The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [Nheterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC.For decades, carbenes, which feature a neutral divalent carbon atom with two nonbonding electrons, were considered prototypical reactive intermediates (1). Today, thanks to the availability of stable carbenes (2,3), these molecules, especially the so-called N-heterocyclic carbenes (NHCs) (I) (4-6) ( Fig. 1, top left), are recognized as versatile ligands for transition metal-based catalysts (7-10) and as metal-free organic catalysts in their own right (11)(12)(13)(14). As expected, NHCs I usually bind metals via the carbene center (C2) to give η 1 complexes II. However, in 2001, Crabtree and co-workers discovered that 2-pyridylmethylimidazolium salts react with IrH 5 (PPh 3 ) 2 to give 1 with the imidazole ring bound the "wrong way," at C5 and not at C2 (15,16) (Fig. 1, center). Since that time, a few other complexes of type IV featuring the so-called abnormal NHCs (aNHCs) (III) (17-20) as ligands have been prepared (21-23) (Fig. 1, top right). Experimental and theoretical data suggest that aNHCs III are even stronger electron-donor ligands than are NHCs I. In line with these observations, initial catalytic screening of aNHC metal complexes IV reveals promising results for the activation of unreactive bonds such as C-H and H-H (24-26). As an example, an aNHC palladium complex has been reported to be an efficient catalyst in the Heck olefination of aryl bromides, whereas the corresponding NHC analog is virtually inactive under identical conditions (24).* To whom correspondence should be addressed. guy.bertrand@ucr.edu. Supporting Online Material NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptLassaletta and co-workers (27) have shown that the deprotonation of imidazo [1,5-a] pyridinium salts 2 leads to free NHC 3 that can be isolated ( Fig. 1, bottom). In contrast, using C2-substituted precursors, such as 4, Lassaletta et al. did not observe the corresponding free aNHC. However, by performing the deprotonation reaction in the presence of [Rh(COD)Cl] 2 , they were able to isolate the corresponding aNHC complex 5. Because calculations predicted that the parent aNHC III (where R is equal to H) is only about 17 kcal mol −1 ...

The discovery in 1900 by Gomberg that the trityl radical (Ph(3)C(.)) exists at room temperature is often considered to be the beginning of radical chemistry. Since then, persistent and even room-temperature stable radicals based on second-row and heavier elements have been synthesized. However, few of them have been characterized crystallographically, because they are either too reactive or dimerize in the solid state. Here, we show that a P(2) fragment, capped with two bulky, strongly electron-releasing singlet carbenes (dicoordinate carbon compounds with only six valence electrons), can undergo one-electron oxidation, giving rise to room-temperature stable radical cations. Moreover, when N-heterocyclic carbenes are used, two-electron oxidation can also be performed, producing the corresponding stable dicationic diphosphene, which has to be regarded as a P(2)(2+) fragment coordinated by two carbenes. These results reveal a new application of stable singlet carbenes, the stabilization of paramagnetic species and electron-poor fragments.

Recent theoretical studies are reviewed which show that the naked group 14 atoms E = C-Pb in the singlet (1)D state behave as bidentate Lewis acids that strongly bind two σ donor ligands L in the donor-acceptor complexes L→E←L. Tetrylones EL2 are divalent E(0) compounds which possess two lone pairs at E. The unique electronic structure of tetrylones (carbones, silylones, germylones, stannylones, plumbylones) clearly distinguishes them from tetrylenes ER2 (carbenes, silylenes, germylenes, stannylenes, plumbylenes) which have electron-sharing bonds R-E-R and only one lone pair at atom E. The different electronic structures of tetrylones and tetrylenes are revealed by charge- and energy decomposition analyses and they become obvious experimentally by a distinctively different chemical reactivity. The unusual structures and chemical behaviour of tetrylones EL2 can be understood in terms of the donor-acceptor interactions L→E←L. Tetrylones are potential donor ligands in main group compounds and transition metal complexes which are experimentally not yet known. The review also introduces theoretical studies of transition metal complexes [TM]-E which carry naked tetrele atoms E = C-Sn as ligands. The bonding analyses suggest that the group-14 atoms bind in the (3)P reference state to the transition metal in a combination of σ and π∥ electron-sharing bonds TM-E and π⊥ backdonation TM→E. The unique bonding situation of the tetrele complexes [TM]-E makes them suitable ligands in adducts with Lewis acids. Theoretical studies of [TM]-E→W(CO)5 predict that such species may becomes synthesized.

The reaction of [LAlH2 ] (L=HC(CMeNAr)2 , Ar=2,6-iPr2 C6 H3 ) with MeOTf (Tf=SO2 CF3 ) resulted in the formation of [LAlH(OTf)] (1) in high yield. The triflate substituent in 1 increases the positive charge at the aluminum center, which implies that 1 has a strong Lewis acidic character. The excellent catalytic activity of 1 for the hydroboration of organic compounds with carbonyl groups was investigated. Furthermore, it was shown that 1 effectively initiates the addition reaction of trimethylsilyl cyanide (TMSCN) to both aldehydes and ketones. Quantum mechanical calculations were carried out to explore the reaction mechanism.