Architecturally Distinctive Chiral-at-Rhenium Complexes with P═O/N:, P═O/P═O and S═O/S═O Donor Moieties: Syntheses, Structures, Reactivity, and Gas Phase Brønsted Basicities

Abstract

In efforts to develop nucleophilic catalysts for asymmetric organic synthesis, enantiopure mono- and dirhenium complexes with P═O or S═O moieties and a second donor group are prepared. In one tack, the enantiopure methyl complex (S)-(η5-C5H5)Re(NO)(PPh3)(CH3) ((S)-1) is converted to the quinoline-containing phosphine oxides (S)-(η5-C5H4(1-isoquinolinyl))Re(NO)(PPh3)((CH2)nP(═O)Ph2) (n = 0, 1 ((S)-22)). In another, the known phosphonium salt (S)-[(η5-C5H4I)Re(NO)(PPh3)(CH2PPh2H)]+ PF6– is converted to the dirhenium bis(phosphine oxide) (SS)-(Ph2(O═)PCH2)(Ph3P)(ON)Re(η5-C5H4C≡C-η5-C5H4)Re(NO)(PPh3)(CH2P(═O)Ph2). In another, (S)-1 is converted to the dirhenium phosphine oxide (SS)-(H3C)(Ph3P)(ON)Re(η5-C5H4P(═O)(Ph)-η5-C5H4)Re(NO)(PPh3)(CH3) and thence to tris(phosphine oxide) (SS)-(Ph2(O═)P)(Ph3P)(ON)Re(η5-C5H4P(═O)(Ph)-η5-C5H4)Re(NO)(PPh3)(P(═O)Ph2) ((SS)-30). In another, (S)-1 and HS(CH2)3SH are converted to dirhenium dithiol complex (SS)-[(η5-C5H5)Re(NO)(PPh3)(S(H)(CH2)3(H)S)(Ph3P)(ON)Re(η5-C5H5)]2+ 2BF4– and thence (t-BuOK, dimethyl dioxirane) to the bis(″sulfone″) (SS)-(η5-C5H5)Re(NO)(PPh3)(S(═O)2(CH2)3S(═O)2)(Ph3P)(ON)Re(η5-C5H5). Crystal structures of (R)-22, its precursor (R)-(η5-C5H4(1-isoquinolinyl))Re(NO)(PPh3)(CH3), (RR)-30, the DMSO complex (S)-[(η5-C5H5)Re(NO)(PPh3)(S(═O)(CH3)2)]+ BF4– ((S)-37+ BF4–), and a byproduct of the dithiol complex in which two (η5-C5H5)Re(NO)(PPh3) cations bind to trans sulfur lone pairs of 1,2-dithiacyclopentane are determined. DFT calculations show that the gas phase Brønsted basicities of the preceding complexes are greater than many chiral P═O or S═O species that are excellent catalysts for asymmetric aldehyde allylations, but good activities or enantioselectivities are nonetheless elusive.