Spectroscopic Identification of Dihydroxycarbene

Abstract

Klein, aber fein: Das Abspalten von CO2 aus einfachen α‐Ketocarbonsäuren lieferte bisher unbekannte Hydroxycarbene. Die Blitzpyrolyse von Oxalsäure oder ihrem Monomethylester im Hochvakuum machte die Synthese und Charakterisierung von Dihydroxy‐ (1) bzw. Hydroxymethoxycarben möglich; beide werden durch eine Out‐of‐Plane‐Konjugation vom π‐Typ und eine In‐Plane‐σ‐Konjugation stabilisiert (das HOMO von s‐trans, s‐trans‐1 ist gezeigt).

Gas‐Phase Structure Determination of Dihydroxycarbene, One of the Smallest Stable Singlet Carbenes

“…The semi-experimental equilibrium structure derived from five isotopic variants of HOÀC ÀOH contains two very short CO single bonds (ca. [23][24][25][26][27][28][29] It is understood that carbenes are stabilized by nearby electron-donating groups. Detection of HOÀC ÀOH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH 2 O 2 chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH 2 OO.…”

“…1.32 ). [1] Several studies have reported the potential energy surface (PES) of one of the smallest carbenes with two electron-donating groups: the singlet ground state of dihydroxycarbene (HO À C À OH), [3,4,23] which has been suggested to be an intermediate in the photolysis of oxalic acid. Relatively stable singlet carbenes with bulky electron-donating substituents, such as N-heterocyclic carbenes, have long been used in organic synthesis, [1,2] but smaller carbenes are more difficult to isolate and characterize.…”

“…Relatively stable singlet carbenes with bulky electron-donating substituents, such as N-heterocyclic carbenes, have long been used in organic synthesis, [1,2] but smaller carbenes are more difficult to isolate and characterize. The stability of singlet HO À C À OH is provided by the overlap between the electrons in the oxygen p orbitals and the empty p orbital on the carbon, leading to much shorter CO bond lengths (1.32 ) [4,23] than those found in alcohols (such as the 1.42 CO bond length in methanol [36] ). [15,16] Although triatomic carbenes (DCX 2 where X = H, F, Cl, Br, or I) and vinylidene (DC=CH 2 ) have been studied extensively, [17][18][19][20][21][22] fewer experimental studies have examined the properties of small carbenes with substituent groups containting two to three atoms, which have a larger range of possible reaction pathways.…”

“…Numerous studies [3][4][5][6][7] implicate small organic carbenes as intermediates in chemical reactions such as the thermal decomposition of dicarboxylic acids (which are a key component of secondary organic aerosols, [8][9][10] photochemical smog, [11] and interstellar clouds [12] ) and the reactions of carbon dioxide and carbon monoxide with H 2 O/H 2 , thought to be important in prebiotic chemistry, [12][13][14] and in reversible hydrogen storage. [23][24][25][26][27][28][29] It is understood that carbenes are stabilized by nearby electron-donating groups. [23][24][25][26][27][28][29] It is understood that carbenes are stabilized by nearby electron-donating groups.…”

A Formal Carbon–Sulfur Triple Bond: HCSOH

“…In our continuing studies of the new class of hydroxycarbenes [5] we envisaged preparing hydroxy mercaptocarbene [6] (HS À C À OH, cf. potential energy surface (PES) Figure 2) through the generation and photochemical rearrangement of thioformaldehyde S-oxide (H 2 C = S = O (6), Scheme 1), generated thermally by high-vacuum flash pyrolysis (HVFP) from its dimer 5.…”