Reductive elimination with palladium

Enhancing Reductive Elimination with Palladium Diall lhiaiylphosphines... 

Insertion of palladium into the Si-Sn bond generates intermediate 428 that undergoes m-addition on the triple bond (Scheme 108). The resulting vinylpalladium 429 ensures the carbopalladation of the second triple bond followed by reductive elimination with retention of stereochemistry.376... 

Similarly, a double functionalization can be reached when an activating group is present in close vicinity to the triple bond. Tsuji et al. have discovered that with a diphosphine palladium(O) complex, a carbonate function in the a-position of the alkyne provides by decarboxylation a palladium methoxy species on which the alkyne moiety can be isomerized into an al-lenyl a -bonded group. CO insertion in the Pd - C bond, reductive elimination with the methoxy group and further cyclization with incorporation of a second CO molecule give rise to the corresponding cyclopentenone as shown in Scheme 21 [127]. 

The sequence of events is (i) the insertion of the alkene into the palladium-carboxylate bond followed by (ii) CO insertion into die newly generated palladium-carbon bond followed by (iii) the reaction with solvent to give a palladium hydride that undergoes reductive elimination to palladium(O) (Scheme 12). 

These reactions are commonly interpreted to be composed of three main steps, namely a) oxidative addition of an aryl-X species to palladium(0) with formation of an arylpalladiumffi) bond b) insertion of a terminal olefin and c) reductive elimination regenerating palladium(0). To achieve a catalytic cycle, the rates of these steps have to match each other. The basic process was discovered by Heck in 1968. The mechanism has not yet been well defined and several variants have been proposed. A widely accepted scheme is reported in Figure 6. 

One exceptional reaction involving oxidative addition onto divalent palladium complexes has been reported. The spiro trisilane 21 is reacted with Cq3-al-lyl)CpPd(II) complexes at room temperature to afford Tris(organosilyl) CpPd(IV) complexes 23 in good yields (Eq. 7) [21]. The reaction maybe rationalized by oxidative addition of one of the two Si-Si bonds of 21, subsequent reductive elimination with formation of the Si-allyl bond giving mono(si-lyl)CpPd(II) complexes, and then oxidative addition of another Si-Si bond in the molecule onto the Pd(II). 

The stereochemistry of oxidative addition to the palladium(II) palladacycle was studied by Lautens using an enantioenriched secondary alkyl halide (Scheme 9) [32], From alkyl halide 23, product 24 was obtained, showing a net inversion of stereochemistry [33-35], Previous work by Stille showed that reductive elimination from palladium(IV) occurs with retention of stereochemistry [36], suggesting that oxidative addition occurs with an inversion of stereochemistry. This corresponds with the generally accepted SN2 mechanism for the reaction of palladium(O) with alkyl halides [37, 38],... 

Unlike the platinum analogs, direct observation of palladium(IV) alkyls has been limited to those with nitrogen-based ligands such as 2,2 -bipyridine and a-diimines [49]. However, reductive elimination from palladium(IV) complexes has... 

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

Of course, no one wants to make ethane that way (if at all) but many other pairs of ligands can be coupled by reductive elimination. Reductive elimination is one of the most important methods for the removal of a transition metal from a reaction sequence, leaving a neutral organic product. We will see many examples as the chapter develops but here is an indole synthesis that depends on a reductive elimination at palladium as a last step. In the starting material, palladium has two a bonds sharing electrons with C, and is Pd(II). In the reaction the two C substituents bond together to form the indole ring and a Pd(0) species is eliminated. 

It has been shown earlier that the presence of electron-withdrawing ligands favors the reductive elimination in palladium 77 -allyl complexes. Benzoquinone has been used to perform the clean reductive elimination in -allyl aryl palladium complexes, that otherwise decompose by other competing routes such as /3-H elimination. The intermediate complex with a coordinated benzoquinone molecule has been characterized in solution (Scheme 72). Benzoquinones have also been used to promote the reductive elimination of chloro and an -allyl Cl as a nucleophile toward Pd 77 -allyls usually attacks in a trans- (or exo-) fashion. A different stereochemistry (cis- or /rrfo-attack) has been achieved in this work. It was found that the more electron withdrawing the quinone the more favored the m-attack, as supported by theoretical calculations. Allyltin trichloride can be obtained by a reductive elimination reaction from Pd(SnCl3)(77 -allyl)L. The reaction is accelerated by addition of an electron-withdrawing olefin such as allyl chloride. Tin dichloride also increases the reaction rate. ... 

With the electTOTi-poor allenic esters, palladium(0) is able to catalyze the reaction without gold. The reactiOTi then is initiated at the other end, after oxidative addition of the aryl halide to the electrophilic palladium(II) species cycloisomerizes the allenic ester and then forms the product by reductive elimination. With o-alkynylbenzoates, the intermediate vinylgold species contains an enol ether substructure and is able to directly intercept the activated allyl donors, even in the absence of palladium. In both cases, by careful trace analysis (ICP), the presence of the other metal was excluded [78]. 

All proposed reaction pathways end with reductive elimination of palladium(O) and formation of simple, condensed, or polyheterocyclic compounds. 

---