Trisoxazolinylborate

Tris(oxazolinyl)borate compounds are a class of tridentate ligands; often abbreviated To^R, where R is the substituent on the oxazoline ring. Most commonly the substituent is either a methyl, propyl, tert-butyl or hydrogen. The formation of anionic boron backbone with addition of a phenyl group on boron allows the ligand to strongly bind to the metal center. It results in a more robust complex.

Tris(oxazolinyl)borates can be characterised as scorpionate ligands and may be compared to tris(pyrazolyl)borate and trisoxazoline ligands. In bulky pyrazolylborate (Tp) derivatives, isomerization may occur via 1,2-shifts; additionally B–N bond cleavage is a common decomposition pathways for the Tp ligands. The oxazoline-based ligands with B-C linkages avoid these decomposition problems.

Synthesis

The first example of a trisoxazolinylborate ligand was tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (To^M). This was prepared by a reaction of dichlorophenylborane with 3 equivalents of 2-lithio-4,4-dimethyl-2-oxazolide. Later variants, such as tris(4S-isopropyl-2-oxazolinyl)phenylborate (To^P) have been prepared in an analogous manner.

PhBCl₂ + 3 Li(Oxaz-Me₂) → PhB(Oxaz-Me₂)₃ + 3 LiCl

Complexes of To^R

The first coordination complexes made using To^M ligands were based around zirconium (IV), as the sterically bulky ligands were able to stabilise the highly reactive metal centers. To^MZr(IV) complexes were prepared by salt metathesis using LiTo^M and TlTo^M and ZrCl₄. The formed complex To^MZrCl₃ was found to be quite robust and showed C₃V symmetry in both solution and solid state.

Lithium tris(4,4-dimethyl-2-oxazolin-2-yl) phenyl borate (LiTo^M) is used as a transfer agent. However TlTo^M frequently is as a more effective transfer agent than LiTo^M because of the higher solubility of the Tl salt and the insolubility of thallium chloride by-products. In contrast, lithium halide byproducts from preparations employing LiTo^M can cause purification problems.

Another example for the coordinating chemistry of To^M is the formation of To^MMgMe by the reaction of equimolar amounts of HTo^M and MgMe²(O₂C₄H₈)₂. In addition, the reaction of two equivalents of HTo^M with MgMe₂(O₂C₄H₈)₂ gives the homoleptic To^M₂Mg compound. This compound can also be obtained by the reaction between one equivalent of HTo^M and To^MMgMe revealing that Mg in To^MMgMe is an active center for the chemical reactions. According to 1H NMR spectroscopic data, To^M₂Mg shows C_s symmetry. In these reactions HTo^M is used as the transfer agent. Coordination chemistry of iridium(I) centers with To^P has been shown by the preparation of [Ir(To^P)(COD)] (COD =1,5-C₈H₁₂) by the reaction of LiTo^P and 0.5 equivalent of [Ir(μ-Cl)(COD)]₂.

Catalysis

To^MMgMe is an effective precatalyst for the cross-dehydrocoupling of Si-H bonds in organosilanes and N-H bonds in amines to give Si-N bonds and H₂. Furthermore, tris(oxazolinyl)borate yttrium alkyl and amide compounds (To^MYR₂) can be used as precatalysts for the cyclization of aminoalkenes.

See also

• Trisoxazolines
• Trispyrazolylborate

References