Synthesis and Crystal Structure of an Enantiomerically Pure, Internally Coordinated Alkylchloroborane. The Boron-Centered Anomeric Effect


The synthesis and crystal structure of (S)-(O-B)-chloro[(1S, 2R, 3S, 5S)-2-[2(R)-methoxyethyl]-6, 6-dimethyl-3- norpinyl]borane, an enantiomerically pure, internally coordinated alkylchloroborane monomer, are described. This is the first crystallographic structure determination for a chiral organoborane, and the first complete X-ray analysis of a monoborane-ether complex. The crystals are monoclinic of space group P21with Z = 2 in a unit cell of dimensions a = 7.468 (3) Å, b = 10.887 (3) Å, c = 7.898 (3) Å, and β = 95.29 (3)°. The structure was determined by a heavy atom method from 1676 unique reflections and refined to a final R value of 0.046. Key features of the structure, including the configuration of the stereogenic boron atom, the unusually long B-Cl bond (d = 1.890 Å), and the geometry of the trivalent oxygen, are manifestations of a novel boron-centered anomeric effect. An improved preparation of BH2Cl•SMe2 has also been devised. © 1985, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society