Azolium Control of the Osmium-Promoted Aromatic C–H Bond Activation in 1,3-Disubstituted Substrates

Abstract

The hexahydride complex OsH 6 (P i Pr 3 ) 2 promotes the C–H bond activation of the 1,3-disubstituted phenyl group of the [BF 4 ] − and [BPh 4 ] − salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ 2 - C,N - or κ 2 - C,C -chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ 3 - C,C,N -pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ 2 - C,N - and κ 2 - C,C -chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF 4 ] − inhibits the azolium reactions, [BPh 4 ] − favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C–H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Homogeneous catalysis with polyhydride complexes

“…68 Complex 4 also promotes the dehydrogenation of linear amines such as Nphenylbenzylamine and N-methylbenzylamine, but the respective imines are formed in low yield after 48 h. 69 Trihydrides 5-7 and the bimetallic hexahydride 8 (Chart 3) also promote the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline, in p-xylene, at 140 1C. 70 In this case, between 58% and 87% of the H 2 capacity of the heterocycle is released after 48 h, in the presence of 7 mol% of metal. In contrast to the reactions performed in the presence of 3 and 4, the dehydrogenation is sequential, with the release of the first hydrogen molecule being faster than the liberation of the second one (Scheme 4).…”

“…Osmium-trihydride complexes 5-7 and the homo-binuclear osmium-hexahydride derivative 8 also catalyze the dehydrogenation of secondary and primary alcohols, such as 1-phenylethanol, benzyl alcohol, and 1,2-phenylenedimethanol, in toluene, at 100 1C. 70 With 7 mol% of metal, the dehydrogenation of 1-phenylethanol affords the expected acetophenone in yield lower than 60%, after 24 h. The dehydrogenation of benzyl alcohol in the presence of the mononuclear trihydrides gives benzaldehyde also in low to moderate yields. On the other hand, the binuclear compound 8 promotes the transformation of the alcohol into a mixture of benzylbenzoate (13%) and benzaldehyde (47%).…”

“…78 The dehydrogenation of 1,2-phenylenedimethanol in the presence of 8 leads to 1-isobenzofuranone and molecular hydrogen in a quantitative yield after 24 h, whereas the mononuclear trihydrides yield less than 60% of lactone (Scheme 7). 70 Dehydrogenative homo-coupling of aliphatic primary alcohols has been selectively achieved with the rhenium-pentahydride ReH 5 {k 3 -P,P,P-[(PPh 2 CH 2 ) 3 CMe]} (17) 79 and the osmium-tetrahydride OsH 4 {k 3 -P,N,P-[HN(CH 2 CH 2 P i Pr 2 ) 2 ]} (18). The pentahydride works in 1,4-dioxane, at 120 1C, 80 whereas the reactions with the osmium-tetrahydride were performed in the alcohol, as solvent, at temperature above 120 1C.…”

“…Compounds 68 and 69 are also active for the ethylene coupling with both ketones, in agreement with Scheme 32. 154 Their catalytic efficiency is significantly higher than that of RuH{k 2 -C,O-[C 6 H 4 C(O)-Me]}(CO)(PCy 3 ) 2 (70), which bears a carbonyl group instead of a dihydrogen ligand at the same position. Such difference in behavior has been assigned to the higher lability of the dihydrogen ligand with regard to the carbonyl ligand, which facilitates the olefins coordination and thus the insertion step.…”

“…78 The dehydrogenation of 1,2-phenylenedimethanol in the presence of 8 leads to 1-isobenzofuranone and molecular hydrogen in a quantitative yield after 24 h, whereas the mononuclear trihydrides yield less than 60% of lactone (Scheme 7). 70…”