Alkyl derivatives palladium reactivity

The use of organozirconium compounds as carbanion equivalents is greatly facilitated by trans metallations to the more reactive aluminum [11,100], copper [104—106], nickel [96—98], and palladium [99] derivatives. Copper-catalyzed carbon—carbon bond-forming reactions of alkyl- and alkenylzirconocene compounds have been particularly well studied, and have found considerable application in organic synthesis [107,108]. 

D.1. Reactions with Nucleophiles. Previously, a jr-allylic palladium complex was generated by reaction of palladium reagents with allylic hydrocarbons prior to reaction with nucleophiles. In the catalytic version of this reaction, an allylic halide or an allylic acetate is used with a palladium(O) reagent. Why use a palladium complex when enolate alkylation is a well-known process (sec. 9.3.A) A typical enolate coupling reaction is the conversion of 2-methylcyclopentane-l,3-dione (373) to the enolate anion by reaction with NaOH, allowing reaction with allyl bromide. Under these conditions only 34% of 374 was obtained. When allyl acetate was used in place of allyl bromide in this reaction and tetra w(triphenylphosphino)palladium was used as a catalyst, a 94% yield of 374 was obtained.224 in this reaction, formation of the Jt-allyl palladium complex facilitated coupling with the nucleophilic enolate derived from 373, which exhibited poor reactivity in the normal enolate alkylation sequence. 

The reason why certain bromides are more active than iodides in the attack to palladium(II) is probably connected to steric effects. We ascertained that at room temperature the order of reactivity of aryl iodides and bromides with palladium(O) and paUadiiun(II) is the same (I>Br), so the difference is hkely to lie in the easier accessibility of the reaction center of the palladacycle to suitably activated bromo derivatives than to o-alkyl-substituted aryl iodides, which are considerably bulkier. We ascertained that several groups are compatible as shown in Table 4, referring to the reaction of o-substituted aryl iodides, substituted aryl bromides, norbornene, methyl acrylate, K2CO3, and Pd(OAc)2 in the molar ratio 50 50 50 80 120 1. 

Aryl-, alkenyl-, and alkynylpalladium complexes (Table 9) are generated in manners similar to the preparation of alkyl- and benzylpalladium derivatives. However, oxidative addition is much more widely applicable and satisfactory than in the preparation of alkyl-palladium derivatives, since aryl, alkenyl, and alkynyl halides are not only much more reactive toward Pd(0) complexes than their alkyl counterparts but are also uncomplicated by j8-elimination. 

The reactivity of furan derivatives in palladium-catalyzed desulfitative arylation was studied. Alkyl-substituted furan derivatives were successfully coupled with a variety of benzenesulfonyl chlorides using a phosphine-free catalyst regioselective arylation at C-5 of the furan was observed in all cases (14S2515). 

Many arylations are assisted by functional groups that promote ortho-metallation. Thus, for example, acetanihdes react with arylsilanes at the ortho position via pal-ladacycles 78 and 79 to form derivatives 80 (Scheme 11.27) [84]. Mechanistically, this transformation is similar to a cross-coupling reaction, in which the oxidative addition step is replaced by the ortho-metallation, although in this case the Pd(0) intermediate must be oxidized in situ to generate the reactive Pd(ll) species. Unsubstituted benzylamines and N-methylbenzylamine are ortho-arylated with Pd(OAc)2 in the presence of trifluoroacetic acid (TFA) and silver acetate [85], and a mechanism which differs from the usual Pd(0)/Pd(II) catalytic cycle was suggested for this. Ortho-alkylation was also observed in the palladium-catalyzed... 

Abstract Square-planar palladium(II) and platinum(II) complexes with a high-lying filled d 2 orbital can act as metalloligands for Lewis-acidic metal centers such as d ° and s cations. This behavior is promoted by hard ligands such as a-bound hydrocarbyl ligands. A wide diversity of structural motifs based on this kind of donor-acceptor metal-metal bonds has been discovered in the last decades. This chapter reviews the coordination chemistry of metalloligands derived from alkyl, aryl, alkynyl, and carbene complexes of palladium(II) and platinum(II). The specific reactivity of the resulting bimetallic complexes is also addressed. 

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