Synthesis and reactivity of 1,2- and 1,3-diphosphanes that contain four chiral rhenium fragments: Architecturally novel tetrametallo-DMPE and -DMPP species that are unprivileged ligands for enantioselective catalysis
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Reactions of enantiopure (S)-[(5-C5H 5)Re(NO)(PPh3)-(=CH2)]+ PF 6- [(S)-2] and PH2CH2(CH 2)nCH2PH2 (0.5 equiv.) give (S ReSRe)-[(5-C5H 5)Re(NO)(PPh3){CH2PH2CH 2(CH2)n-CH2PH2CH 2}(Ph3P)(ON)Re(5-C5H 5)]2+ 2PF6- [n = 0/1, (S ReSRe)-3/4; 65-62/77-58%]. Reaction of racemic 2 (BF 4- salt) and PH2(CH2) 2PH2 (0.5 equiv.) gives the meso and rac diastereomers of 3 (BF4- salts) in 28% and 38% yields after crystallization. Treatments of (SReSRe)-3/4 with fBuOK and then (S)-2 give the tetrarhenium complexes (SReSReS ReSRe)[{(5-C5H 5)Re(NO)(PPh3)(CH2)}2{PHCH 2(CH2)n-CH2PH}{(CH 2)(Ph3P)(ON)Re(5-C5H 5)}2]2+ 2PF6- [n = 0/1, (SReSReSReSRe)-7/8; 89-88/98-87%]. The crystal structure of (SReSReSReS Re)-7 is determined and its conformation analyzed. Reactions of (SReSReSReSRe)-7/8 and tBuOK give air-sensitive diphosphanes (SReSReSReS Re)-{(5-C5H5)Re(NO)-(PPh 3)(CH2)}2{PCH2(CH2) nCH2P}{(CH2)(Ph3P)(ON) Re(5-C5H5)}2 [n = 0/1, (S ReSReSReSRe)-9/10; 92/62%]. Additions of (a) PhIO give the corresponding dioxides (72/62 %), and (b) [Rh(NBD)2]+ PF6- give the corresponding chelates [(P-P)-Rh(NBD)]+ PF6- (75/82%) (NBD = norbornadiene). These catalyze hydrogenations of protected dehydroamino acids and hydrosilylations of propiophenone with only modest enantioselectivities. Similar results are obtained when (SReS ReSReSRe)-9/10 are applied in rhodium-catalyzed conjugate additions of aryl boronic acids, or palladium-catalyzed allylic alkylations. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.
