ABSTRACT
The enantioselective silylation of racemic alcohols, where one enantiomer reacts faster than the other, is an alternative approach to established enzymatic and non-enzymatic acylation techniques. The existing art is either limited to structurally biased alcohols or requires elaborate catalysts. Simple substrates, such as benzylic and allylic alcohols, with no coordinating functionality in the proximity of the hydroxy group have been challenging in these kinetic resolutions. We report here the identification of a broadly applicable chiral catalyst for the enantioselective dehydrogenative coupling of alcohols and hydrosilanes with both the chiral ligand and the hydrosilane being commercially available. The efficiency of kinetic resolutions is characterized by the selectivity factor, that is, the ratio of the reaction rates of the fast-reacting over the slow-reacting enantiomer. The selectivity factors achieved with the new method are good for acyclic benzylic alcohols (≤170) and high for synthetically usefully cyclic benzylic (≤40.1) and allylic alcohols (≤159).
ABSTRACT
A mild method for the regioselective C2-bromination of fused azine N-oxides is presented, employing tosic anhydride as the activator and tetra-n-butylammonium bromide as the nucleophilic bromide source. The C2-brominated compounds are produced in moderate to excellent yields and with excellent regioselectivity in most cases. The potential extension of this method to other halogens, effecting C2-chlorination with Ts(2)O/TBACl is also presented. Finally, this method could be incorporated into a viable one-pot oxidation/bromination process, using methyltrioxorhenium/urea hydropgen peroxide as the oxidant.
Subject(s)
Cyclic N-Oxides/chemistry , Hydrocarbons, Brominated/chemical synthesis , Catalysis , Halogenation , Hydrocarbons, Brominated/chemistry , Molecular Structure , StereoisomerismABSTRACT
Stereoselective Si-O couplings are auspicious processes for the synthesis of both chiral alcohols and chiral silanes. Attractive facets of this theme are currently enjoying a renaissance, and the several significant contributions are summarised in this Emerging Area.
ABSTRACT
Silicon- and tin-containing molecules are versatile building blocks in organic synthesis. A stalwart method for their preparation relies on the stoichiometric use of silicon- and tin-based cuprates, although a few copper(I)-catalyzed or even copper-free protocols have been known for decades. In this Concept, we describe our efforts towards copper(I)-catalyzed carbon--silicon and also carbon--tin bond formations using soft bis(triorganosilyl) and bis(triorganostannyl) zinc reagents as powerful sources of nucleophilic silicon and tin. Conjugate addition, allylic substitution, and carbon--carbon multiple bond functionalization is now catalytic in copper!
Subject(s)
Carbon/chemistry , Copper/chemistry , Organometallic Compounds/chemistry , Silicon/chemistry , Tin/chemistry , Catalysis , Molecular Structure , Stereoisomerism , Zinc/chemistryABSTRACT
A remarkable tert-butoxide-catalyzed coupling of alcohols and silanes is reported. Dihydrogen and not hydrochloric acid (generated in the prevalent, related coupling of alcohols and chlorosilanes) is formed as the sole by-product. A comprehensive survey of common silanes provides a reliable tool for the predictability of their reactivity under defined reaction conditions. The debated mechanism of this transformation is investigated monitoring the stereochemical course at the silicon atom by means of a silicon-stereogenic silane. On this basis, a transition state for the enantiospecific Si-O coupling step is suggested rationalizing the observed frontside attack and thus retention at the silicon atom.
