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Organic Syntheses via Transition Metal Complexes, 35. - (C-Amino)Ketene Imines, 2-Imidazolin-5-ones, and Α-Amino Acids from Aminocarbene Chromium Complexes and Isocyanides
Aumann, Rudolf,Heinen, Heinrich
, p. 1139 - 1146 (2007/10/02)
C-(Alkylideneamino)ketene imines (1,4-diaza-1,2,4-pentatrienes) 8 are easily accessible by C=C coupling of isocyanides R1-NC (2, R1 = t-C4H9, c-C6H11, CH3) with the (alkylideneamino)carbene (= 2-azaallenyl) ligand of (CO)5Cr-+=C(C6H5)OCOC6H5> (6).The tendency for an insertion of 2 into M=C bonds of aminocarbene complexes strongly depends on the electronic character of the aminocarbene ligand.Electron-rich aminocarbene chromium complexes like (CO)5Cr=C(NR2)C6H5 (1, R = H, CH3), show only little tendency for an insertion but mainly give cis-(R1-NC)(CO)4Cr=C(NR2)C6H5 (3) by susbtitution of CO.Electron-poor aminocarbene complexes like 6 (readily available on N-benzoylation of 1a) undergo a facile insertion of 2 into the M=C bond with smooth formation of 7.From 7 ketene imine 8 is spontaneously disengaged by a second equivalent of 2.The reaction works especially well with bulky isocyanides 2a, b.In the case of methyl isocyanide (2c) a chromium complex 15 is obtained as a minor product besides 8c. 15 results from a cycloaddition of 2c at the ketene imine ligand of 7c.C-(Alkylideneamino)ketene imines 8 are easily accessible by our method.They prove to be very thermolabile in solution and spontaneously isomerize to give 2-imidazolin-5-ones 10 and C-amidoketene imines 11.Under the influence of wet silica gel ketene imine derivatives 8 rapidly form α-amino acid amides 16 by hydrolysis with concomitant migration of a benzoyl group.Under the same conditions 11 also gives the "normal" α-amino acid amides 17. - Keywords: C-(Alkylideneamino)ketene imines; Isocyanides, C=C coupling with aminocarbene ligands; (2-Azaallenyl)chromium complexes; 2-Imidazolin-5-ones; α-Amino acids
