Organische Synthesen mit Übergangsmetall‐Komplexen, 58. Heptafulvenylcarben‐Komplexe von Chrom und Wolfram. – Azaspiro[4]‐Anellierung an 1‐Metalla‐1,3‐diene mit Isocyaniden; Liganden‐Abspaltung mit Pyridin oder durch Thermolyse

Abstract

Organic Syntheses via Transition Metal Complexes, 58[1]. – Heptafulvenylcarbene Complexes of Chromium and Tungsten. – Azaspiro[4] Anellation to 1‐Metalla‐1,3‐dienes with Isocyanides; Displacement of Ligands by Pyridine or by Thermolysis*(Cycloheptatrien‐1‐ylmethyl)carbene complexes LnM = C(OEt)CH2‐c‐C7H7 3 [LnM = Cr(CO)5 (a), W(CO)5 (b)] are transformed into heptafulven‐8‐ylcarbene complexes LnM = C(OEt)CH = c‐C7H7 5 in two steps. The first step involves the formation of (tropyliummethyl)carbene complexes [LnM = C(OEt)CH2‐c‐C7H6]+ [BF4]‐ 4 by a hydride abstraction from the cycloheptatriene ring of 3 with [Ph3C]+ [BF4]‐ (4a, 4b: 70%). 4 is deprotonated with Et3N to give 5 (5a: 89%; 5b: 92%). 5 adds one equivalent of cyclohexyl isocyanide (6) to the LnM = C – C = C unit via the intermediate ketenimine complexes LnM[c‐C6H11N = C = C(OEt) – CH = c‐C7H6] 7, which cyclize spontaneously and form the (thermolabile) azaspiro[4.6]undecatetraene complexes 8 (8a: 91%; 8b: 93%). The reaction of 8a, b with pyridine leads to the disengagement of a pyrrole 10 in almost quantitative yields. Thermolysis of 8a, b in the absence of pyridine gives an azulene 12.