2-PROPYLVALERONITRILE (13310-75-3) 's Synthetic route

Triazacyclohexane chromium triflate complexes as precursors for the catalytic selective olefin trimerisation and its investigation by mass spectrometry

K?hn, Randolf D.  Coxon, Alexander G.N.  Chunawat, Samaphon  Heron, Callum  Mihan, Shahram  Lyall, Catherine L.  Reeksting, Shaun B.  Kociok-K?hn, Gabriele

A novel set of linear α-olefin trimerisation pre-catalysts is presented. The R₃TACCr(OTf)₃ (R₃TAC = 1,3,5-trialkyl-1,3,5-triazacyclohexane) complexes produce an active system after facile activation with less than 10 equivalents of trialkylaluminium. Isomerisation was observed in many cases, which is proposed to occur via the π-allyl complex mechanism, though this was minimised by optimisation of the reaction conditions. The pre-catalysts can be synthesised from R₃TACCrCl₃ in neat TfOH or by addition of R₃TAC to Cr(OTf)₃ or better Cr(OTf)₃(THF)₃, the synthesis and structure of which is presented here. The use of this highly defined system allowed the identification of 2-methyl-1-hexene as a side product of activation with AlMe₃, in agreement with the proposed metallacyclic mechanism. Isomer distribution of the trimer product is similar to R₃TACCrCl₃/MAO activated systems and depends mostly on the ligand substituent R. ESI mass spectrometry of an ortho-difluorobenzene solution of the activated catalyst was analysed at different stages of reaction. A series of signals was observed that matched those expected for cationic chromium species predicted by the metallacyclic mechanism. In particular, [R₃TACCrMe₂]⁺ was observed to form immediately after alkylation with AlMe₃, while [R₃TACCr(olefin)_n]⁺ (n = 0, 1, 2, 3, 4 and olefin = 1-hexene or 1-octene) formed after addition of 1-hexene. Absence of any detected tetramer and observation of [R₃TACCr(olefin)₄]⁺ leads to the conclusion that a metallacycloheptane-olefin complex may lead to β-H abstraction rather than insertion and may even be required to avoid formation of [R₃TACCr(olefin)₁]⁺ which is observed in only very small amounts indicating a low stability. Chromium(I) complexes [R₃TACCr(arene)]⁺ with the arene solvent can also be observed and their signal intensity indicate a relative stability of o-C₆H₄F₂ < cymene < benzene < PhF, toluene < p-xylene.