TETRAHYDRO-ARYL-4H-1,3,6,2-DIOXAZABORACINES. CRYSTAL STRUCTURE OF TETRAHYDRO-2-BIPHENYL- 4H-1,3,6,2-DIOXAZABORACINE

Intramolecular B-N coordination occurs in tetrahydro-2-biphenylyl-4//-l,3,6,2-dioxazaboracine, (2: R = o-PhC6H4, R' = H) both in the solid state and in CDCl3 solution. Molecules are linked, in the solid state, into spiral chains, via N-H—Ο hydrogen bonds utilising only one of the two oxygen atoms present in each molecule. Enantiomeric pairs of two different rotamers are present in the solid state structure. The intramolecular B-N bond length in the rotamers are 1.658(4) and 1.666(5) Ä; other geometric parameters are B-O bond lengths between 1.448(4) and 1.464(4) A, BC bond lengths 1.612(5) and 1.620(5) A, and bond angles at Β between 100.9(3) and 114.1(3)°. The δ π Β value for (2: R = o-PhQHL,, R = H) at 25 °C = 10.7 ppm): NMR spectra of other (2: R =aryl, R = H) compounds are also reported. INTRODUCTION Compared to simple boronic acid esters, RB(OR')2 (1), tetrahydro-aryl-4/7-1,3,6,2dioxazaboracine compounds, (2), derived from diethanolamines and mono-organoboranes, show much enhanced hydrolytic stabilities.' This is generally considered to be a consequence of the N3 7 · 11 Β intramolecular co-ordination " revealed both in solution, e.g. by Β NMR spectroscopy for (2: R=alkyl or aryl), and in the solid state, e.g. by X-ray crystallography for (2: R = Ph, R = H). The 4-co-ordinate boron centre in 2 in solution was suggested by the δ π Β values of ca 10-15 ppm, compared to lower field values for the three co-ordinate compounds, 1, e.g. values of δ π Β are 10.65 and 28.6 ppm for (2: R = Ph, R = H) and (1: R = Ph; R = Et), respectively.