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Trisoxazolinylborate

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Class of chemical compounds
Trisoxazolinylborate ligand

Tris(oxazolinyl)borate compounds are a class of tridentate ligands; often abbreviated To, 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

Tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (To)
Tris(4S-isopropyl-2-oxazolinyl)phenylborate (To)

The first example of a trisoxazolinylborate ligand was tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (To). 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) have been prepared in an analogous manner.

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

Complexes of To

The first coordination complexes made using To ligands were based around zirconium (IV), as the sterically bulky ligands were able to stabilise the highly reactive metal centers. ToZr(IV) complexes were prepared by salt metathesis using LiTo and TlTo and ZrCl4. The formed complex ToZrCl3 was found to be quite robust and showed C3V symmetry in both solution and solid state.

Preparation of (To)ZrCl3 ligand

Lithium tris(4,4-dimethyl-2-oxazolin-2-yl) phenyl borate (LiTo) is used as a transfer agent. However TlTo frequently is as a more effective transfer agent than LiTo 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 can cause purification problems.

Another example for the coordinating chemistry of To is the formation of ToMgMe by the reaction of equimolar amounts of HTo and MgMe(O2C4H8)2. In addition, the reaction of two equivalents of HTo with MgMe2(O2C4H8)2 gives the homoleptic To2Mg compound. This compound can also be obtained by the reaction between one equivalent of HTo and ToMgMe revealing that Mg in ToMgMe is an active center for the chemical reactions. According to 1H NMR spectroscopic data, To2Mg shows Cs symmetry. In these reactions HTo is used as the transfer agent. Coordination chemistry of iridium(I) centers with To has been shown by the preparation of (COD =1,5-C8H12) by the reaction of LiTo and 0.5 equivalent of 2.

Preparation of (To)2Mg ligand

Catalysis

ToMgMe 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 (ToYR2) can be used as precatalysts for the cyclization of aminoalkenes.

See also

References

  1. ^ Dunne, J.F. (2011). Stoichiometric and catalytic reactivity of tris(oxazolinyl)borate main group metal compounds (PhD dissertation). Iowa State University. doi:10.31274/etd-180810-27.
  2. ^ Dunne, J.F.; Neal, S.R; Engelkemier, J; Arkady, E; Sadow, A.D. (2011). "Tris(oxazolinyl)boratomagnesium-Catalyzed Cross-Dehydrocoupling of Organosilanes with Amines, Hydrazine, and Ammonia". J. Am. Chem. Soc. 133 (42): 16782–5. doi:10.1021/ja207641b. PMID 21958426.
  3. ^ Baird, Benjamin; Pawlikowski, A.V; Su, J; Wiencg, J.W; Pruski, M; Sadow, A.D. (2008). "Easily Prepared Chiral Scorpionates: Tris(2-oxazolinyl)boratoiridium(I) Compounds and Their Interactions with MeOTf". Inorg. Chem. 47 (22): 10208–10. doi:10.1021/ic801637s. PMID 18921996.
  4. Dunne, J.F.; Su, J; Ellern, A; Sadow, A.D. (2008). "A New Scorpionate Ligand: Tris(4,4-dimethyl-2-oxazolinyl)borate and Its Zirconium(IV) Complexes". Organometallics. 27 (11): 2399. doi:10.1021/om800252p.
  5. Pawlikowski, A.V.; Ellern, A; Sadow, A.D. (2009). "Ligand Exchange Reactions and Hydroamination with Tris(oxazolinyl)borato Yttrium Compounds". Organometallics. 27 (16): 8020–9. doi:10.1021/ic900689k. PMID 19586044.
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