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Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium tert-Butoxyimido Compound

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journal contribution
posted on 23.12.2013, 00:00 by Laura R. Groom, Andrew D. Schwarz, Ainara Nova, Eric Clot, Philip Mountford
We report the first detailed reactivity study of a group 4 alkoxyimido complex, namely Cp*Ti­{PhC­(NiPr)2}­(NOtBu) (19), with heterocumulenes, aldehydes, ketones, organic nitriles, ArF5CCH, and B­(ArF5)3 (ArF5 = C6F5). Compound 19 was synthesized via imide/alkoxyamine exchange from Cp*Ti­{PhC­(NiPr)2}­(NtBu) and tBuONH2. Reaction of 19 with CS2 and Ar′NCO (Ar′ = 2,6-C6H3iPr2) gave the [2 + 2] cycloaddition products Cp*Ti­{PhC­(NiPr)2}­{SC­(S)­N­(OtBu)} and Cp*Ti­{PhC­(NiPr)2}­{N­(OtBu)­C­(NAr′)­O}, respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti­{PhC­(NiPr)2}­{OC­(NTol)­N­(Tol)­C­(NOtBu)­O} following a sequence of cycloaddition–extrusion and cycloaddition–insertion steps. Net NOtBu group transfer was observed with both tBuNCO and PhC­(O)­R, yielding the oxo-bridged dimer [Cp*Ti­{PhC­(NiPr)2}­(μ-O)]2 and either the alkoxycarbodiimide tBuNCNOtBu or the oxime ethers PhC­(NOtBu)­R (R = H (25a), Me (25b), Ph (25c)). DFT studies showed that in the reaction with PhC­(O)­R (R = H, Me) the product distribution between the syn and anti isomers of PhC­(NOtBu)­R was under kinetic control. Reaction of 19 with ArCN gave the TiNα insertion products Cp*Ti­{PhC­(NiPr)2}­{NC­(Ar)­NOtBu} (Ar = Ph (28), 2,6-C6H3F2 (27), ArF5 (26)) containing tert-butoxybenzimidamide ligands. Reaction of 19 or 26 with an excess of ArF5CN gave Cp*Ti­{PhC­(NiPr)2}­{NC­(ArF5)­NC­(ArF5)­N­(C­{ArF5}­NOtBu)} (29) following net head-to-tail coupling of 2 equiv of ArF5CN across the TiNα bond of 26. Reductive Nα–Oβ bond cleavage was observed with ArF5CCH, forming Cp*Ti­(OtBu)­{NC­(ArF5)­C­(H)­N­(iPr)­C­(Ph)­N­(iPr)} (30). Addition of 2 equiv of [Et3NH]­[BPh4] to 19 in THF-d8 resulted in protonolysis of the amidinate ligand, forming [PhC­(NHiPr)2]­[BPh4] and the cationic alkoxyimido complex [Cp*Ti­(NOtBu)­(THF-d8)2]+. In contrast, reaction with B­(ArF5)3 resulted in elimination of isobutene and formation of Cp*Ti­{PhC­(NiPr)2}­{η2-ON­(H)­B­(ArF5)3}.