Reaction site diversity in the reactions of titanium hydrazides with organic nitriles, isonitriles and isocyanates: Ti=Nα cycloaddition, Ti=Nα insertion and Nα-Nβ bond cleavage
Résumé
We report a range of new transformations of the diamide–amine supported TiNNPh2 functional group with a variety of unsaturated substrates, along with DFT studies of the key mechanisms. Reaction of [Ti(N2Npy)(NNPh2)(py)] (4, N2Npy=(2-NC5H4)CMe(CH2NSiMe3)2; py=pyridine) with MeCN gave the dimeric species [Ti2(N2Npy)2{μ-NC(Me)(NNPh2)}2] through a [2+2] cycloaddition process. Reaction of 4 or [Ti(N2NMe)(NNPh2)(py)] (5, N2NMe=MeN(CH2CH2NSiMe3)2) with fluorinated benzonitriles gave the terminal hydrazonamide complexes [Ti(N2NR){NC(Armath image)NNPh2}(py)] (R=py or Me; Armath image=2,6-C6H3F2 or C6F5). DFT studies showed that this proceeds through an overall [2+2] cycloaddition–reverse cycloaddition, resulting in net insertion of Armath imageCN into the TiNα bonds of the respective hydrazides. Reaction of 4 with a mixture of MeCN and PhCCMe gave the metallacycle [Ti(N2Npy){NC(Me)C(Ph)C(Me)NNPh2}] by sequential coupling of TiNNPh2 with PhCCMe and then MeCN. A related product, [Ti(N2Npy){NC(Me)C(ArF)C(H)NNPh2}], was formed by insertion of MeCN into the TiC bond of the isolated azatitanacyclobutene [Ti(N2Npy){N(NPh2)C(H)C(ArF)}] (ArF=3-C6H4F). Reaction of 4 with two equivalents of B(Armath image)3 (Armath image=C6F5) formed the zwitterionic borate [Ti(N2Npy){η2-N(NPh2)B(Armath image)3}] by electrophilic attack at Nα. Compounds 4 and 5 reacted with tBuNC and/or XylNC (Xyl=2,6-C6H3Me2) to give the NαNβ bond cleavage products, [Ti(N2NR)(NCNR′)(NPh2)] (R=py or Me; R′=tBu or Xyl), containing metallated carbodiimide ligands. DFT studies of these reactions found an initial addition of RNC across TiNα followed by Nβ coordination, and finally complete Nα transfer from the NNPh2 to the RNC fragment. Reaction of 5 with Ar′NCE (E=O, S, Se; Ar′=2,6-C6H3iPr2) gave the [2+2] cycloaddition products [Ti(N2NMe){N(NPh2)C(NAr′)O}(py)] and [Ti(N2NMe){N(NPh2)C(NAr′)E}] (E=S or Se), which did not undergo further transformation of the TiNNPh2 moiety.