Carbon-palladium bonds allenes

As described in the preceding sections, many domino reactions start with the formation of vinyl palladium species, these being formed by an oxidative addition of vinylic halides or triflates to Pd°. On the other hand, such an intermediate can also be obtained from the addition of a nucleophile to a divalent palladium-coordinated allene. Usually, some oxidant must be added to regenerate Pd11 from Pd° in order to achieve a catalytic cycle. Lu and coworkers [182] have used a protonolysis reaction of the formed carbon-palladium bond in the presence of excess halide ions to regenerate Pd2+ species. Thus, reaction of 6/1-386 and acrolein in the presence of Pd2+ and LiBr gave mainly 6/1-388. In some reactions 6/1-389 was formed as a side product (Scheme 6/1.98). 

Larock and co-workers reported a stoichiometric Pd(II)-promoted cyclization of ortho-thalliated benzoic acid 79 with allenes to afford cyclic benzoannelated 5-valeroIactones 80 and 81 (Scheme 28). In this reaction the first carbon-palladium bond is formed in a transmetallation reaction from T1 to Pd. 

The insertion of allenes in the palladium-carbon a bond of cyclopalladated pyridine derivative 295 (cf. 00CRV3067) affords stable, isolable (ry -allyl) palladium complexes (e.g., 296) (03JOM(687)313). The ideally located imine unit when depalladated reacts selectively with the allyl functionality to yield methylene morphanthridizinium salts 297a-c. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

The Pd(0)-catalyzed reactions of propargylic compounds so far discovered can be classified into four types, I, II, III, and TV, from a mechanistic viewpoint. The allenyl intermediate complex 8 undergoes three types of transformation, depending on reactants. The reactions of Type I proceed by insertion of unsaturated bonds into the a-bond between palladium and sp carbon in 8. This a-bond has a reactivity similar to the a-bond formed by the oxidative addition of alkenyl halides to Pd(0) in the Heck reaction [3]. Therefore, reactions similar to those observed in the Heck reaction are expected to occur witli the intermediate 8. Alkenes and carbon monoxide are known to insert into the palladium-carbon a-bond. The allene derivatives 9 are formed by these reactions (Scheme 11.3). 

Active methylene compounds can be added to polar double bonds such as those in acrylate esters and methyl vinyl ketone as has been described in the previous section. Active methylene compounds can also be added to carbon-carbon multiple bonds in allenes and alkynes with the aid of the transition metal complexes as the catalyst. The addition of methylmalononitrile to 3-phenyl-l,2-butadiene takes place in the presence of Pd2(dba)3-CHCl3 to give the corresponding addition product with E-stereochemistry (Eq. 67) [137 a]. The C-C bond formation occurs exclusively at the terminal position of the allenes. Trost et al. independently reported the similar results with respect to palladium-catalyzed addition of C-H bonds in active methylene compounds to allenes [137 b. ... 

The addition of C-H bond of active methine compounds to carbon-carbon double bond in the allene moiety proceeds in intramolecular fashion in the presence of palladium catalyst, leading to the five- or six-membered carbocycles (Eq. 69) [142]. Similar intramolecular carbocyclization can be applied to the methine compounds having the acetylene moiety, leading to the five-membered exo-methylene cyclopentanes in good to excellent yields [143]. 

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. 

Palladium-catalyzed addition of heteroatom compounds bearing heteroatom-heteroatom bond (X—X) or heteroatom-hydrogen bond (X—H) to carbon-carbon unsaturated bonds, such as aUcynes, alkenes, and allenes, is one of the most useful methods for introducing heteroatom functions into organic molecules. The reaction may involve the formation of the species bearing a heteroatom-palladium bond as a key intermediate and proceed via heteropalladation of unsaturated compounds (or alternatively via hydropalla-dation by a palladium hydride species (H—Pd— X) formed in situ). The following two processes can be operative for the heteropalladation (Scheme 1). While the former process, that is, anh-addition process, proceeds by the attack of the heteroatom nucleophile (X ) to the unsaturated bond coordinated by palladium, the later process involves the i yn-addition of X—PdL to the unsaturated bonds. Whereas the onh-addition process is widely known, 5yn-heteropalladation has been rare. 

An example of an intramolecular palladium-catalyzed oxidation of an allene involving carbonylation was used in the synthesis of pumilotoxin 251 D (equation 32)65. Intramolecular aminopalladation of the allene followed by carbonylation of the palladium-carbon bond and subsequent oxidative cleavage of the acylpalladium intermediate by CuCE afforded pyrrolidine 72 in which the chirality at the carbon at the 2-position was established. 

A unique system for catalytic silaboration of allenes, in which a catalytic amount of organic halide is used as a crucial additive, has been reported (Equation (86)).232 In the presence of Pd2(dba)3 (5 mol%) with 3-iodo-2-methyl-2-cyclohexen-l-one (10mol%), reactions of terminal allenes with a silylborane afford /3-silylallylboranes in good yields with excellent regioselectivity. It is worth noting that the addition takes place at the terminal C=C bond in contrast to the above-mentioned palladium-catalyzed silaboration. The alkenyl iodide can be replaced with iodine or trimethylsilyl iodide. The key reaction intermediate seems to be silylpalladium(n) iodide, which promotes the insertion of allenes with Si-C bond formation at the central -carbon. 

Reaction of the stannylborane 9 with an allenyne gives a cyclization product, in which the boryl and stannyl groups are introduced to the acetylenic terminus and the allenic central carbon, respectively (Equation (104)).159 Based on the assumption that an unsaturated functionality initially inserts into the Pd-B bond of (boryl)(stannyl)palladium(n) species, it seems likely that the alkyne moiety is more reactive than the allene moiety in this reaction. 

Similarly, 1,2-cyclononadiene in methanol with 10% palladium on carbon catalyst gave cis-cyclononene122. The cis isomer is not necessarily the primary product of allene hydrogenation, since the initially formed trans isomer is rapidly isomerized under the reaction conditions. Bond and Sheridan showed that allene resembles acetylene in its ease of hydrogenation123. They suggested that it is selectively adsorbed and held more strongly by the catalyst than 1-propene. Allene was selectively hydrogenated with Pd, Pt and Ni in the presence of 1-propene without its further reduction. 

By using the same catalytic system, alkylations of 1,3-dimethylbarbituric acid with alcohols were also accomplished (Scheme 5.31) [68]. The Cp lr-catalyzed alkylation using 2-iodobenzyl alcohol, followed by palladium-catalyzed carbon-carbon bond formation with allene, gave spirocyclic barbituric acid derivatives in a one-pot process. 

In the coupling of Z-3-iodo-3-trimethylsilylacrylic acid and allenyltin reagents the carbon-carbon bond was established in a Stille coupling, followed by the attack of the carboxylic acid on the allene moiety, probably promoted by palladium (4.39.). 

The initial Pd(PPh3)4-catalyzed addition of the Si-Sn bond of Me3Si SnMe3 to 1,1-dimethylallene selectively places the silicon on the central allenic carbon, but no selectivity is observed for placement of the tin atom. However, heating the reaction mixture (90°C) in the presence of the same palladium catalyst results in an 80% preference for the regioisomer having the stannyl group on the less hindered carbon.69,723... 

The palladium-catalyzed silylboration of typical allenes exhibited a strong tendency to occur at the internal double bond giving allylsilanes, which undergo allylation of aldehydes in the presence of a Lewis acid (Equation (44)).254 257 The selectivity can be varied by changing the substituents, but the boron atom always added to the central carbon. Enantioselective silylboration of terminal monosubstituted allenes was demonstrated by double asymmetric induction using chiral silylboranes and chiral catalysts (Equation (45)). 

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