Preparation of Functionalized Dialkylzincs via a Boron−Zinc Exchange. Reactivity and Catalytic Asymmetric Addition to Aldehydes

Abstract

The hydroboration of olefins with Et(2)BH provides diethyl(alkyl)boranes 2 which readily undergo a boron-zinc exchange with Et(2)Zn providing a range of polyfunctional primary, secondary, and benzylic diorganozincs. The resulting diorganozincs 3 have been reacted with various electrophiles (allylic halides, acid chlorides, alkylidenemalonates, ethyl propiolate, nitroolefins) in the presence of CuCN.2LiCl with excellent yields. With secondary dialkylzincs prepared from diastereomerically pure diethyl(alkyl)boranes, the boron-zinc exchange occurs with loss of stereochemistry. The asymmetric addition of 3 to aldehydes in the presence of the chiral catalyst 55 furnishes optically active polyfunctional secondary alcohols (50 to over 96% ee).

Self-Assembly of 1,1′-Biphenyl-2,2′,6,6′-tetracarboxylic Acid: Formation of an Achiral Grid with Chiral Compartments

“…Tetrafunctional molecules with tetrahedrally arranged substituents have been investigated in this context and have shown to be endowed with the fascinating faculty of building diamond-like networks, [3] with methanetetraacetic acid 1 being a prototypal example. [4] We are interested in the hydrogen bond driven self-assembly of 2,2',6,6'-tetrasubstituted biaryls [5,6] that are topologically related to the tetrasubstituted methanes. For example, if we stretch the D 2d symmetry conformer of the tecton 1 along its axis a framework of 1,1'-biphenyl-2,2',6,6'-tetracarboxylic acid (2) gradually (on further elaboration) emerges (Scheme 1).…”

Peptidic Amidomonophosphane Ligand for Copper‐Catalyzed Asymmetric Conjugate Addition of Diorganozincs to Cycloalkenones

“…Table 2. Preparation of Dialkylzinc [35] In a Schlenk tube, hydroboration was performed by mixing alkene (10 mmol) and diethylborane, freshly prepared by mixing triethylborane (1.4 mL, 10 mmol) and borane-dimethyl sulfide complex (0.47 mL, 5 mmol) in a Schlenk tube at 0 8C for 5 min. After completion by stirring at room temperature, the solvent was carefully distilled off (0.5 mm Hg, 0 8C) for 0.5 h to afford a crude product, which was treated with a hexane solution of diethylzinc (2.0 mL, 20 mmol) at 0 8C.…”

Catalytic Asymmetric Alkylation of Aldehydes with Grignard Reagents

“…[1] Less reactive reagents, typically diorganozinc derivatives, [2,3] are used to achieve this transformation because a direct, noncatalyzed reaction would deminish the enantioselectivity. [4][5][6] The reaction of Grignard reagents with carbonyl compounds is one of the most reliable methods for forming carbon-carbon bonds.…”