Abstract
Two synthetic protocols for the conversion of 1-phenylphosphinan-4-ones to novel P-stereogenic 1-phenylphosphin-2-en-4-ones by enantioselective deprotonation followed by oxidation and by asymmetric organocatalytic halogenation accompanied by elimination have been developed. These two-step one-pot transformations provide convenient access to optically active 1-phenylphosphin-2-en-4-one 1-sulfide and 1-phenylphosphin-2-en-4-one 1-oxide of 96 and 55% enantiomeric purities, respectively.
Highlights
Cyclic nonracemic phosphines constitute an important group of organophosphorus compounds that are sought for their advantageous performance as organocatalysts and as ligands in various asymmetric processes.1 Numerous chiral five-membered2 and four-membered ligands3 have been developed to meet the demand
The desymmetrization by path A involves asymmetric deprotonation of 1-phenylphosphinan-4-one (1) by a chiral base and conversion of the resulting lithium enolate to the silyl enol ether 2 by quenching with TMSCl.10a10b The desymmetrization by path B entails transformation of phosphinanone 1 into a chiral α-halogenated derivative 3, which could be achieved by organocatalytic asymmetric α-halogenation.10f10g Both synthetic procedures make use of the ketone functionality of phosphinanone 1, and both result in the overall asymmetric transformation of the remote prochiral phosphorus center in ketone 1 into a P-stereogenic one in enone 4 via intermediate 2 or 3
Since the time the enantioselective deprotonation of cyclic ketones by a chiral lithium amide was first demonstrated in 1986,10a10b the method has been widely utilized in asymmetric synthesis for generating chirality centers in cyclic ketones by Received: December 29, 2020
Summary
Cyclic nonracemic phosphines constitute an important group of organophosphorus compounds that are sought for their advantageous performance as organocatalysts and as ligands in various asymmetric processes. Numerous chiral five-membered (phospholane) and four-membered (phosphetane) ligands have been developed to meet the demand. The corresponding chiral six-membered carbon-phosphorus heterocycles (phosphinanes) have received relatively little attention due, most probably, to scarcity of convenient methods enabling their synthesis in suitably functionalized and nonracemic forms.. Synthesis of cyclic phosphine derivatives by this route can start either from an acyclic,4e4i or from a cyclic precursor. In the latter case, the reported precedents included phosphol-3-ene oxide and its epoxide, phosphetane sulfide,3d and phospholane sulfide2f as well as phospholane borane and phosphinane boranes.4a In this paper, we wish to report our results on evaluation of enantioselective desymmetrization of 1-phenylphosphinan-4one (1) by employing its carbonyl function in two independent two-step processes designed to lead to the formation of optically active 1-phenylphosphin-2-en-4-one derivatives 4 (Figure 1C). The target phosphin-2-en-4-one, equipped with a versatile enone functionality, represents a novel phosphinane scaffold potentially amenable to rich chemistry further downstream
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