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Catalytic methods deprotonation

In this chapter, catalytic methods for ligand synthesis are described in detail. In spite of that, the enantioselective deprotonation of tert-butyldimethylpho-sphine borane with a catalytic amount of (—)-sparteine or a (+)-sparteine surrogate, reported by O Brien and co-workers, is included for convenience in Chapter 5, Section 5.4.2, following the discussion on the general strategy of desymmetrisation by enantioselective deprotonation. The coverage of Section 6.2 is mainly limited to systems in which the chiral catalyst acts in the step where the... [Pg.292]

Enantioselective deprotonations of meso substrates such as ketones or epoxides are firmly entrenched as a method in asymmetric synthesis, although the bulk of this work involves stoichiometric amounts of the chiral reagent. Nevertheless, a handful of reports have appeared detailing a catalytic approach to enantioselective deprotonation. The issue that ultimately determines whether an asymmetric deprotonation may be rendered catalytic is a balance of the stoichiometric base s ability... [Pg.294]

Enantioselective catalytic alkylation is a versatile method for construction of stereo-genic carbon centers. Typically, phase-transfer catalysts are used and form a chiral ion pair of type 4 as an key intermediate. In a first step, an anion, 2, is formed via deprotonation with an achiral base this is followed by extraction in the organic phase via formation of a salt complex of type 4 with the phase-transfer organocata-lyst, 3. Subsequently, a nucleophilic substitution reaction furnishes the optically active alkylated products of type 6, with recovery of the catalyst 3. An overview of this reaction concept is given in Scheme 3.1 [1],... [Pg.13]

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. [Pg.89]

This is the first volume of Inorganic Syntheses that has a special section (Chap. 1) devoted to syntheses of compounds that are of particular interest in the solid state. We hope that more syntheses of this type will appear in future volumes. This volume is also notable for the inclusion of reliable methods for preparing certain coordination compounds which are presently of great interest because of their catalytic activity. (See S3Titheses 18, 19, and 20.) Another unique feature is Chap. 4, which is devoted entirely to organometallic syntheses that illustrate the use of potassium hydroxide as a deprotonating agent. [Pg.236]

Dehydrogenation. Reetz and Eibach have developed a new method for dehydrogenation of dihydroarenes based on deprotonation-hydride elimination. Potassium fencholate (formed by reduction of fenchone with KH) serves as base and fenchone (1) as hydride acceptor. Since potassium fencholate is regenerated in the aromatization, only catalytic quantities are required. Yields of arenes are 70-85% (isolated). The method is not useful for dehydrogenation to form alkenes. Fenchone is particularly suitable since it is reduced by hydride transfer because of steric reasons,... [Pg.119]

The cross-coupling of aryl halides and enolates is a powerful method to generate new C-C bonds and it has been extensively investigated using various palladium catalysts [70]. Extremely active NHC/Pd systems have been reported for the a-arylation of esters at room temperature [71] and for the arylation of amides [72]. Ketones that possess a-protons can be deprotonated in the presence of strong bases, a-Arylation of ketones can be performed in the presence of a catalytic amount of (NHC)Pd(allyl)Cl as catalyst and NaO Bu as base (Scheme 19). [Pg.259]


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