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studied with a substrate/catalyst ratio from 100:1 to 2000:1
which corresponds to a TOF up to 2000 hÀ1 under optimized
conditions. The similarity of these results with our data con-
firms the excellent activities of P-arylphospholanes and phos-
pholaniums, which do not require heating or pressure of dihy-
drogen gas, to attain complete substrate conversion with satis-
fying TOF values.[19]
Preparation of enantiopure P-aryl-2,5-diphenylphospholane
(3a–i)
Method A (from direct reduction of phosphine oxide 1a–j): To the
appropriate P-aryl-phospholane oxide 1a–j (1 mmol) dissolved in
distilled DME (10 mL), were added methyl trifluoromethanesulfo-
nate (1.1 mmol) under argon atmosphere. After 2 h the mixture is
cooled down to 08C and lithium aluminum hydride (1.5 mmol)
were added. The mixture was allowed to warm to room tempera-
ture and stirred for additional 15 h. After hydrolysis with a mini-
mum of water, the mixture was filtered under argon through Celite
via cannula. DME was evaporated under vacuum to give the free
P-aryl-phospholane 3a–j.
We were able to gather some interesting information by
comparing the activity data for phosphonium salts 4a–i with
that of free phosphanes 3a–i. We observed only an usual but
very slight slowdown of the catalyst activity when the reaction
was performed in the presence of phosphonium as ligand pre-
cursor. This could be explained by a short latency time before
the beginning of the consumption of dihydrogen. It probably
corresponds to the time necessary to liberate the free phos-
phine and form the active complex. However, the TOF1/2 values
are very close to the values at complete conversion for each
ligand. This provides further evidence of the high effectiveness
and stability of the catalytic system during the entire reaction
duration. Moreover, the enantiomeric excesses recorded using
as ligands free phosphanes 3a–i are very similar with those ob-
served by utilizing the corresponding phosphonium salts 4a–i.
Overall, these results confirm that the rhodium complex is
formed by the coordination of the rhodium precursor with the
phosphane obtained from of the dissociation of the salt in
polar solvent in absence of any base.[9a]
Method B (from deprotection of borane complex 2a–j): To a solu-
tion of phosphane-borane complex 2a–j (1 mmol) in freshly de-
gassed dichloromethane was added tetrafluoroboric acid dimethyl
ether complex (4 mmol) at 08C. The mixture was allowed to warm
to room temperature and stirred overnight at ambient tempera-
ture. The mixture is cooled down to 08C and a solution of saturat-
ed NaHCO3 was added carefully under argon atmosphere until the
effervescence ceased. The aqueous phase was extracted three
times with degassed dichloromethane under argon via cannula.
The organic layers were combined and washed with brine. The
aqueous phase was eliminated via cannula and the organic layer
died over magnesium sulfate, filtrated under argon. The solvent
was evaporated by vacuum to give the free P-aryl-phospho-
lane 3a–j.
Preparation of P-aryl-2,5-diphenylphospholanium
tetrafluoroborate salt (4a–i)
To a solution of enantiopure P-aryl-2,5-diphenylphospholane 3a–
i (1 mmol) in dry degassed diethyl ether (5 mL) were added three
equivalents of HBF4·Me2O complex at 08C. After a few minutes the
precipitate was collected by filtration, washed with diethyl ether
(3ꢁ10 mL) and dried, to give the phospholanium salt as a white
powder.
Conclusion
P-aryl-2,5-diphenylphospholaniums salts 4a–i are useful pre-
cursors ligands of enantiopure P-aryl-2,5-diphenylphospho-
lanes. These air-stable salts were conveniently used in rhodi-
um-catalyzed enantioselective hydrogenation: Methyl-(Z)-2-
acetamidocinnamate is hydrogenated to provide methyl-N-
acetyl-phenylalaninate in very short reaction time, and good
enantiomeric excesses for a monophosphine ligand (up to
93% ee). We also demonstrated that by using the phosphoni-
um salts as ligands, same yields and enantioselectivities as for
the use of free phosphanes are obtained. The salt can be used
in the absence of base. TOF calculations show that the P-aryl
phospholane ligand offers an efficient and stable rhodium-cat-
alytic system during all the advancement of reaction without
significant loss of activity.
Rhodium-catalyzed asymmetric hydrogenation
General method: A Schlenk tube was charged with free P-aryl-
phospholane 3a–i or phospholanium salt 4a–i (0.024 mmol) and
bis(1,5-cyclooctadiene) rhodium(I)tetrafluoroborate (10 mmol). The
tube was purged with argon and degassed, anhydrous methanol
(5 mL) was added. The solution was stirred for 10 min, and the
yellow solution obtained was cannulated into a Schlenk tube con-
taining methyl-(Z)-a-acetamidocinnamate (1 mmol) under a hydro-
gen atmosphere. The uptake of hydrogen began immediately
upon stirring. The conversion was calculated by direct measure-
ment of consumption of dihydrogen by using graduated glass-
ware. After completion of the reaction (no further hydrogen
uptake), the resulting solution was concentrated in vacuo, taken
up in dichloromethane (10 mL) and stirred with activated carbon
for 1.5 h. Filtration over celite and removal of the solvent afforded
the hydrogenated product. The conversion was evaluated by
1H NMR analysis. Enantiomeric excesses were determined by chiral
HPLC on a Chiralcel OD-H column, with hexane/iPrOH (9:1) as
eluent.
Experimental Section
Data for compounds 3a and 4a were already described in Referen-
ces [9b] and [9a] respectively. Data for compounds P-aryl-1-oxo-
2,5-diphenylphospholane 1a–j were already described in our previ-
ous paper.[14] The experimental procedure for the preparation of
phosphane-borane complex 2a–j is described in Ref. [14]. Data for
compounds 3b–i and 4b–i were collected in the Supporting Infor-
mation.
Keywords: asymmetric catalysis · homogeneous catalysis ·
hydrogenation · P-ligands · turnover frequency
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Dobrota, Cristian
