Intermolecular Reactions of Foiled Carbenes with N−H Bonds: Evidence for an Ylidic Pathway

Abstract

The chemistry of endo-tricyclo[6.2.1.0 (2.7)]undec-9-en-11-ylidene (10), an archetypal foiled carbene, has been investigated. The intermolecular reactions of 10 are most conveniently performed with oxadiazoline 6 because the corresponding diazirine can be obtained only in very low yield. Furthermore, the aziridinyl imine is difficult to decompose and the tosylhydrazone sodium salt poorly soluble in common organic solvents. Photolysis of 6 in diethylamine leads merely to a reduction of the diazo group and regeneration of acetyl hydrazone 5, whereas thermolysis cleanly affords tertiary amine 12(anti) in 77% yield. Calculations show that even stabilized-nucleophilic carbenes react with amines through an ylidic pathway and not by a concerted insertion into the N-H bond. Nevertheless, in the gas phase, norbornen-7-ylidene (13) is predicted to be stabilized by one molecule of NH3 more efficiently through a hydrogen bond than by ylide formation.

2‐Methoxy‐Δ3‐1,3,4‐oxadiazoline, a Multipurpose Precursor for the Generation of a Carbene, an Ylide, or a Diazo Compound

“…[4] These electronic processes explain why foiled carbenes could not be trapped intermolecularly in synthetically use-ful yields by alkenes or hydrocarbons. In fact, the only efficient intermolecular reactions known so far concerns the insertion into the N-H bond of diethylamine [5] and the O-H bond of methanol. [6] For the latter reaction, the ease of etherification can be related to the high proton affinity (PA) of 5.…”

“…[5] endo,syn-Tricyclo[6.2.1.0 2,7 ]undec-9-en-11-ol (9): Oxadiazoline 1a (302 mg, 1.22 mmol) was thermolyzed overnight in 4 mL of methanol in a pressure tube at 155°C. After removal of the solvent, syn alcohol 9 was separated from the main product, anti methyl ether 10me by preparative TLC with hexane/2-propanol (39:1).…”

“…The difference in the stability of oxadiazolines 13 and 18 probably is caused by the ring strain of polycyclic compound 13 and its decomposition products 14, 15, and 16. In 13, C(7) is about sp 3 -hybridized (preferred angle in unstrained systems: 109.5°; actual angle from the crystal structure of 1b [5] is 94.3°) whereas for the formation of the products a double bond has to be formed leading to sp 2 hybridization (preferred angle: 120°), a geometry requiring an even larger angle. A similar behavior is found for example for ketals of bicyclo[3.2.1]octan-8-one derivatives which are more stable than ketals of acyclic ketones.…”

Three Ways Isolable Carbenes Can Modulate Emission of NH-Containing Fluorophores

“…Herein, we report on the reaction products of two representative isolable carbenes, IPr and Et CAAC , with two well-known NH-containing fluorophores, carbazole and 2-phenylbenzimidazole ( Ph-BIM ). These studies were initiated during our efforts to ascertain the mechanisms for fluorescence quenching of 2-thiophene-1,3-benzazaphosphole by IPr . , While the reactivity of NHCs and CAACs has been investigated with various NH-containing systems, ,− it appears that little work has been directed at examining the photophysical impact of carbenes on NH-containing fluorophores in metal-free systems.…”

A Versatile, Unexpected, One-Pot Regioselective Synthesis of a New Class of 1,3-Diazepinoindolones by Reaction of Pyranoindolones with Monosubstituted Ureas

“…1,2 However, the isolation of products 3 led to the conclusion that the reaction followed a different pathway (Scheme 2), most probably as a result of the weak nucleophilic character of the urea nitrogens. According to the proposed mechanism the reaction is initiated by electrocyclic ring-opening of the pyrone ring of 1 leading to the formation of a vinylogous acylketene inter- 7 into the N-H bond of the urea to form intermediate 9 which undergoes cyclization and elimination of water to yield product 3.…”