This was because a previously reported hydrogenation of
the same substrate under 1 atm of hydrogen pressure using
a chiral Ir complex with BARF required a relatively high
catalyst loading (2 mol %) and a long reaction time (4 h) to
give a moderate yield (39%) of the product.2u
Table 1. Effects of Counterions
Encouraged by this result, we prepared an iridium(I)
complex of (S,S)-1,2-bis(tert-butylmethylphosphino)ethane
((S,S)-t-Bu-BisP*) bearing BARF as the counterion because
the phosphine ligand was proved to show pronounced
efficiency in the rhodium-catalyzed hydrogenation of dehy-
droamino acids and related substrates,6 and we envisioned
that the ligand could also be successfully used in the iridium-
catalyzed asymmetric hydrogenation of imines. The prepara-
tive route to the desired complex [Ir((S,S)-t-Bu-BisP*)(cod)]-
[BARF] is shown in Scheme 1. In this preparation, we used
entry
Ir complex
yield (%)a
ee (%)
1
2
3
4
5
[IrCl(cod)]2/(S)-BINAP
[Ir((S)-binap)(cod)][BF4]
[Ir((S)-binap)(cod)][OTf]
[Ir((S)-binap)(cod)][PF6]
[Ir((S)-binap)(cod)][BARF]
0
0
0
0
93
16
a Isolated yield.
ethyl)phenyl)borate ([BARF]-)4 remarkably promoted the
reduction even under 1 atm of hydrogen pressure to give
the product in 93% yield after 1.5 h, albeit with low ee (16%)
of the product (entry 5). Although the pronounced rate
acceleration effect of BARF as the counterion was observed
in many reactions including Ir-catalyzed hydrogenation
reactions,2m,p,r,u,w,5 we were surprised to obtain this result.
Scheme 1. Preparation of [Ir(t-Bu-BisP*)(cod)][BARF]
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