Palladium catalysis Alkene alkylation

The mechanism of propene/CO copolymerisation by palladium catalysis is essentially analogous to those of ethene and styrene (i. e., chain propagation proceeds via alternating insertions of CO into Pd-alkyl and alkene into Pd-acyl controlled by p-chelates) [1]. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

In contrast, the closely related palladium acetate-promoted intramolecular alkylation of alkenes by tri-methylsilyl enol ethers (Scheme 4)6,7 has been used to synthesize a large number of bridged carbocyclic systems (Table 1). In principle, this process should be capable of being made catalytic in palladium(II), since silyl enol ethers are stable to a range of oxidants used to carry the Pd° -> Pd11 redox chemistry required for catalysis. In practice, catalytically efficient conditions have not yet been developed, and the reaction is usually carried out using a full equivalent of palladium(II) acetate. This chemistry has been used in the synthesis of quadrone (equation 2).8 With the more electrophilic palladium(II) trifluoroace-tate, methyl enol ethers underwent this cyclization process (equation 3).9... 

The appropriate interpretation may depend on the range of structures considered, the catalyst and the conditions employed. The interpretations differ in that in one, the rate of desorption is assumed to be fast relative to the conversion of adsorbed alkene to the alkyl intermediate, while in the other, desorption is assumed to be relatively slow. Catalysis by palladium may conform more closely to the reversible adsorption model than does platinum. 

Wacker oxidation11 provides a way to add water to an alkene 8 and oxidise the product to a ketone 72 all in the one step using oxygen under palladium (II) catalysis. The key to the difference between these two superficially rather similar sequences lies in the great tendency for palladium to undergo p-elimination 70. Oxypalladation 69 gives an unstable alkyl-palladium o-complex which decomposes at once to regenerate the double bond. 

The alkene, allyl, and alkyne complexes of Pd and Pt have been discussed to a large extent in earlier Sections, as have the use of palladium species in catalysis (Chapters 23 and 24). Here we deal mainly with complexes of the type MXHL2 and MXRL2 where X is halogen, R is an alkyl or aryl group and L is usually a tertiary phosphine. 

The pivotal difference is the presence of excess formic acid, at least equimolar to added base, as the reducing agent The catalysis commences with an oxidative addition of the C(sp )—X bond of 148 to a paUadium(0) catalyst (148—>149). The thus-formed electrophiUc C(sp )—Pd(ll) complex 149 will then capture the alkene 150, followed by syn selective migratory insertion (149—>151). C(sp )—Pd(II) intermediates are normally prone to facile P-hydride elimination, yet no conformation-ally accessible C—H bond, neither at Cp nor at Cp-, is available in 151 (vide supra). Instead, salt elimination with formate (151—>152), followed by the extrasion of carbon dioxide, occurs (152 153). This detour establishes an alternative route to the o-alkyl palladium(ll) hydride intermediate 153, which could otherwise emerge from 151 if conventional P-hydride elimination were possible. Reductive... 

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