Allyl Derivatives of Various Metals

Many metal allyls and their derivatives may be prepared from an allyl Grignard reagent.1-4 However, the conditions for the 

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Chemical shifts have been reported for [Me3CCH2HgX] 203) (see Table XII), tr-allylic derivatives (see Table XIII), o-cyclopentadienyl derivatives (see Table XIV), and miscellaneous compounds (see Table XV). In the case of fluxional a-cyclopentadienyl derivatives, it has been shown that chemical shifts can be used to differentiate between fluxional o-cyclopentadienyl derivatives and w-cyclopentadienyl derivatives 103). The NMR spectrum of [Sb(cyclopropyl)s] has only two carbon resonances, showing that the molecule is fluxional (56) and the NMR spectrum of the naturally occurring organometallic compound, 5 -deoxyadenosylcobalamin, has been reported (65). For a number of norbornane and allylic derivatives of palladium and platinum, a number of linear relationships have been found between chemical shifts of various carbon atoms. It was suggested that a term due to paramagnetic shielding by the metal was dominant 52b). 

Dienes and allylarcncs can be prepared by the Pd-catalyzcd coupling of allylic compounds with hard carbon nucleophiles derived from alkenyl and aryl compounds of main group metals. Allylic compounds with various leaving groups can be used. Some of them are unreactive with soft nucleophiles, but... 

Monoanions derived from nitroalkanes are more prone to alkylate on oxygen rather than on carbon in reactions with alkyl halides, as discussed in Section 5.1. Methods to circumvent O-alkylation of nitro compounds are presented in Sections 5.1 and 5.4, in which alkylation of the a.a-dianions of primary nitro compounds and radial reactions are described. Palladium-catalyzed alkylation of nitro compounds offers another useful method for C-alkylation of nitro compounds. Tsuj i and Trost have developed the carbon-carbon bond forming reactions using 7t-allyl Pd complexes. Various nucleophiles such as the anions derived from diethyl malonate or ethyl acetoacetate are employed for this transformation, as shown in Scheme 5.7. This process is now one of the most important tools for synthesis of complex compounds.6811-1 Nitro compounds can participate in palladium-catalyzed alkylation, both as alkylating agents (see Section 7.1.2) and nucleophiles. This section summarizes the C-alkylation of nitro compounds using transition metals. 

The metal-catalyzed asymmetric epoxidation of allylic alcohols with various enan-tiomerically pure hydroperoxides has been studied by several groups. This approach has been employed in the Ti- and V-mediated epoxidation of this class of substrates, in the presence of different achiral additives with modest enantioselectivities (ee ee < 46% ), which turned satisfactory (ee 72%) in the presence of the TADDOL-derived hydroperoxide TADOOH 73 . This oxidant has been recently employed in the oxovanadium sandwich-type POM [ZnW(V0)2(ZnW9034)2] catalyzed epoxidation of various allylic alcohols with very high catalytic efficiency (42000 turnovers) and enantiomeric ratios up to 95 5 98. 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

A unique feature of the transition-metal-catalyzed allylic alkylation is its ability to convert starting materials of various symmetry types, such as racemic, meso- and achiral compounds, into optically pure material, Strategies to effect such transformations derive from recognition of the stereochemical courses in each step of the catalytic cycle and analysis of symmetry elements in the substrate or intermediate. Figure 8E.1 summarizes potential sources of enantio-discrimination in transition-metal-catalyzed allylic alkylation. 

Emphasis on the employment of transition metal catalysts to achieve useful synthetic transformations is illustrated by three procedures in this volume. The reaction of allylic alcohols with aryl halides in the presence of a palladium derived catalyst can be used to prepare various /3-arylaldehydes. This is illustrated by the preparation of 2-METHYL-... 

Several papers are concerned with the threo-erythro stereoselectivity of the reaction of allylic organozinc reagents with carbonyl compounds. The addition (involving allylic rearrangement) of crotylzinc derivatives to various aldehydes occurs stereoselectively, and the relative amount of tAreo-alcohol increases with increasing steric demand of the group R tert-Bu, S4% f-Pr, 707o n-Bt, 46%) and in the sequence of metals Mg < Zn < Cd 3, 7). The temperature or the polarity of the solvent... 

A number of methods to increase the thermal stability of Kdbrich reagents have centered around replacement of the lithium counterion normally present with a variety of transition metals, chiefly titanium, hafnium and copper." For example, the reagent derived from transmetalladon of lithium dichlorometh-ane with titanium isopropoxide could be reacted with various ketones and aldehydes to afford halohydrin adducts in good yields at temperatures as high as 0 °C. Competition experiments established the chemoselectivity of this reagent (Scheme 14). The direct activation of severid allylic halides has been accomplished with tin and chromium ° reagents (equation 26). Transmetallation of an alkyllead precursor has also been noted. 

Much of the recent interest in the arylstannanes, particularly the aryltrialkylstannanes, however, derives from their use in the Stille reaction in which an aryl-C bond is generated by reaction with a halide or sulfonate RX, where R = vinyl, allyl, benzyl, or aryl, to give Ar-R under catalysis by palladium68 or another transition metal.69-72 The reactions are very tolerant of various functional groups, and are finding wide application in organic synthesis, and are considered in Section 22.2. 

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