Nitrogen Heterocycles from Carbene Complexes and Alkenyl Isocyanides

Abstract

Ketenimine complexes are readily accessible in good yields by the addition of isocyanides to carbene complexes.[I1 They may be used, after isolation or in situ, for further transformations. Their reactivity patterns are completely different from those of ketenimines free of metals. Herein we describe novel template syntheses of four-, five-, and six-membered N-heterocycles by means of such complexes.Complex 1 reacts rapidly with alkenyl isocyanides 2 at 0°C in hexane to form the 1 : 1 adducts (3-aza-1,2,4-pentatriene complexes) 3. These can be trapped as aminocarbene complexes with protic nucleophiles."I In the absence of a suitable reactant, 3 adds a second molecule of 2 . The I-azafulvene complexes 4 and 4' and the azetidin-2-ylidene complexes 5 are thereby formed competitively by Complexes 414' precipitate directly from the reaction mixture as red crystals. They are by far more stable than metal-free 1 -azafulvenes.['] However, 4/4' cannot be separated, because they undergo interconversion in solution. The structures of the complexes were established on the basis of spectroscopic data ( Table 1).Upon hydrolysis in the presence of pyridine, 4b/4'b afford 3-amino-2-benzoyl-4-methylpyrrole 6 in 90% yield and pentacarbonyl(pyridine)tungsten 7. Thermolysis of 4a/4'a gives the pyrrolo[3,2-b]pyridine complex 9 by a metal-promoted cyclization. The reaction presumably proceeds via 8 as an intermediate.The formation of 4/4' from 1 and 2 corresponds to the synthesis of indoles from carbene complexes and aromatic isocyanides.['I The formation of azetidin-2-ylidene complexes 5, on the other hand, must be regarded as a prototype of a novel [3 + I ] cycloaddition of a ketenimine ligand to an isocyanide. The reaction is favored by the migration of the W(CO)s group from the N atom to the central C atom of the C = C = N moiety, since this results in the formation of an especially stable aminocarbene structure.Complexes 5 can be easily isolated by chromatography of the mother liquor after separation of the 4/4' crystals.Complex 5b was synthesized starting from a ca. 10 : 1 rnixture of cis-and trans-2b.I') A 15 : 1 : I mixture of three stereoisomers, which differ in the relative configurations of the C = C bonds, was obtained. As expected, the main product has a cis configuration at both C = C bonds [5b(cis/cis) : R,(petroleumether)=O. 14, m.p. = 107°C; Sb(cis/rruns): R,(petroleum ether) =0.42, m.p. = 101 " C ; 5b(trans/cis): R,(petroleum ether/ether= 50/1)= 0.7, m.p. = 80"Cl.The structure of the 3-methyl-3-ethoxyazetidin-2-ylidene-W(CO), 512, which is analogous to 5a (CH, instead of C,Hs in 5a), was determined by an X-ray crystal structure analysis (Fig. I). CI-W 2.044(6), W-C6 2.137(5), C6-C7 1.558(7), C6-NI 1.360(6), C7-C8 1.546(7), NI-C14 1.408(7), CI4-Cl5 1.295(9), NI-C8 1.430(7), C8-N2 1.260(7), N2-Cl2 1.408(8), Cl2-Cl3 1.299(9); W-C6-C7 133.0(3), W-C6-NI 137.0(4), C7-C6-N I 89.9(4).Oxidation of 5a with pyridine N-oxide 10 in toluene leads to the p-lactam 11 in 88% yield.

The Synthesis of η2-β-Vinylpyrrole Complexes and Their Conversion to Highly Substituted Indoles

“…Although azafulvene and azafulvenium derivatives have been identified as important reaction intermediates, relatively few reports of transition metal azafulvenes and azafulvenium complexes exist in the literature. An N -bound tungsten azafulvene was recently reported, and a second report described a tin-metalated azafulvene dimer …”

Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

“…Metal vinylidene complexes, often generated in situ from carbene precursors or through prototropic rearrangement of η 2 -coordinated terminal alkynes, − are promising reactants for β-lactam syntheses − via [2+2] cycloaddition reactions of imines. Following a report of Fischer (eq 20), Barrett et al .…”

Four-membered rings from isocyanides: Developments since the mid 1980s

“…58 As the compounds 84 readily undergo oxidative demetalation, they are useful precursors to functionalized -lactams ( 85a-c,f). 56,58 EtO C Ph M(CO) n + RN C Ph …”