Enantioselective Dehydrogenation of Alkan‐2‐ols by Gaseous (BINOLate)Ni⁺

Abstract

Dedicated to Professor Robert G. Bergman on the occasion of his 60th birthday Electrospray ionization of methanolic solutions of nickel(II) nitrate, 1,1'-binaphthalene-2,2'-diol (BINOL), and secondary alcohols (ROH) inter alia affords monocationic complexes of the typeþ , where BINOLate stands for singly deprotonated BINOL. Upon collisioninduced dissociation (CID), the mass-selected ions undergo competing fragmentations involving loss of the alcohol ligand and expulsion of the corresponding carbonyl compound. The latter reaction leads to the hydride complex [(BINOL)Ni(H)] þ and can thus be regarded as the reversal of the reduction of ketones with metal hydrides. The possibility of the occurrence of enantioselective gas-phase reactions is probed for combinations of chiral BINOLate ligands with chiral alkan-2-ols. Whereas aliphatic alkan-2-ols do not show pronounced chiral effects, enantioselective bond activation is observed for 1-phenylethanol, indicating an interaction of the aromatic ring of the alkanol with the positively charged metal center.Introduction. -While mass spectrometry has elucidated a wealth of mechanistic details about gas-phase reactions of transition-metal ions, even relevant in the context of homogeneous and heterogeneous catalysis [1], only few examples of enantioselective gas-phase reactions have been reported so far. This is indeed a crucial lack of insight, considering the enormous relevance of transition-metal catalysts in asymmetric synthesis, on the one hand, and the often rather limited mechanistic information about these reactions, on the other. In particular, the molecular origin of enantioselectivity in transition-metal-mediated reactions is often unknown, and catalyst optimization thus by and large still based upon mere trial-and-error procedures, instead of relying on more systematic approaches. One major reason for the only few examples of enantioselective reactions in gas-phase chemistry is associated with the use of mass spectrometry, which a priori is an achiral method of detection. Moreover, as the desired, usually organic product is released as a neutral molecule after bond-formation has taken place, it escapes detection in conventional mass-spectrometric experiments. The challenge in the discovery of enantioselective gas-phase reactions is thus not only to find appropriate systems which bear chiral effects, but also to identify those systems in which these effects can be measured by monitoring the ionic products of a reaction.

Generation and Reactivity of Enantiomeric (BINOLato)Ni+ Complexes with Chiral Secondary Alcohols in the Gas Phase

“…[16,17] However, examples of chiral effects in bond-activation processes are very rare, [18] and in many cases no significant enantioselectivities are observed. Here [19] we report a detailed mechanistic study on the enantioselective oxidation of chiral secondary alcohols-CH 3 CH(OH)R (with R = CH 3 , C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 , n-C 5 H 11 , n-C 6 H 13 , c-C 6 H 11 , and C 6 H 5 )-to the corresponding ketones. In analogy to well known asymmetric hydrogenation catalysts involving a transition metal (particularly Ru) bound to a ligand with a binaphthyl skeleton, [20] the chiral BINOLato ligand attached to a Ni II center has been chosen, where BINOLato stands for singly deprotonated 1,1'-bis-2-naphthol (BINOL).…”

Gas Phase Ligand Fragmentation to Unmask Reactive Metallic Species

“…Path A involves the formation of a T-shaped transition state, 5, while Path B involves a side-on S N 2 transition state, 6. Metal alkoxides generated via ESI also fragment to form metal hydrides as demonstrated by a number of studies [75,76]. In contrast, the transition state energies for both pathways for the silver congener are above the energy-separated reactants.…”

Modeling the interactions between peptide functions and Sr2+: formamide–Sr2+ reactions in the gas phase

“…These difficulties are particularly severe for transition metals having rather high second ionization energies. Most often in those cases, the only ions experimentally detectable are singly charged [M(L-H)] + species, [1][2][3][4][5] in which the neutral ligand has lost a proton. This is not the case however for alkaline-earth metals, such as Ca, Sr or Ba, which have much smaller second ionization energies and yield easily detectable ML 2+ dications [6][7][8][9][10][11][12][13][14] and whose unimolecular reactivity upon collision could be explored.…”