pd(II) hydride species 16, followed by reductive elimination gives the non-ring-opened products 15. It is noteworthy to mention that the palladium-catalyzed intermolecular or intramolecular addition of nitriles to carbon-carbon multiple bonds can be explained by the hydropalladation mechanism, except for the intramolecular addition to the C=C triple bond of alkynes (vide infra). 

Pd-catalyzed C-H transformation at nitriles to methylenecydopropanes 8 affords the terminal 9 and internal alkenes 10, either exclusively or as a mixture of both (Scheme 2, Eq. 4) [9]. The selectivity of product formation depends on the mode of hydropalladation of the carbon-carbon double bond of the methylene-cyclopropane and ring opening. Ring opening normally occurs at the distal bond. Also, the mode of ring opening of methylenecydopropane depends on the chain length of the substituent at the exomethylene carbon. In the addition of malononi-trile lb to 8b, both monoalkylation and dialkylation products are obtained as a 1 1 mixture (Eq. 10). 

Part I of Figure 17.75 shows the beginning and the cw-selective path of the hydrogenation. First, an H2 molecule dissolved in the liquid phase is bonded covalently to the surface of (or absorbed into) the metal catalyst. The alkene B also is bonded to that surface. This bonding is accomplished by reversible -complex formation. Occasionally, an alkene will thus be bonded to the catalyst surface in proximity to a Pd—H bond as shown in C. What follows is a kind of cis-selective hydropalladation of the alkene. A Pd atom binds to one end of a C=C bond and an H atom that was bonded to a proximate Pd adds to the other end of the C=C bond. The hydropalladation product is described by stereostructure E. Compound E can react further... 

Part II of Figure 17.75 shows the side reactions that occur when the Pd-catalyzed hydrogenation is not completely cis-selective. The start is the formation of the -complex F from the hydropalladation product E. In a way, this reaction is the reverse of the reaction type that formed E from the -complex C (cf. part I of Figure 17.75). In an equilibrium reaction, the isomerized -complex F subsequently releases the alkene iso-B, which is a double bond isomer of the substrate alkene B—and represents a type of compound that could well be the side product of an alkene hydrogenation, too. 

Research on the mechanism of the Pd(II)-catalyzed ene reaction points to a hydropalladation cycle shown in Scheme 12.27, which first involves compl-exation of Pd to both n ligands of the substrate to yield 82 (step a). Migratory 1,2-insertion of the alkyne into the Pd-H bond provides 83 (step b), and then 1,2-insertion of the rf-coordinated double bond occurs to yield 84 (step c). Finally, step d is -elimination, which yields two possible regioisomers (85 and 86). 

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... 

Pd(OAc)2 can catalyze the reaction of 2,3-allenoic acids with non-allylic alkenyl bromides with a terminal C=C bond leading to P-alkenyl butenolides 61 and 62 (Scheme 29). The reaction proceeded via oxypalladation-carbopal-ladation, repeated P-dehydropalladation/hydropalladation-dehalopalladation, in which Pd(II) is the catalytically active species [21]. 

Due to the higher reactivity of the allene moiety toward hydropalladation in 1,6-allenynes, the reaction may proceed via a hydropalladation of the allene moiety of 146 affording a vinylic palladium intermediate 147. Subsequent intramolecular carbopalladation of the C-C triple bond moiety would lead to the 1,3-dienyl palladium formate 148. Releasing of CO2 and reductive elimination afford the final product 149 and Pd(0). Pd(0) would react with HCO2H to afford HC02PdH, which is the catalytically active species (Scheme 61) [36]. 

The insertion of alkene to Pd-H, which is called hydropalladation of an alkene, affords the alkylpalladium complex 18, and insertion of alkyne to Pd-R bonds... 

In this section, Pd(0)-catalyzed reactions of allenes with nucleophiles are treated, which are clearly different mechanistically from the reactions explained in the above. Attack of nucleophiles may occur at C-1, C-2, and C-3 carbons of the allenes 63. Among them, attack at C-3 to give 64 is predominant. Most importantly, reactions of allenes with pronucleophiles start by the oxidative addition of pronucleophiles to Pd(0) to generate H-Pd-Nu 65. The formation of 64 by hydro-carbonation can be explained in two ways in the case where Nu-H is the carbon pronucleophile. As one possibility, hydropalladation of one of the two double bonds occurs to afford the terminal palladium intermediate 66, which is stabilized by the formation of 7r-allyl complex 67, and reductive elimination provides the C-3 adduct 68. Another possibility is carbopalladation to generate 69, and subsequent reductive elimination provides 68. Of these two possibilities, the hydropalladation mechanism is preferable. 

Allylic acetate 110 was obtained by regioselective hydroacetoxylation of phenylal-lene at the terminal double bond. DPPF is the most effective ligand. The reaction is explained by hydropalladation of allene with H-Pd-OAc to form 7r-allylpalladium acetate 111 and reductive elimination [33]. 

Hydropalladation and carbopalladation can proceed very readily with a variety of alkenes and alkynes, and they share some common critical features. Thus, they generally involve sttict syn addition of H— Pd and C— Pd bonds, respectively. These features are in agreement with a concerted mechanism involving interaction of an empty orbital of Pd with a TT-bond of alkenes or alkynes and that of H— Pd or C—Pd <7-bond with a 7r -orbital, as shown in Scheme 8. It should be noted that the overall synergistic bonding schane for hydropalladation or carbopalladation is very closely related to the Dewar-Chatt-Dun-canson (DCD) model " for Tr-complexation. In the schemes for hydropalladation and carbopalladation, the nonbonding Pd d orbital of the DCD model is substituted with a H— Pd and C—Pd <7-orbital, respectively. In all of these concerted processes, the presence or ready availability of a Pd empty orbital is critically important. 

Various metal-metal bonded compounds containing relatively electronegative metals, such as Si, Ge, Sn, B, Al, and Zn, can undergo Pd-catalyzed metallometallation, which mostly involves yn-addition to alkynes. One plausible mechanistic scheme involves (i) oxidative addition of metal-metal bonded compounds to Pd, (ii) metallopalladation (pattern 6) leading to yn-addition of metal-Pd bonds, and (iii) reductive elimination (Scheme 12). As such, the overall mechanism resembles that of Pd-catalyzed hydrogenation or hydrosilation, and the critical metallopalladation step must be mechanistically closely related to those of hydropalladation and carbopalladation. These reactions are discussed in detail in Sect. Vn.5. 

The reactivity of the C—Pd bond is very diverse. In addition to the previously discussed j8-elimination and reductive elimination, alkyl-, benzyl-, aryl-, alkenyl-, and alkynyl-Pd complexes can undergo syn addition of the C—Pd bond across the C—C bond of alkenes and alkynes, that is, carbopalladation. This reaction can generate alkyl- or alkenyl-Pd complexes and is occasionally used as a preparative method, as exemplified in Scheme 31.Closely related is the hydropalladation of alkenes and alkynes. However, these processes are mostly observed under catalytic conditions. 

Reaction mode A (Scheme 16) has recently also been realized for the Pd-catalyzed hy-drofurylation of alkylidenecyclopropanes 63 in which a hydropalladation is the initial step (Scheme 17). Mechanistic investigations of this reaction using a labeled 2-alkyl-5-deuteriofuran demonstrated this transformation to really proceed via intermediates 66 and 67 rather than by a direct insertion of a furylpalladium species into the distal bond of the methylenecyclopropane 63. The deuterium content at the methyne position of 65 was, however, only 44%. 

In the Pd-catalyzed hydrocarbonation of methylenecyclopropanes 68 with pronucleophiles of type 69 (Scheme 18), the direction of the initial hydropalladation depends craciaUy on the electron density distribution in the double bond of 68. Thus, a competition of the reactions along both pathways A and B can be observed. When R = alkyl or substituted alkyl, the reaction proceeds mainly via intermediates I and II furnishing the hydrocarbonation products 70 in good to very good yields. This type of reaction was performed both as an inter- as well as an intramolecular version. The reaction proceeded... 

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