Reaction site diversity in the reactions of titanium hydrazides with organic nitriles, isonitriles and isocyanates: Ti=N_α cycloaddition, Ti=N_α insertion and N_α-N _β bond cleavage

摘要

We report a range of new transformations of the diamide-amine supported Ti=NNPh_2 functional group with a variety of unsaturated substrates, along with DFT studies of the key mechanisms. Reaction of [Ti(N _2N~(py))(NNPh_2)(py)] (4, N_2N ~(py)=(2-NC_5H_4)CMe(CH_2NSiMe _3)_2; py=pyridine) with MeCN gave the dimeric species [Ti_2(N_2N~(py))_2{μ-NC(Me)(NNPh _2)}_2] through a [2+2] cycloaddition process. Reaction of 4 or [Ti(N_2N~(Me))(NNPh_2)(py)] (5, N _2N~(Me)=MeN(CH_2CH_2NSiMe _3)_2) with fluorinated benzonitriles gave the terminal hydrazonamide complexes [Ti(N_2N~R){NC(Ar F x)NNPh _2}(py)] (R=py or Me; Ar F x=2,6-C_6H_3F _2 or C_6F_5). DFT studies showed that this proceeds through an overall [2+2] cycloaddition-reverse cycloaddition, resulting in net insertion of Ar F xCN into the Ti=N_α bonds of the respective hydrazides. Reaction of 4 with a mixture of MeCN and PhCCMe gave the metallacycle [Ti(N_2N~(py)){NC(Me)C(Ph)C(Me)NNPh_2}] by sequential coupling of Ti=NNPh_2 with PhCCMe and then MeCN. A related product, [Ti(N_2N~(py)){NC(Me)C(Ar~F)C(H) NNPh_2}], was formed by insertion of MeCN into the Ti-C bond of the isolated azatitanacyclobutene [Ti(N_2N~(py)){N(NPh _2)C(H)C(Ar~F)}] (Ar~F=3-C_6H _4F). Reaction of 4 with two equivalents of B(Ar F 5)_3 (Ar F 5=C_6F_5) formed the zwitterionic borate [Ti(N _2N~(py)){?·~2-N(NPh_2)B(Ar F 5)_3}] by electrophilic attack at N_α. Compounds 4 and 5 reacted with tBuNC and/or XylNC (Xyl=2,6-C_6H_3Me _2) to give the N_α-N_β bond cleavage products, [Ti(N_2N~R)(NCNR′)(NPh_2)] (R=py or Me; R′=tBu or Xyl), containing metallated carbodiimide ligands. DFT studies of these reactions found an initial addition of RNC across Ti=N _α followed by N_β coordination, and finally complete N_α transfer from the NNPh_2 to the RNC fragment. Reaction of 5 with Ar′NCE (E=O, S, Se; Ar′=2,6-C _6H_3iPr_2) gave the [2+2] cycloaddition products [Ti(N_2N~(Me)){N(NPh_2)C(NAr′)O}(py)] and [Ti(N_2N~(Me)){N(NPh_2)C(NAr′)E}] (E=S or Se), which did not undergo further transformation of the Ti-N-NPh_2 moiety. Ti-hydrazide transformations: A range of transformations of the Ti=NNPh_2 functional group with unsaturated organic substrates leading to Ti=N_α [2+2] cycloaddition, Ti=N_α insertion or N_α-N_β bond cleavage products (see scheme) is reported. A DFT study of the mechanisms of the last two reaction types is described.