Alkyl derivatives center formation

Two major mechanisms have to be taken into consideration for the alkylation of Co -corrins. The classical mechanism of a bimolecular nucleophilic substitution reaction at carbon (the Co -corrin acts as a nucleophile) leads to /3-aUcylated Co -corrins with high diastereoselectivity. Secondly, an electron transfer-induced radical process (where the Co -corrin acts as a one-electron reducing agent) may also lead to cobalt alkylation. The observed formation of incomplete a-aUcylated Co -corrins under kinetically controlled conditions has been proposed to occur via this path. The high nucleophilic reactivity of Co -corrins and their diastereoselective nucleophilic reaction on the ( upper ) /3-face are not increased by the nucleotide function on the ( lower ) a-face rather they appear to be an inherent reactivity of the corrin-bound tetracoordinate Co -center. Among the organometallic B12 derivatives prepared to date, neopentylcobalamin, benzylcobalamin, and... 

However, it is noteworthy that oxygen atom transfer to the magnesium alkyl derivative does not occur rapidly at room temperature. This is presumably one of the factors that is responsible for the observation that the reactions of [Tp ]MgR with O2 are selective in the formation of alkylperoxo complexes, rather than the alkoxo derivatives. It is likely that this is a consequence of the sterically encumbered nature of the magnesium centers in these complexes. 

Fig. 4 Derivatives of the parent macrocycles O-alkylation of hexahomocalix[3]arenes (top) and hexahomotrioxacalix[3]arenes (center) formation of tetrahomodiazacalix[4]arene betaine (bottom).

Due to the equivalence of the phenolic units, it is now possible to construct in an unambiguous way all kinds of 0-alkylation products (from mono- to tetraethers), as with achiral calix[4]arenes. Of course, the symmetry is reduced in partially 0-alkylated derivatives. Mono- and triether derivatives are asymmetric (Ci) while 1,3-diether derivatives with the usual jyn-arrangement of the O-alkyl groups have C2 symmetry. An anri-1,3-diether (the formation of which is not observed under usual reaction conditions) would have an inversion center in its 1,2-altemate conformation, hence being achiral. 

Electrophilic substitution of thianthrene takes place at C-2. No examples of even minor amounts of 1-mono-substituted product have been reported. Disubstitution gives 2,7- (usually) or 2,8-products. In a few cases, 2,6-derivatives have been claimed. The presence of a sulfoxide or sulfone unit greatly reduces the susceptibility of either ring to electrophilic substitution. Carbon-centered electrophilic addition to sulfur to produce 5-R-thianthrenium salts has been described rarely most examples of the formation of such salts have involved the thianthrene radical ion(l-t-). Treatment of thianthrene with alkyl/aryllithiums produces the 1-lithio-species, and these organometallic derivatives allow the introduction of substituents at this position. 

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

Stereoselective formation of 3-alkyl-6-methoxy-2,5-piperazine-dione derivatives by the addition of methanol in the presence of NBS to 3-alkyl-6-alkylidene-2,5-piperazinediones was recently reported by Shin et al. 232 The asymmetric induction in this reaction was accomplished by the chiral center of a derivative of the natural proteinogenic chiral amino acid threonine. 

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