elucidate the mechanism of reactions such as hydrophos-
2
CN) ), a mixture of cis- and trans-2a isomers was obtained
in a 1:1 ratio but with almost no racemization.
6
7
15
phinylation and hydrophosphorylation. Regarding these
complexes, only addition processes such as chemoselective
Exchange of chloride to acetate using 2 equiv of AgOAc
resulted in irreversible formation of a neutral Pt(acetato)
complex 3a in 65% yield accompanied by a loss of acetic
8
9
hydrolysis of nitrile to amide or hydroformylation have
been reported to date. However, no catalytic effect on
carbon-carbon bond formation has ever been mentioned
involving platinum-SPO complexes. Recently, we disclosed
an unusual [2 + 1] cycloaddition of terminal alkynes with
norbornene derivatives catalyzed by phosphinous acid-
palladium complexes.1 In connection with these studies,
we decided to examine the catalytic activity of platinum-
SPO complexes in the coupling reaction of norbornadienes
with alkynes. The extension of this method to platinum would
be the first example of carbon-carbon bond formation
3
1
acid. Indeed, P NMR only reveals the exclusive formation
of a cis complex, and mass spectrometry data are in
agreement with a monomeric structure containing one acetate
ligand (Scheme 3).
0,11
Scheme 3. Synthesis of 3a from PtCl (t-BuPhPHO) 2a
2
2
catalyzed by this class of complex. In this letter, we report
2
the synthesis of a novel Pt(η -acetato){[(R)(Ph)PO]
2
H}
complex 3 and its application to the benzylidenecyclopro-
panation of norbornene derivatives. First, we studied the
1
6
Finally, X-ray analysis of the meso complex confirmed
2
2
a Pt(η -acetato){[(t-Bu)(Ph)PO] H} structure for 3a. The
planar geometry around the platinum atom and the distance
between the two oxygen atoms O1 and O2 (2.391 Å) suggest
a symmetrical O-H-O hydrogen bonding (Figure 1). The
formation reaction of this complex involving PtCl
2 3 2
(CH CN)
and 2 equiv of (R)-(+)-tert-butylphenylphosphine oxide (R)-
1
2
(
+)-1a. Complete conversion was achieved after 12 h in
17
31
THF at 60 °C. The P NMR spectrum of the crude mixture
was consistent with the structure PtCl [(t-Bu)(Ph)P(OH)]
a, in which the metal is coordinated by two SPOs 1a in
2
2
2
13
the trans configuration (Scheme 2). Noteworthy in this case
Scheme 2. Synthesis of Platinum Complex 2 from Pt(II) and
SPOs 1
Figure 1. X-ray structure of (meso)-3a. Selected bond lengths [Å]
and angles [°]: Pt1-P1 2.2298 (9), Pt1-P2 2.2267 (9), P1-O1
1
.540 (3), P2-O2 1.553 (3), O4-Pt1-O3 60.34 (10), O4-Pt1-
P2 103.21 (7), 03-Pt1-P1 104.08 (7), P2-Pt1-P1 92.38 (3).
is that the coordination of chiral SPO (R)-(+)-1a to the metal
proceeded with partial racemization.14 Interestingly, when
the reaction was carried out at room temperature for 12 h in
reaction was extended to other SPO ligands. For example,
1
b could be converted to complex 3b with an overall yield
2 2 2 2 3
CH Cl , with PtCl (COD) (more soluble than PtCl (CH -
of 70%.
(
6) Han, L.-B.; Choi, N.; Tanaka, M. Organometallics 1996, 15, 3259-
(12) NMR analyses became easier by monitoring the reaction with
3
1
261.
enantiopure 1. For the enantioselective synthesis of SPOs, see: (a) Leyris,
A.; Nuel, D.; Giordano, L.; Achard, M.; Buono, G. Tetrahedron Lett. 2005,
46, 8673-8676. (b) Leyris, A.; Bigeault, J.; Nuel, D.; Giordano, L.; Buono,
G. Tetrahedron Lett. 2007, 48, 5247-5250.
(7) (a) Han, L.-B.; Tanaka, M. J. Am. Chem. Soc. 1996, 118, 1571-
572. (b) Han, L.-B.; Mirzaei, F.; Zhao, C.-Q.; Tanaka, M. J. Am. Chem.
Soc. 2000, 122, 5407-5408. (c) Han, L.-B.; Zhao, C.-Q.; Onozawa, S.-y.;
Goto, M.; Tanaka, M. J. Am. Chem. Soc. 2002, 124, 3842-3843.
(13) The structures of complexes were deduced from coupling constants
1
(8) (a) Cobley, C. J.; van den Heuvel, M.; Abbadi, A.; de Vries, J. G.
between platinum and phosphorus atoms. For the cis isomer, the J195Pt-P
Tetrahedron Lett. 2000, 41, 2467-2470. (b) Ghaffar, T.; Parkins, A. W. J.
Mol. Catal. A. Chem. 2000, 160, 249-261. (c) Jiang, X.-B.; Minnaard, A.
J.; Feringa, B. L.; de Vries, J. G. J. Org. Chem. 2004, 69, 2327-2331. (d)
Herzon, S. B.; Myers, A. G. J. Am. Chem. Soc. 2005, 127, 5342-5344.
coupling constant is larger (≈3700 Hz) than that for the trans isomer (≈2500
Hz). For 31P NMR study of tertiary phosphine complexes of platinum(II),
see: (a) Grim, S. O.; Keiter, R. L.; McFarlane, W. Inorg. Chem. 1967, 6,
1133-1137. (b) Cobley, C. J.; Pringle, P. G. Inorg. Chim. Acta. 1997, 265,
107-115.
(14) The observation in the NMR spectrum of two doublets for the trans,
meso, and D,L isomers of 2a revealed a racemization process during the
conversion of PtCl2(CH3CN)2 to 2a. The racemization of (R)-(+)-t-BuPhP-
(O)H in the presence of PtCl2 was recently reported: ref 8c.
(15) For similar observations, see: Jiang, X.-B. Ph.D. Thesis, University
of Groningen, 2004.
(16) The crystallographic data for complex 3a have been deposited with
the Cambridge Crystallographic Data Centre (CCDC 644335). These data
can be obtained free of charge at www.ccdc.cam.ac.uk/data_request/cif.
(17) Naik, D. V.; Palenik, G. J.; Jacobson, S.; Carty, A. J. J. Am. Chem.
Soc. 1974, 96, 2286-2288.
(
9) (a) van Leeuwen, P. W. N. M.; Roobeek, C. F.; Wife, R. L.; Frijns,
J. H. G. J. Chem. Soc., Chem. Commun. 1986, 31-33. (b) van Leeuwen,
P. W. N. M.; Roobeek, C. F. New J. Chem. 1990, 14, 487-493.
(
10) Bigeault, J.; Giordano, L.; Buono, G. Angew. Chem., Int. Ed. 2005,
4
4, 4753-4757.
(11) For recent examples of transition metal catalyzed vinylcyclopro-
panation of norbornene derivatives, see: (a) Tr e´ panier, V. E.; Fillion, E.
Organometallics 2007, 26, 30-32. (b) Tseng, N.-W.; Mancuso, J.; Lautens,
M. J. Am. Chem. Soc. 2006, 128, 5338-5339. (c) Miura, T.; Sasaki, T.;
Harumashi, T.; Murakami, M. J. Am. Chem. Soc. 2006, 128, 2516-2517.
(
d) Tenaglia, A.; Marc, S. J. Org. Chem. 2006, 71, 3569-3575 and
references therein.
3568
Org. Lett., Vol. 9, No. 18, 2007