Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium <i>tert-</i>Butoxyimido Compound
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Abstract
We report the first detailed reactivity
study of a group 4 alkoxyimido
complex, namely Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}(NO<sup>t</sup>Bu) (<b>19</b>), with heterocumulenes, aldehydes, ketones,
organic nitriles, Ar<sup>F<sub>5</sub></sup>CCH, and B(Ar<sup>F<sub>5</sub></sup>)<sub>3</sub> (Ar<sup>F<sub>5</sub></sup> = C<sub>6</sub>F<sub>5</sub>). Compound <b>19</b> was synthesized via imide/alkoxyamine
exchange from Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}(N<sup>t</sup>Bu) and <sup>t</sup>BuONH<sub>2</sub>. Reaction of <b>19</b> with CS<sub>2</sub> and Ar′NCO (Ar′ = 2,6-C<sub>6</sub>H<sub>3</sub><sup>i</sup>Pr<sub>2</sub>) gave the [2 + 2] cycloaddition
products Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}{SC(S)N(O<sup>t</sup>Bu)} and Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}{N(O<sup>t</sup>Bu)C(NAr′)O},
respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}{OC(NTol)N(Tol)C(NO<sup>t</sup>Bu)O} following
a sequence of cycloaddition–extrusion and cycloaddition–insertion
steps. Net NO<sup>t</sup>Bu group transfer was observed with both <sup>t</sup>BuNCO and PhC(O)R, yielding the oxo-bridged dimer [Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}(μ-O)]<sub>2</sub> and either the alkoxycarbodiimide <sup>t</sup>BuNCNO<sup>t</sup>Bu or the oxime ethers PhC(NO<sup>t</sup>Bu)R (R = H (<b>25a</b>), Me (<b>25b</b>), Ph (<b>25c</b>)). DFT studies showed that in the reaction with PhC(O)R
(R = H, Me) the product distribution between the <i>syn</i> and <i>anti</i> isomers of PhC(NO<sup>t</sup>Bu)R was
under kinetic control. Reaction of <b>19</b> with ArCN gave
the TiN<sub>α</sub> insertion products Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}{NC(Ar)NO<sup>t</sup>Bu} (Ar = Ph (<b>28</b>), 2,6-C<sub>6</sub>H<sub>3</sub>F<sub>2</sub> (<b>27</b>),
Ar<sup>F<sub>5</sub></sup> (<b>26</b>)) containing <i>tert</i>-butoxybenzimidamide ligands. Reaction of <b>19</b> or <b>26</b> with an excess of Ar<sup>F<sub>5</sub></sup>CN gave Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}{NC(Ar<sup>F<sub>5</sub></sup>)NC(Ar<sup>F<sub>5</sub></sup>)N(C{Ar<sup>F<sub>5</sub></sup>}NO<sup>t</sup>Bu)} (<b>29</b>) following net head-to-tail coupling of 2 equiv of Ar<sup>F<sub>5</sub></sup>CN across the TiN<sub>α</sub> bond
of <b>26</b>. Reductive N<sub>α</sub>–O<sub>β</sub> bond cleavage was observed with Ar<sup>F<sub>5</sub></sup>CCH, forming
Cp*Ti(O<sup>t</sup>Bu){NC(Ar<sup>F<sub>5</sub></sup>)C(H)N(<sup>i</sup>Pr)C(Ph)N(<sup>i</sup>Pr)} (<b>30</b>). Addition of 2 equiv
of [Et<sub>3</sub>NH][BPh<sub>4</sub>] to <b>19</b> in THF-<i>d</i><sub>8</sub> resulted in protonolysis of the amidinate
ligand, forming [PhC(NH<sup>i</sup>Pr)<sub>2</sub>][BPh<sub>4</sub>] and the cationic alkoxyimido complex [Cp*Ti(NO<sup>t</sup>Bu)(THF-<i>d</i><sub>8</sub>)<sub>2</sub>]<sup>+</sup>. In contrast, reaction
with B(Ar<sup>F<sub>5</sub></sup>)<sub>3</sub> resulted in elimination
of isobutene and formation of Cp*Ti{PhC(N<sup>i</sup>Pr)<sub>2</sub>}{η<sup>2</sup>-ON(H)B(Ar<sup>F<sub>5</sub></sup>)<sub>3</sub>}
