Synthesis and reactivity of subvalent compounds Part 11. Oxidation, hydrogenation and hydrolysis of stable diamino carbenes

The reactivities of the two stable diamino carbenes 1,3-di-tert-butylimidazol-2-ylidene (1) and 1,3-di-tert-butylimidazolin-2-ylidene (2) toward hydrogen, oxygen, water and carbon monoxide were investigated. The carbenes do not react with O-2 or CO but are attacked by water to give the respective hydrolysis products Bu-t-N=CHCH2-N(CHO)Bu-t (7) and Bu-t-NH-CH2CH2-N(CHO)Bu-t (8). While 2 is hydrolyzed instantaneously, the aromatically stabilized 1 reacts only very slowly and can be handled in air for brief periods of time. The carbenes 1 and 2 are unreactive towards H-2 alone but can be catalytically hydrogenated to the respective aminals 2,3-dihydro-1,3-di-tert-butylimidazole (5) and 1,3-di-tert-butylimidazolidine (6). The reaction products were characterized by single crystal X-ray crystallography, multinuclear NMR spectroscopy (H-1, C-13, O-17), IR spectroscopy and DFT methods. The extent of aromatic delocalization in 1 was investigated through isodesmic hydrogenation reactions at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level. At this level of theory, the 'carbene oxide' (2,3-dihydro-imidazol-2-one) retains ca. 50% of the aromatic delocalization energy of the carbene. The oxidation of the diamino carbenes to the 'carbene oxides' (ureas) is calculated to be exothermic by - 79.1 kcal mol(-1) (1) and - 86.4 kcal mol(-1) (2). The oxygen affinity of the carbenes resembles that of trimethylphosphine (- 79.7 kcal mol(-1)) and triethyl phosphite (- 88.6 kcal mol(-1)) and is significantly higher than that of CO (- 67.6 kcal mol(-1)). The aminals 5 and 6 are structurally related to Thauer's hydrogenase (TH2/TH+) but do not show hydrogenase activity. (C) 2001 Elsevier Science B.V. All rights reserved.