C O M M U N I C A T I O N S
Table 2. Elaboration of Coupling Products 1b and 10ba
(Grant RO1-GM069445) for partial support of this research.
Umicore is acknowledged for generous donation of [Rh(cod)Cl]2.
Supporting Information Available: Experimental procedures,
spectral data for all new compounds, and HPLC data. This material is
References
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a Cited yields are of isolated material 10c: 10b, Boc2O, DMAP, MeCN,
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Through a systematic assay of Brønsted acids, m-nitrobenzoic
acid was identified as the optimal cocatalyst. A potential role for
the Brønsted acid additive has been outlined for analogous couplings
of acetylene to carbonyl compounds.16b The collective data suggest
that Brønsted acids facilitate coupling by circumventing highly
energetic 4-centered transition structures for σ-bond metathesis, as
required for direct hydrogenolysis of metallacyclic intermediates,
with 6-centered transition structures for hydrogenolysis of rhodium
carboxylates derived upon protonolysis of the metallacycle.15
Gratifyingly, the first chiral ligands screened, (S)-MeO-BIPHEP
and (S)-Cl,MeO-BIPHEP, were both found to promote equally
favorable levels of asymmetric induction. Selection of the latter
was arbitrary.
Under these optimized conditions aromatic and aliphatic N-
arylsulfonyl aldimines 1a-12a were found to engage in highly
enantioselective couplings to gaseous acetylene to furnish (Z)-dienyl
allylic amines 1b-12b as single geometrical isomers (Table 1).
Benzaldimines possessing ortho substitution do not react efficiently.
The choice of arylsulfonyl protecting group was made in response
to issues of solubility and reactivity. If the p-toluenesulfonyl
derivative was not sufficiently reactive, the p-nitrophenylsulfonyl
was used. If the p-toluenesulfonyl derivative was not sufficiently
soluble, the benzenesulfonyl derivative was used. Increased yields
are observed upon introduction of Na2SO4, which presumably
mitigates imine hydrolysis and the production of catalytically
inactive hydroxy-bridged dimers of rhodium. To illustrate the unique
features of the (Z)-dienyl side chain, adducts 1b and 10b were
selectively transformed to 1c, 1d, and 10c-j, which embody a
variety of useful functional group arrays (Table 2). The absolute
stereochemical assignment of coupling products 1b-12b is based
upon correlation of 1d to an authentic sample of optically enriched
material, as described in the Supporting Information.
In summary, we report the first highly enantioselective catalytic
vinylation of aldimines.13c,14,16a The present protocol does not
employ preformed organometallic reagents nor does it generate
stoichiometric byproducts. Reductions employing hydrogen as
terminal reductant, termed “hydrogenations,” are used extensively
in academia and industry. As further demonstrated by the results
herein, C-C bond formations employing hydrogen as terminal
reductant, termed “C-C bond forming hydrogenations,” evoke
boundless possibilities in terms of innovative methodologies and
applications to arise in the future.
(13) (a) Oblinger, E.; Montgomery, J. J. Am. Chem. Soc. 1997, 119, 9065. (b)
Huang, W.-S.; Chan, J.; Jamison, T. F. Org. Lett. 2000, 2, 4221. (c) Patel,
S. J.; Jamison, T. F. Angew. Chem., Int. Ed. 2004, 42, 1364.
(14) Catalyzed addition of vinylzirconocenes to imines is known, but enanti-
oselective variants have not been developed: (a) Kakuuchi, A.; Taguchi,
T.; Hanzawa, Y. Tetrahedron Lett. 2003, 44, 923. (b) Wipf, P.; Kendall,
C.; Stephenson, C. R. J. J. Am. Chem. Soc. 2003, 125, 761.
(15) For a recent review of C-C bond-forming hydrogenations, see: Ngai,
M.-Y.; Krische, M. J. J. Org. Chem. 2007, 72, 1063.
(16) (a) Kong, J.-R.; Cho, C.-W.; Krische, M. J. J. Am. Chem. Soc. 2005, 127,
11269. (b) Kong, J.-R.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 16040.
Acknowledgment. Acknowledgment is made to the Welch
Foundation, Johnson & Johnson, Merck and the NIH-NIGMS
JA0715896
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J. AM. CHEM. SOC. VOL. 129, NO. 23, 2007 7243