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Chiral Diazaphospholene Organocatalysts
Phosphenium ion promoted asymmetric imine reduction demonstrates highest enantioselectivity
Transition-metal catalysts involved in homogenous hydrogenation reactions is well studied, however, the development of alternative procedures employing main group element-based, or metal free catalysts is currently a major challenge. Not only to reduce or even avoid the usage of precious-metals, but mainly to reach higher reactivity and enantioselectivities of those catalysts.
Hydridic diazaphospholenes are an emerging class of reductive catalysts. Prof. Alexander W. H. Speed from the Dalhousie University, Halifax, Canada, has developed chiral diazaphospholenes as a new class of organocatalysts with unprecedented enantioselectivity in the hydroboration of alkyl imines with pinacolborane reported to date [3]. His group has synthesized diazaphospholene based phosphenium triflates, which are commercially available from the Strem Catalog Part of Ascensus Specialties. (Fig. 1)
Figure 1. (R) 15-1285 and (S) 15-1292 enantiomers of chiral diazaphospholene organocatalysts
These new chiral diazaphospholene organocatalysts are able to catalyze the hydroboration or hydro–silylation of cyclic imines with enantiomeric ratios of up to 97:3 [4]. Catalyst loadings are as low as 0.2 mol %. Imines containing functional groups such as thiophenes or pyridyl rings that can challenge transition-metal catalysts were also reduced employing these systems.
Figure 2. Diazaphospholene-based, organocatalytic enantioselective imine reduction.
This is a first example of an asymmetric reaction promoted by a phosphenium cation and it is anticipated that this technology will be useful for the preparation of many cyclic secondary amines.
References
- Coord. Chem. Rev. 2016, 324, 123.
- J. Am. Chem. Soc. 2015, 137, 4038.
- Angew. Chem. Int. Ed. 2017, 56, 16660.
- J. Am. Chem. Soc. 2019, 141, 14083.
Featured Products
15-1285 1,3-Bis[(1R)-1-(1-naphthalenyl)ethyl]-2,3-dihydro-1H-1,3,2-diazaphosphol-2-yl trifluoromethanesulfonate, min. 98% (2377935-66-3)
15-1292 1,3-Bis[(1S)-1-(1-naphthalenyl)ethyl]-2,3-dihydro-1H-1,3,2-diazaphosphol-2-yl trifluoromethanesulfonate, min. 98%
