Electron-deficient palladium catalysis

Z. Karpinski, Catalysis by supported, unsupported and electron-deficient palladium, Adv. Catal. 37, 45-100 (1981). 

Catalysis by Supported, Unsupported, and Electron-Deficient Palladium... 

Even though the versatility of palladium in hydrogenation reactions is recognized, the explanation of its catalytic properties is still far from being satisfactory. The chapter by Z. Karpinski gives a comprehensive survey of Catalysis by Supported, Unsupported, and Electron-Deficient Palladium. ... 

Intermediates which are involved in heterogeneous catalysis could have ionic character, which require an extention of the general treatment of complex reactions. As an example we can consider the catalytic hydrogenation over oxides and sulphides, where intermediates of cationic character were proposed. Ionic intermediates are also possible in catalysis over metals, for instance in the case of neopentane transformations over electron deficient palladium, which occur via formation of carbocations. If we consider olefin hydrogenation over oxides or sulphide with a heterolytic dissociation of hydrogen, the mechanismn of this reaction can be presented in the following form... 

Bicyclopropylidene (1) also reacts with activated alkenes under transition-metal catalysis. With electron-deficient alkenes under nickel(O) catalysis, the [2-1-2] cycloadduct 263 was the main component in the reaction mixture [2b, 150]. Under palladium(O) catalysis, formal [3-1-2] cycloaddition of electron-deficient (Scheme 60) as well as strained alkenes can be achieved exclusively... 

Scheme 60. Reactions of bicyclopropylidene (1) with electron-deficient alkenes under nickel(O) and palladium(O) catalysis [2b, 150]...

Synthetic methods for the preparation of dienes and enynes abound, yet the use of alkenyl iodonium salts offers distinct advantages. Their coupling reactions with electron-deficient alkenes and alkenyl- or alkynylstannanes constitute a valuable extension to the previously existing methodology, because of mild conditions, ease of operation, high stereoselectivity and good yields. The simplest reaction of this category is with unsaturated carbonyl compounds and requires palladium catalysis. 

As shown in Section 2.2.2.3.1., bicyclopropylidene (1) is capable of undergoing [2-1-2] cycloadditions with electron-deficient alkenes such as diethyl fumarate under nickel(O) catalysis. The [3 -I- 2] cyclodimer and a cyclotrimer are obtained only as minor products from this reaction. In contrast, exclusive [3 -I- 2] cycloaddition can be achieved with many other substrates when palladium(O) catalysts are employed. These cycloaddition products are also produced with phosphite-modified nickel(O) catalysts, but both yields and selectivities are markedly lowered. The reactions of 1 with norbornadiene and norbornene serve as examples for the reaction with strained hydrocarbons, providing the cyclodimers 2 and 3 in 61% and 66% yield, respectively. ... 

Such substitutions follow the same mechanistic route as the displacement of halide from 2- and 4-halo-nitrobenzenes, i.e. the nucleophile first adds and then the halide departs. By analogy with the benzenoid situation, the addition is facilitated by (i) the electron-deficiency at a- and y-carbons, further increased by the halogen substituent, and (ii) the ability of the heteroatom to accommodate negative charge in the intermediate thus produced. A comparison of the three possible intermediates makes it immediately plain that this latter is not available for attack at a p-position, and thus p nucleophilic displacements are very much slower - for practical purposes they do not occur (see, however, reactions with palladium catalysis, 4.2)... 

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

The highly strained and thns unnsually reactive tetrasubstituted alkene bicyclopropylidene also turned out to cleanly undergo cocyclizations under palladium catalysis. The TMM species generated from 50 underwent formal [3 + 2] cycloadditions onto electron-deficient (Scheme 14) as well as strained alkenes (Scheme 15).[ ... 

Contrary to heteroarenes, only few examples of intermolecular alkylations of the less reactive arenes have been reported. In a seminal example, perylene bisimides have been meta-alkylated with various alkyl halides under palladium catalysis and using CS2CO3 as the base (Scheme 19.25) [39]. The C-H activation step was again proposed to occur through the base-induced CMD mechanism (Scheme 19.18), and the exclusive functionaU2ation at the meta position can be correlated to the higher acidity of the meta C-H bond, as already shown in the related C-H arylation of electron-deficient arenes [29b,c]. 

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