An intermolecular dative B←N bond in 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-thiazole

The title compound, C9H14BNO2S, is in an unusual bend conformation and the B atom of one molecule within the crystal forms an intermolecular dative bond with the N atom of a neighbouring molecule, an infrequent phenomenon in boronic derivative crystals.

The title compound, C 9 H 14 BNO 2 S, is in an unusual bend conformation and the B atom of one molecule within the crystal forms an intermolecular dative bond with the N atom of a neighbouring molecule, an infrequent phenomenon in boronic derivative crystals.
For the molecular modelling software, we thank the CRIHAN, the 'Ré gion Haute-Normandie' and the European Community (FEDER).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: DN2507).

Comment
The thiazole ring is a widespread heterocycle found in various biologically active natural products like vitamin B1 and penicillins (Dondoni & Merino, 1996;Kalgutkar et al., 1996;Hutchinson et al., 2000), and also in many peptides and peptolides isolated from marine organisms (Ogino et al., 1996;Williams & Jacobs, 1993;Faulkner, 1998). The search of new regioselective methods for the preparation of new thiazole derivatives is always a matter of interest.
As a part of our study of 1,3-azolylboronic derivatives, we are focused on the synthesis of the new thiazol-5-ylboronic acid pinacol ester (Primas et al., 2009). Indeed, this new boronic ester permits a facile synthetic route to 5-(het)arylthiazoles via a Suzuki-Miyaura cross-coupling reaction with various (het)halides. This boronic ester was stable under aqueous conditions in the Suzuki process (Primas et al., 2008). To date only a few crystallographic studies have been published on such heterocyclic compounds. We carried out this study with the aim to confirm the structure of the title compound and to find an explanation to its greater stability than its imidazole analogue (Primas et al., 2008).
The structure shows that the molecule is in an unusual bent conformation, thus the thiazole cycle and the dioxaborolane ring of the boronic ester forms an angle of about 55.0(0.05)° (Fig. 1). Usually, in the boronic esters deposed in Cambridge Structural Database (CSD, Version; Allen 2002) as well as in the ones solved previously in our laboratory, the ester ring is coplanar to the aromatic ring (Sopková-de Oliveira Santos et al., 2003a,b).
In the crystal structure the boron atom is the peak bending, and it is committed to the B←N dative bond with N of the neighbouring molecule (-x,y -1/2, -z + 1/2), which leads to a tetracoordinated B atom in the crystal. As it was already published (Hall, 2005), the formation of tetracoordinate B influences all bond lenghts in the boron vicinity. The observed B-O bond lengths, 1.4351 (13)Å and 1.4447 (13) Å, are in agreement with the ones reported when B is tetracoordinated (Hall, 2005), between 1.43-1.47 Å. The observed C-B distance is about 1.6207 (15)Å which is closed to the usually observed value for tetracoordinate boron, 1.613Å (Rettig & Trotter, 1975). However, the B←N dative bond observed in the crystal is shorter with respect to the published value, its length is about 1.6354 (14) Å. Furthermore, the calculated parameter describing the tetrahedral character of boron (THC DA ; Höpfl, 1999) is in the title compound of 81% which is a high value and shows that the formed B←N dative bond is a strong one. The existence of this strong B←N dative bond could explain the stability of this boronic ester even not only in the solid state but also in the solution. Further studies concerning this phenomenon are currently in progress.
The dioxaborolane ring of the boronic ester is in a half-chair conformation with an O1-C6-C7-O2 torsion angle of about -37.74(0.10)°.
The crucial element of the crystal packing is of course this intermolecular dative B←N bond. The interacting neighbouring molecules form a strand along the b axis (Fig. 2). Some electrostatic interactions occur between these strands, the strongest seems to be an electrostatic interaction between O1 and H2-C2 of symmetrically related molecule ( The title compound was synthesized from 2-trimethylsilylthiazole using the method described by Primas et al. (2009).
Crystals of (I) suitable for X-ray analysis were obtained by slow evaporation from diethyl ether at room temperature.

Refinement
All non-hydrogen atoms were refined anisotropically. All H atoms were determined via difference Fourier map and refined with isotropic atomic displacement parameters with exception on H atoms on methyl groups which were calculated and fixed on the atoms in the ideal geometry (distance 0.96 Å). Fig. 1. View of the title compound showing the labelling scheme of the non-hydrogen atoms. Thermal ellipsoids are shown at the 50% probability levels; hydrogen atoms are drawn as small circles of arbitrary radii. 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-thiazole  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.