Regioselectivity metal-mediated reactions

Other metal-mediated reactions of azide reagents to terminal alkynes have also been reported. Indium(ll) triflate catalyzed tandem azidation/l,3-dipolar cycloaddition of various (o,(o-dialkoxyalkynes 134 with trimethylsilyl azide yielded fused 1,2,3-triazoles 135 <05TL8639>. A new ruthenium-catalyzed process for the regioselective synthesis of 1,5-disubstituted-1,2,3-triazoles has been developed <05JA15998>. 

Development of a highly regioselective metal-mediated allylation reaction IN AQUEOUS MEDIA... 

In conclusion, a general regioselective metal-mediated allylation reaction has been developed. It has been demonstrated that indium, tin and zinc can be used to carry out this reaction, giving moderate to good yields and selectivities. Table 8.1 gives a brief summary of the different aldehydes and allyl bromides which are used to synthesize homoallylic alcohols in this procedure. 

Transition metal-catalyzed carbenoid transfer reactions, such as alkene cyclopro-panation, C-H insertion, X-H insertion (X = heteroatom), ylide formation, and cycloaddition, are powerful methods for the construction of C-C and C-heteroatom bonds [1-6]. In contrast to a free carbene, metallocarbene-mediated reactions often proceed stereo- and regioselectively under mild conditions with tolerance to a wide range of functionalities. The reactivity and selectivity of metallocarbenes can be... 

Recently, Zhu and coworkers have investigated the reaction of the zinc reagent from ethyl 3-bromodifluoromethyl-3-benzyloxyacrylate. The exclusive zinc reagent formed in DMF was the -carbon metallated product (equation 88)82. These workers utilized this reaction to prepare -dilluorovinyl substituted / -hydroxyesters in high yields and high regioselectivity via the reactions of aldehydes mediated by zinc (equation 89)82. 

It allows reaction products to be observed from reactions under either kinetic or thermodynamic control. Numerous reports are available, where ion trap MS is applied in ion chemistry studies [68], e.g., involving reactions between 1,4-benzodiazepines and dimethyl ether ions [77], dissociation of [Alanine + Alkali cation]+-ions to study the role of the metal cation [78], or regioselective ion-molecule reactions to enable MS differentiation of protonated isomeric aromatic diamines [79]. The three-dimensional ion trap mass spectrometer has even been described as a complete chemical laboratory for fundamental gas-phase studies of metal-mediated chemistry [80]. 

Qualitatively, most metal-mediated allylation reactions in aqueous medium display similar regio- and stereoselectivities. Such studies have been carried out most extensively on the indium-mediated allylations. For regioselectivities of the allylic moiety, both electronic and steric effects are important. Usually, the carbon-carbon bond formation occurs at the more substituted carbon of the allyl halide, irrespective of the position of halogen in the starting material (Eq. 4.28). However, the carbon-carbon bond forms at the less substituted carbon when the y-substituents of allyl halides are large enough (e.g. trimethylsilyl or tert-butyl) as shown by Chan and Isaac. 

Metal-mediated carbonyl allylation, allenylation, and propargylation of optically pure azetidine-2,3-diones were investigated in aqueous environments. The regioselectivity of these reactions depended on the type of metal promoters that were used. The configuration of the new C3-substituted C3-hydroxy quaternary center was controlled by placing a chiral auxiliary at C4. 

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