No abstract

The chiral racemic methyl complex (η5-C5H5)Re(NO)(PPh3)(CH3) is converted to the rhenium-containing phosphorus donor ligands (η5-C5H5)Re(NO)(PPh3)((CH2) n PR2) (n/R = 3a, 0/Ph; 3b, 0/t-Bu; 3c, 0/Me; 5a, 1/Ph; 5b, 1/t-Bu) and (η5-C5H4PR2)Re(NO)(PPh3)(CH3) (7; R = a, Ph; b, t-Bu) via standard reactions (3, TfOH/CH2Cl2 or HBF4/chlorobenzene, then PR2H, then t-BuOK; 5, Ph3C+X-, then PR2H, then t-BuOK; 7, n-BuLi, then PR2Cl). (η5-C5H4PR2)Re(CO)3 (R = Ph, t-Bu) is prepared from (η5-C5H5)Re(CO)3 analogously to 7. Most of these species are effective ligands for palladium-catalyzed Suzuki couplings. Typical conditions involve toluene solvent, an aryl bromide (1.0 equiv), phenylboronic acid (1.5 equiv), K3PO4 (2.0 equiv), Pd(OAc)2 (1 mol %), the rhenium/PR2 species (4 mol %), and 60−100 °C. In the cases of 3 and 5, the rhenium/PR2 species are generated in situ from indefinitely stable conjugate acids [rhenium/PR2H]+ and t-BuOK (2 equiv or 8 mol %). The bulkier and more electron-rich rhenium/P(t-Bu)2 systems generally give more active catalysts than the rhenium/PPh2 analogues. Under many conditions, the activities of 3a and 3b approach (but do not exceed) those of the corresponding organophosphines PPh3 and P(t-Bu)3, the latter being a benchmark ligand for Suzuki couplings. Turnover numbers of >1000 are easily realized. Chloroarenes can be coupled, but at much slower rates and in lower yields. The crystal structures of 5b and 7b are determined. The trigonal phosphorus atoms become increasingly pyramidalized in the series 5b < 5a < 7b.

Rhodium chelate complexes of the title diphosphines, [(η 5 -C 5 H 4 PPh 2 )Re(NO)(PPh 3 )(µ-, are catalyst precursors (1.0 mol %, 24-26 °C) for additions of Ph 2 SiH 2 to the ketones PhCOCH 2 R (R ) H, CH 3 , CH 2 CH 3 , CH 2 CH 2 Cl; 93-77% conversions). Hydrolyses give the alcohols (R)-HOCH(Ph)CH 2 R in 83-60% isolated yields; configurations correlate with the relative rhenium configurations of the catalysts. The ee values with (S)-6 (63/62/60/92%) exceed those of (S)-5 (44/38/47/48%); ee values of (S Re R C )-7 (41/46/62/58%) are higher than those of the diastereomer (S Re S C )-7 (23/11/15/35%). The alkenes (Z)-RCHdC(NHCOCH 3 )-CO 2 R′ (R/R′ ) H/H, Ph/H, Ph/CH 3 ) and H 2 (1 atm) react in the presence of (S Re S C )-and (S Re R C )-7 (0.5 mol %, 30 °C) to give the protected amino acids RCH 2 CH(NHCOCH 3 )CO 2 R′ in 85-92% yields. The ee values with (S Re S C )-7 (37/62/61%) are lower than those found earlier for (S)-6 (62/72/65%) or (S)-5 (92/93/88%), with S enantiomers dominant in all cases. Surprisingly, (S Re R C )-7 gave much higher ee values (93/94/97%), but with R enantiomers dominant. Hence, the carbon stereocenter of 7 controls the product configuration.

The rhenium complexes (eta 5-C5H5)Re(NO)(PPh3)((CH2)nPR2:) (n/R = 0/Ph, 0/t-Bu, 0/Me, 1/Ph, 1/t-Bu), which contain electron-rich and sterically congested phosphido moieties, give active catalysts for the title reaction; typical conditions (toluene, 60-100 degrees C): aryl bromide (1.0 equiv.), PhB(OH)2 (1.5 equiv.), K3PO4 (2.0 equiv.), Pd(OAc)2 (1 mol%), and a Re(CH2)nPR2: species or a 1:2 [Re(CH2)nPR2H]+X-/t-BuOK mixture (4 mol% rhenium).

The less stable ReC isomer of the enantiomerically pure benzylidene complex (S)-[(η5-C5H5)Re(NO)(PPh3)(CHPh)]+BF4 - ((S)-(sc)-6) and PPh2H react to give the phosphonium salt (S Re S C)-[(η5-C5H5)Re(NO)(PPh3)(CH(Ph)PPh2H)]+BF4 - ((S Re S C)-7; 91%), as confirmed by a crystal structure of the racemate. Reaction with t-BuOK gives the phosphine (S Re S C)-(η5-C5H5)Re(NO)(PPh3)(CH(Ph)PPh2) ((S Re S C)-8; 73%). An analogous sequence with the more stable ReC isomer ((S)-(ac)-6) but at −60 °C gives (S Re R C)-8 (58%). NMR experiments show that the intermediate (S Re R C)-7 epimerizes to (S Re S C)-7 above −20 °C, and the mechanism has been analyzed. Reactions of (S Re S C)- and (S Re R C)-8 with t-BuLi and then PPh2Cl give the title diphosphines (S Re S C)- and (S Re R C)-(η5-C5H4PPh2)Re(NO)(PPh3)(CH(Ph)PPh2) (84−89%). Additions of [Rh(NBD)2]+PF6 - give the heterobimetallic chelate complexes (S Re S C)- and (87−91%). In efforts to access similar compounds with ReCPh2PPh2 linkages, the chlorobenzene complex [(η5-C5H5)Re(NO)(PPh3)(ClPh)]+BF4 - and NNCPh2 have been reacted. Instead of the target ReCPh2 adduct, the diphenyldiazomethane complex [(η5-C5H5)Re(NO)(PPh3)(NNCPh2)]+BF4 - (73%) has been isolated and crystallographically characterized.