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8776
J. S. Fossey, C. J. Richards / Tetrahedron Letters 44 (2003) 8773–8776
racemic.16 To further study the mode of catalysis by
the NCN-pincer complexes, we combined 1a and benz-
aldehyde in a 1:0.9 ratio in CD2Cl2, and observed no
change in chemical shift of the carbonyl hydrogen
References
1. For recent reviews see: (a) Steenwinkel, P.; Gossage, R.
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1
(PhCHO) in the H NMR spectrum. In contrast, we
have previously noted the downfield chemical shift in
the 1H NMR of acetonitrile when combined with
approximately one equivalent of group 10 NCN pincer
complexes in CD2Cl2 (e.g. 2.66 ppm with 2a versus
1.93 ppm).3c This points to catalysis proceeding via
activation of TMSCN rather than the aldehyde. The
1H/13C NMR spectra of 1:1 TMSCN/1a in CD2Cl2
were complex and inconclusive. Significantly, the suit-
ability of both aldehyde and imine substrates to catal-
ysis by 1a correlate to the basicities of the carbonyl
oxygens and imine nitrogens respectively, and are
opposite to their intrinsic reactivity towards nucle-
ophiles. This points to a requirement for Lewis acid
activation in the catalytic cycle, and we tentatively
suggest the involvement of intermediate 25 in the silyl-
cyanation of aldehydes catalysed by 1a. The lack of
enantioselectivity with 3b suggests silylcyanation can-
not be an intramolecular process, and in the corre-
sponding intermediate to 25 arising from 3b, the five
bond distance between the metal and the carbonyl
carbon is too great for the oxazoline substituents to
control facial selectivity.
2. Fossey, J. S.; Richards, C. J. Organometallics, in press.
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10. To a solution of 4 (3.6 mmol) and 0.024 g (0.036 mmol)
of 1a in CH2Cl2 (10 mL not specifically dried) was added
TMSCN (0.6 mL, 4.5 mmol) and the resulting solution
stirred at room temperature for 24 h. Following cautious
addition of 3N hydrochloric acid solution (10 mL) utilis-
ing a sodium hypochlorite trap, and subsequent vigorous
stirring for 12 h, the organic fraction was separated, dried
(MgSO4) and evaporated in vacuo. Purification by
column chromatography (gradient elution, 10% EtOAc/
petroleum ether to 100% EtOAc) gave 5 (0.40 g, 84%)
with 1H/13C NMR data consistent with that previously
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Imines are known to coordinate to platinum com-
plexes 2, but organolithium addition to these requires
a
stoichiometric quantity of pincer complex.4c
Attempts to use 1a as a catalyst for the reaction
between 18 and 2-trimethylsilyloxycyclohexene proved
unsuccessful, suggesting that catalysis of imine silyl-
cyanation is mechanistically similar to aldehyde silyl-
cyanation.
11. Crystallographic data (excluding structure factors) for the
structure in this paper, have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication number CCDC 217546. Copies of the
data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax:
+44(0)-1223-336033 or e-mail: deposit@ccdc.com.ac.uk).
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In conclusion, we have demonstrated that platinum
and palladium NCN-pincer complexes are efficient cat-
alysts for the synthesis of many (racemic) cyanohy-
drins and a-amino nitriles. This is a further example
of the growing use of these stable complexes in cataly-
sis, which are ideal for immobilisation on a solid or
dendrimer support.1b
13. (a) Kobayashi, S.; Ishitani, H.; Ueno, M. Synlett 1997,
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Acknowledgements
16. Sample filtered through a short plug of silica to remove
catalyst eluting with CH2Cl2. Column=CP-Chirasil-Dex
CB: Initial temperature 75°C, initial time 1 min, ramp 1.5
deg./min to 195°C. 38.5 and 39.0 min corresponding to
enantiomers of silylated 5 and 66.3 and 67.5 min corre-
sponding to 5.
We thank the EPSRC for support (J.S.F.) and Majid
Motevalli for the X-ray crystal structure determina-
tion. In addition we are indebted to Mark Stark for
carrying out preliminary experiments in this area.