Reactivity of Keto-Substituted (η⁵-Cyclohexadienyl)Mn(CO)₃Complexes toward Hydrides

Abstract

Reaction of hydrides with (η 5 -ketocyclohexadienyl)Mn(CO) 3 complexes yielded the corresponding alcohols as major products but also cyclohexadienes due to the addition of hydride to the C 2 carbon of the C 1 -C 5 π-system. This unexpected regioselectivity has been established by different labeling experiments. The structures of one of the starting keto-substituted (η 5 -cyclohexadienyl)Mn(CO) 3 complexes and its corresponding alcohol have been determined by X-ray crystallography.

Chromium-Templated Benzannulation of (η5-Cyclohexadienyl)Mn(CO)3-Methoxy-Cr(CO)5Carbenes

“…The Mn− C1 distance represents the longest Mn−C bond and the Mn− C2 distance the shortest (2.145 Å), which can be compared favorably with that of complex 1 (2.119 Å), bearing a ketone at the C1 carbon. 7 The sp 3 C6 carbon is eclipsed by one of the Mn−CO bonds, in agreement with what is observed in η 5 -Mn complexes. 12 The dihedral angle between the C1, C2, C3, C4, C5 and C1, C6, C5 planes is 41.8(1)°.…”

“…2a,c,6 This has greatly expanded the scope of the (η 5 -cyclohexadienyl)Mn(CO) 3 complexes, allowing to substitute the η 5 -π-system by different groups such as keto functions. 7 Thus, when ketone 1 reacted with hydrides, the two η 5 -diastereoisomeric alcohols 2 were recovered as well as a third compound, 3, due to the unexpected exo-1,4-addition of hydride to the C2 carbon of the π-system of the starting material (Scheme 1). This experiment shed light on the particular reactivity of a πbond conjugated to an electron-withdrawing group at the end of the η 5 π-system toward the addition of a nucleophile.…”

“…For these reasons, in the past few years the study of the chemistry of (η 5 -cyclohexadienyl)Mn(CO) 3 complexes has been emerging. They are soluble in most commonly used organic solvents and easily purified by silica gel chromatography column. , Their easy preparation permits the discovery of new methods of functionalization involving Pd cross-coupling, lithiation reactions, and indirect nucleophilic cine and tele substitutions in contrast to direct ipso S N Ar. ,, This has greatly expanded the scope of the (η 5 -cyclohexadienyl)Mn(CO) 3 complexes, allowing to substitute the η 5 -π-system by different groups such as keto functions . Thus, when ketone 1 reacted with hydrides, the two η 5 -diastereoisomeric alcohols 2 were recovered as well as a third compound, 3 , due to the unexpected exo -1,4-addition of hydride to the C2 carbon of the π-system of the starting material (Scheme ).…”

Réactivité des complexes (η5-cyclohexadiényl)Mn(CO)3 substitués par des groupes électroattracteurs vis-à-vis d'hydrures

“…Très récemment, il a été montré que les complexes 1 substitués par une fonction cétone à l'une des extrémités du système p et par un groupe phényle ou ortho-tolyle sur le carbone sp 3 réagissaient avec le borohydrure de sodium de la manière suivante (Schéma 2) [8]. Après addition de 4 équivalents de NaBH 4 le complexe 1a donne lieu à la formation de 3 composés.…”

“…Afin de déterminer le mécanisme de formation des cyclohexadiènes 3a,c, le complexe marqué 1b [8] préparé et engagé dans la même séquence réac-tionnelle. Traité par un excès de NaBH 4 , il conduit aux alcools diastéréoisomères 2b, 2b 0 (69%, rapport 60:40) et au cyclohexadiène 3b dont l'analyse des données RMN 1 H montre clairement la disparition des signaux correspondant aux protons H3 et H5 du composé 3a.…”