# 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.
    PhBCl$_2$ + 3 Li(Oxaz-Me$_2$) → PhB(Oxaz-Me$_2$)$_3$ + 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$^M$Zr(IV) complexes were prepared by salt metathesis using LiTo$^M$ and TlTo$^M$ and ZrCl$_4$. The formed complex To$^M$ZrCl$_3$ was found to be quite robust and showed C$_3$V 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$^M$MgMe by the reaction of equimolar amounts of HTo$^M$ and MgMe$^2$(O$_2$C$_4$H$_8$)$_2$. In addition, the reaction of two equivalents of HTo$^M$ with MgMe$_2$(O$_2$C$_4$H$_8$)$_2$ gives the homoleptic To$^M$$_2$Mg compound. This compound can also be obtained by the reaction between one equivalent of HTo$^M$ and To$^M$MgMe revealing that Mg in To$^M$MgMe is an active center for the chemical reactions. According to 1H NMR spectroscopic data, To$^M$$_2$Mg shows C$_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-C$_8$H$_{12}$) by the reaction of LiTo$^P$ and 0.5 equivalent of [Ir(μ-Cl)(COD)]$_2$.

## Catalysis

To$^M$MgMe 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 H$_2$. Furthermore, tris(oxazolinyl)borate yttrium alkyl and amide compounds (To$^M$YR$_2$) can be used as precatalysts for the cyclization of aminoalkenes.