Cobalt‐Catalysed Asymmetric Addition and Alkylation of Secondary Phosphine Oxides for the Synthesis of P‐Stereogenic Compounds

Abstract

The catalytic asymmetric synthesis of P-chiral phosphorus compounds is an important way to construct Pchiral ligands. Herein, we report a new strategy that adopts the pyridinyl moiety as the coordinating group in the cobaltcatalysed asymmetric nucleophilic addition/alkylation of secondary phosphine oxides. A series of tertiary phosphine oxides were generated with up to 99 % yield and 99.5 % ee, and with broad functional-group tolerance. Mechanistic studies reveal that (R)-secondary phosphine oxides preferentially interact with the cobalt catalysts to produce P-stereogenic compounds.Chiral phosphorus compounds play important roles as ligands [1] or organic catalysts [2] in asymmetric catalysis. Compared to the synthesis of axial-, planar-, and carboncentered chiral phosphorus compounds, the synthesis of Pstereogenic phosphines has been less investigated. The main recent strategies for the preparation of P-chiral compounds involve the use of chiral auxiliary materials or the resolution of racemates. [3] These classical methods require stoichiometric chiral agents or generate an equivalent amount of compound with an opposite configuration. [4] Recently, several asymmetric catalytic methods were developed based on the desymmetrization of phosphine oxides via CÀH bond activation, [5] asymmetric addition, [6] ring-closing metatheses, [7] and [2+2+2] cycloaddition processes. [8] In addition, secondary phosphines have been used as nucleophiles in asymmetric conjugate addition, [9] alkylation, [10] and arylation reactions. [11] However, in this process, the strong coordination ability of the secondary phosphines may lead to deactivation of the metal catalyst, thereby reducing the enantioselectivity. Therefore, the development of alternative methods for the synthesis of P-chiral compounds is still highly necessary.Compared with secondary phosphines, secondary phosphine oxides (SPOs) are more stable in air and have low toxicity to metal catalysts. Nonetheless, only a few synthetic examples have been reported for the preparation of optically active tertiary phosphine oxides (TPOs) from SPOs. In 2016, Gaunt and co-workers reported a new strategy in which Cu combined with chiral pyridine-2,6-bis(oxazolines) ligand (pybox) was applied for the enantioselective arylation of SPOs with diaryliodonium salt, affording P-chiral tertiary phosphorus compounds with up to 98 % ee (Scheme 1 a). [12] The Cai group reported a Pd-catalysed arylation of SPOs with aryl iodides with moderate to high ee (Scheme 1 b). [13] Recently, Zhang and co-workers also reported Pd-catalysed asymmetric arylation and alkenylation reactions of SPOs (Scheme 1 c), in which products were obtained in up to 99 % ee. [14] In addition to the above examples, the Qing-Wei Zhang group developed a Ni-catalysed kinetic resolution of SPOs via an allylic substitution process, providing a series of Pstereogenic compounds with high enantioselectivity (Scheme 1 d). [15] However, the SPOs in the abovementioned examples usually contain one aryl and one alkyl group. Substrates b...

Cobalt-catalyzed asymmetric phospha-Michael reaction of diarylphosphine oxides for the synthesis of chiral organophosphorus compounds

“…On the basis of some related literature reports 12,13 and our control experiment results, a plausible mechanism is suggested in Fig. 2.…”

“…Cobalt, a cheap and low toxic element in comparison with other transition metals, was found to exhibit excellent activity in asymmetric catalysis and has received growing attention from the synthetic community in the last two decades. 11 Recently, Duan's group 12 reported a cobalt-catalysed asymmetric phospha-Michael addition for the synthesis of P-stereogenic compounds, in which a pyridyl moiety was adopted as the coordinating group in secondary phosphine oxides in order to obtain good enantioselectivity (Scheme 1a). In 2019, we reported a visible-light-induced cobalt-catalyzed enantioselective radical conjugate addition when the novel chiral Co-centered octahedral complex was used as the catalyst and an imidazolyl moiety as the coordinating group in the Michael acceptors (Scheme 1b).…”