Synthesis of optically active imidazopyridinium salts and the corresponding NHCs

Article abstract of DOI:10.1016/j.tetlet.2006.10.157

A convergent synthesis of chiral imidazo-[1,5-a]-pyridinium salts is described by facile introduction of a stereogenic center via the N2 substituent. Conversion of these optically active salts to the corresponding N-heterocyclic carbenes (NHCs) and their trapping with sulfur followed by optical activity measurements are discussed.

Full text of DOI:10.1016/j.tetlet.2006.10.157

Tetrahedron Letters 48 (2007) 101–104
Synthesis of optically active imidazopyridinium salts
and the corresponding NHCs
Michael A. Schmidt and Mohammad Movassaghi*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 17 October 2006; accepted 30 October 2006
Available online 21 November 2006
Abstract—A convergent synthesis of chiral imidazo-[1,5-a]-pyridinium salts is described by facile introduction of a stereogenic center
via the N2 substituent. Conversion of these optically active salts to the corresponding N-heterocyclic carbenes (NHCs) and their
trapping with sulfur followed by optical activity measurements are discussed.
ꢀ 2006 Elsevier Ltd. All rights reserved.
The isolation of stable N-heterocyclic carbene (NHC)
derivatives by Arduengo has led to recent studies of their
utility and chemistry.1 Imidazolium, triazolium, and
thiazolium salts are employed in a wide range of trans-
formations for organic synthesis. NHCs have served as
superb ligands in metal-catalyzed reactions2 in addition
to providing an unique chemistry enabling the use of
these heterocycles as organic catalysts.3 Importantly,
the use of optically active NHCs (Fig. 1) has led to the
development of a variety of new catalytic asymmetric
transformations for organic synthesis.3,4 Subtle stereo-
electronic effects are known to have a dramatic impact
on the chemistry of NHCs. Lassaletta5a and Glorius5b
have reported the synthesis of NHC derivatives based
on the imidazo-[1,5-a]-pyridinium ring system 10 (R0@H)
and Miyashita has disclosed the use of imidazopyridi-
nium iodide 11 as a highly reactive catalyst.6 Herein we
describe a convergent synthesis of optically active imid-
azo-[1,5-a]-pyridinium salts and the in situ trapping of
the corresponding carbene derivatives.
O
O
O
N
R
R
N
N
+
N
–
N
+
N
N
+
R'
Ar
8
1
Me
Me
9
7
2
–
Me
–
Me
BF
4
BF
OTf
Me
Me
4
N
R
–
N Me
+
R"
N
N
6
+
1, R = Phenyl
4
3
–
I
5
X
3
4
2, R = 1-Napthyl
10
11
Me
O
O
Many of the transformations catalyzed by NHCs3,4
employ the necessary catalyst by the in situ deprotonation
of the corresponding azolium salt. Excellent examples of
the applications of this strategy in accessing the NHCs
of interest are seen in Enders and Rovis’s studies
employing chiral triazolium salt precatalysts (e.g., 7
and 3, Fig. 1) in a variety of catalytic asymmetric reac-
tions. Additionally, the representative use of optically
active imidazolium salts (e.g., 8 and 9, Fig. 1) by
in situ deprotonation and organometallic complex
formation is seen in the preparation of catalysts by
Hoveyda4k and Grubbs4w for the olefin metathesis reaction.
N
N
+
S
–
N
N
S
N
+
OTf
Ph
Me
Me
+
Br
Me
–
–
OTBS
N
–
Me BF
4
5
6
7
Ph
Ph
Me
Cl
i
-Pr
N
+
i
-Pr
N
N
+
Me
Me
HO
i-Pr
–
BF
4
i
-Pr
8
9
Figure 1. Representative chiral azolium salts.
We envisioned a convergent and practical synthesis of
optically active imidazopyridinium trifluoromethane-
sulfonate 12 (Scheme 1) by the condensation of a chiral
*
Corresponding author. Tel.: +1 617 253 3986; fax: +1 617 252
1504; e-mail: movassag@mit.edu
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.157

102
M. A. Schmidt, M. Movassaghi / Tetrahedron Letters 48 (2007) 101–104
primary amine 14 with a suitable pyridine carboxalde-
hyde 15 followed by imidazolium formation.7
EtOH
Na SO
H
H
Me
Ph
2
4
Me
+
O
N
23 ºC, 2 h
94%
N
N
H N
Ph
2
Under optimized conditions, the condensation of com-
mercially available pyridine-2-carboxaldehyde 15a with
(S)-alpha-methylbenzylamine (14a, 98% ee) provided
imine 13a in a 97% yield (Scheme 2). The treatment of
imine 13a8 with 2,2-dimethyl-propionic acid chloro-
methyl ester and anhydrous silver trifluoromethane-
sulfonate in dichloromethane at 40 ꢁC for 15 h in
the dark afforded the desired imidazopyridinium tri-
fluoromethanesulfonate 12a in a 73% isolated yield.
Imidazo[1,5-a]-pyridinium trifluoromethanesulfonate was
readily purified by flash column chromatography (Scheme
2).
15b
14a
13b
Me
Me
NaH
t
KO Bu
Me
Me
N
N
+
–
ClCH OPiv
N
N
2
THF, 3 h
23 ºC
Ph
Ph
OTf
AgOTf, CH Cl
40 ºC,14 h, 72%
2
2
Me
Me
16
83%
(–)-12b
Scheme 3.
the minimization of the steric interactions between the
N2-substituent (alpha-methyl benzyl group) with
the C1-phenyl substituent would result in projection of
the substituents at the stereogenic center toward the
N2–C3–N4 environment. Aza-benzophenone derivative
15c,12 prepared from 2,6-dibromopyridine in two steps
(nBuLi, MeI, THF, ꢀ78 ꢁC; nBuLi, PhCON(Me)OMe),9
was condensed with amine 14a to provide the corre-
sponding imine 13c in a 96% yield as a mixture of keto-
imine isomers (47:53 by 1H NMR). The condensation of
ketone 15c with amine 14a required more forcing condi-
tions as compared to that used with pyridine-2-carbox-
Given the marked enhancement in the stability of the
corresponding carbene derivative of imidazopyridinium
salts with a C5-substituent,5a we examined their synthe-
sis via the optimized conditions described above. The
condensation of 6-methylpyridine-2-carboxaldehyde
15b, prepared from commercially available 2,6-di-
bromopyridine in two steps (nBuLi, MeI, THF, ꢀ78 ꢁC;
nBuLi, DMF),9 with (S)-alpha-methylbenzylamine
(14a, 98% ee) gave the corresponding imine 13b, which
was converted to the desired 5-methylimidazopyridi-
20
aldehyde derivatives 15a,b. Ketoimine 13c, as
a
nium trifluoromethanesulfonate (ꢀ)-12b ð½aꢁD ꢀ43 (c
0.49, CHCl3)) in a 68% overall yield (Scheme 3).10 With
the 5-methyl substituent in place, deprotonation of 12b
(1.0 equiv NaH, 4 mol % KOtBu, THF (0.2 M), 23 ꢁC,
3 h) afforded the corresponding NHC 16 as a viscous
mixture of isomers, was converted to the corresponding
imidazopyridinium salt 12c in a 40% isolated yield
(Scheme 4).13
1
Direct measurement of the enantiomeric excess of the
imidazopyridinium derivatives described above by chiral
HPLC analysis was not optimal. We envisioned trap-
ping of the corresponding in situ generated NHCs, pre-
pared by deprotonation of the imidazopyridinium salts,
in the form of stable thiourea14 derivatives for chiral
HPLC analysis (Scheme 5). The treatment of imidazo-
pyridinium trifluoromethanesulfonates 12b and 12c with
KOtBu in the presence of elemental sulfur provided the
corresponding thiourea derivatives 17a and 17b in 82%
and 78% yield, respectively. The enantiomeric excess of
the thiourea derivatives 17a,b were found to be P98%
ee, thus illustrating that (1) the stereocenter introduced
by the chiral amine is not compromised during the syn-
thesis of the imidazopyridinium salts, and (2) the corre-
sponding NHCs generated by in situ deprotonation are
formed without epimerization.
paste (80–85% yield), with characteristic H NMR reso-
nances11 consistent with those reported by Lassaletta for
a related optically inactive derivative.5a
We also examined the synthesis of the doubly substi-
tuted (C1 and C5) imidazopyridinium derivative 12c
(Scheme 4). Molecular model analysis suggested that
H
H
R*
O
N
R
N R*
+
R
R
N
+
15
N
N
–
OTf
H N R*
2
14
12
13
Scheme 1.
Na SO
-TsOH (10 mol%)
2
4
Ph
Ph Me
N
EtOH
Na SO
2
H
H
Me
Ph
p
Me
4
+
O
Ph
Me
+
O
N
Toluene, 110
24 h
ºC
N
N
H N
Ph
23 ºC, 2 h
97%
2
N
N
H N
2
Ph
15c
14a
13c
96%
Me
Me
15a
14a
13a
Ph
N
Me
Ph
Me
Ph
ClCH OPiv
2
N
+
–
ClCH OPiv
N
+
2
N
AgOTf, CH Cl
2
2
AgOTf, CH Cl
2
2
–
40 ºC,15 h, 73%
OTf
Me
OTf
40 ºC,15 h, 40%
12a
(+)-12c
Scheme 2.
Scheme 4.

M. A. Schmidt, M. Movassaghi / Tetrahedron Letters 48 (2007) 101–104
103
Hedrick, J. L. Org. Lett. 2002, 4, 3587; (k) Van Veldhu-
izen, J. J.; Gillingham, D. G.; Garber, S. B.; Kataoka, O.;
Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 12502; (l)
Mattson, A. E.; Bharadwaj, A. R.; Scheidt, K. A. J. Am.
Chem. Soc. 2004, 126, 2314; (m) Suzuki, Y.; Yamauchi,
K.; Muramatsu, K.; Sato, M. Chem. Commun. 2004, 2770;
(n) Chow, K. Y.; Bode, J. W. J. Am. Chem. Soc. 2004, 126,
8126; (o) Reynolds, N. T.; de Alaniz, J. R.; Rovis, T. J.
Am. Chem. Soc. 2004, 126, 9518; (p) Sohn, S. S.; Rosen, E.
L.; Bode, J. W. J. Am. Chem. Soc. 2004, 126, 14370; (q)
Burstein, C.; Glorius, F. Angew. Chem., Int. Ed. 2004, 43,
6205; (r) Myers, M. C.; Bharadwaj, A. R.; Milgram, B. C.;
Scheidt, K. A. J. Am. Chem. Soc. 2005, 127, 14675; (s)
Kano, T.; Sasaki, K.; Maruoka, K. Org. Lett. 2005, 7,
1347; (t) Movasssaghi, M.; Schmidt, M. A. Org. Lett.
2005, 7, 2453; (u) Fischer, C.; Smith, S. W.; Powell, D. A.;
Fu, G. C. J. Am. Chem. Soc. 2006, 128, 1472; (v) Song, J.
J.; Gallou, F.; Reeves, J. T.; Tan, Z.; Yee, N. K.;
Senanayake, C. M. J. Org. Chem. 2006, 71, 1273; (w)
Funk, T. W.; Berlin, J. M.; Grubbs, R. H. J. Am. Chem.
Soc. 2006, 128, 1840.
S (2.0 equiv)
8
Me
Ph
Me
Ph
KO
t-Bu (1.0 equiv)
N
+
N
N
N
THF, 23
º
C, 15 min
–
S
OTf
82 %
Me
Me
(–)-12b
17a, ≥98% ee
Ph
Ph
Me
N
S (2.0 equiv)
8
KOt-Bu (1.0 equiv)
Me
Ph
N
+
N
N
Ph
THF, 23
ºC, 15 min
–
78 %
S
OTf
Me
Me
(+)-12c
Scheme 5.
17b,≥98% ee
We have described a short and convergent synthesis of
optically active imidazo-[1,5-a]-pyridinium derivatives.
The synthesis of these imidazopyridinium salts and their
in situ deprotonation to the corresponding NHCs occurs
without any loss in optical activity. Trapping of the
in situ generated NHC derivatives as the corresponding
isolable thiourea derivatives allows a simple method for
enantiomeric excess determination. This short synthetic
sequence allows a multi-gram synthesis (e.g., >4 g-scale
of (ꢀ)-12b)10 of these optically active NHC precursors.
5. (a) Alcarazo, M.; Rosenblade, S. J.; Cowley, A. R.;
´
Fernandez, R.; Brown, J. M.; Lassaletta, J. M. J. Am.
Chem. Soc. 2005, 127, 3290; (b) Burnstein, C.; Lehmann,
C. W.; Glorius, F. Tetrahedron 2005, 61, 6207.
6. (a) Miyashita, A.; Suzuki, Y.; Okumra, Y.; Iwamoto, K-i.;
Higashino, T. Chem. Pharm. Bull. 1996, 46, 6; (b)
Miyashita, A.; Suzuki, Y.; Kobayashi, M.; Kuriyama,
N.; Higashino, T. Heterocycles 1996, 43, 509.
7. For related imidazolium syntheses, see: (a) Altenhoff,
G.; Goddard, R.; Lehmann, C. W.; Glorius, F. J. Am.
Chem. Soc. 2004, 126, 15195; (b) Glorius, F.; Altenhoff,
G.; Goddard, R.; Lehmann, C. Chem. Commun. 2002,
2704.
8. The imines may be used without purification in the next
step.
9. Hicks, R. G.; Koivisto, B. D.; Lemaire, M. T. Org. Lett.
2004, 6, 1887.
Acknowledgments
M.M. is a Dale F. and Betty Ann Frey Damon Runyon
Scholar supported by the Damon Runyon Cancer Re-
search Foundation (DRS-39-04). M.A.S. acknowledges
partial graduate fellowship support by the Milas fund.
We acknowledge ACS-PRF (40631G1), NSF (547905),
Firmenich, Amgen, and MIT for the partial support of
this work.
10. Synthesis of 5-methyl-2-(1-(S)-phenylethyl)-imidazo[1,5-a]-
pyridin-2-ium trifluoromethanesulfonate (12b): (S)-a-
methylbenzylamine (14a, 2.09 mL, 16.4 mmol, 1.00 equiv)
and anhydrous sodium sulfate (3.2 g) were added to a
solution of aldehyde 15b (1.99 g, 16.4 mmol, 1 equiv) in
absolute ethanol (32 mL) and the resulting suspension was
vigorously stirred at 23 ꢁC. After 2 h, the reaction mixture
was filtered and the resulting solution was concentrated to
provide the desired crude imine 13b (3.47 g). 2,2-
Dimethyl-propionic acid chloromethyl ester (3.14 mL,
21.7 mmol, 1.40 equiv) and silver trifluoromethan-
sulfonate (4.77 g, 18.6 mmol, 1.2 equiv) were added to a
solution of crude imine 13b (3.47 g) in dichloromethane
(150 mL) and the resulting mixture was heated to 40 ꢁC in
the dark. After 14 h, the dark suspension was cooled to
23 ꢁC, was filtered through a plug of Celite (7.5 cm
dia. · 2.5 cm ht.). The filter cake was rinsed with methanol
(2 · 20 mL) and the filtrate was concentrated in vacuo to
yield a dark purple oil (8.8 g). Purification of the residue
by flash column chromatography (2.5 · 26 cm, 100%
CH2Cl2 to 2% MeOH in CH2Cl2) afforded imidazopyrid-
inium salt 12b as a light beige powder (4.14 g, 72%). Mp:
106–107 ꢁC. TLC (10% MeOH in CH2Cl2), Rf: 0.48 (UV,
References and notes
1. (a) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1991, 113, 361; (b) Arduengo, A. J., III;
Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995,
117, 11027.
2. (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand,
G. Chem. Rev. 2000, 100, 39; (b) Cowley, A. H. J.
Organomet. Chem. 2001, 617–618, 105; (c) Herrmann, W.
A. Angew. Chem., Int. Ed. 2002, 41, 1290.
3. (a) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37,
534; (b) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed.
2004, 43, 5138; (c) Nair, V.; Bindu, S.; Sreekumar, V.
Angew. Chem., Int. Ed. 2004, 43, 5130.
4. For representative examples see: (a) Breslow, R. J. Am.
Chem. Soc. 1958, 80, 3719; (b) Sheehan, J. C.; Hunne-
mann, D. H. J. Am. Chem. Soc. 1966, 88, 3666; (c)
Sheehan, J. C.; Hara, T. J. Org. Chem. 1974, 39, 1196; (d)
Stetter, H.; Kuhlmann, H. Synthesis 1975, 379; (e) Knight,
R. L.; Leeper, F. J. J. Chem. Soc., Perkin Trans. 1 1998,
1891; (f) Enders, D.; Kallfass, U. Angew. Chem., Int. Ed.
2002, 41, 1743; (g) Connor, E. F.; Nyce, G. W.; Myers,
M.; Mock, A.; Hedrick, J. L. J. Am. Chem. Soc. 2002, 124,
914; (h) Kerr, M. S.; de Alaniz, J. R.; Rovis, T. J. Am.
Chem. Soc. 2002, 124, 10298; (i) Grasa, G. A.; Kissling, R.
M.; Nolan, S. P. Org. Lett. 2002, 4, 3583; (j) Nyce, G. W.;
Lamboy, J. A.; Connor, E. F.; Waymouth, R. M.;
20
KMnO4). ½aꢁD ꢀ43 (c 0.49, CHCl3). The ee of 12b was
determined to be 98.6% by conversion to thiourea 17a and
analysis by HPLC (Chirapak AD-H, 30% isopropanol in
hexanes, 1.0 mL/min, tmajor = 6.99 min, tminor = 4.83
1
min). H NMR (500 MHz, CDCl3, 20 ꢁC): d 10.1 (s, 1H,
C3–H), 7.65 (s, 1H, C1–H), 7.52–7.54 (m, 2H, PhH), 7.38–
7.49 (m, 4H, PhH and C8–H), 7.16 (dd, J = 9.5 Hz,
7.0 Hz, 1H, C7–H), 6.86 (dt, J = 7.0 Hz, 1.0 Hz, 1H, C6–

104
M. A. Schmidt, M. Movassaghi / Tetrahedron Letters 48 (2007) 101–104
H), 6.28 (q, J = 7.0 Hz, 1H, MeCH), 2.82 (s, 3H, pyr–
1.40 equiv) and silver trifluoromethanesulfonate (226 mg,
0.878 mmol, 1.20 equiv) were added to a solution of the
imine 13c (220 mg, 0.732 mmol, 1 equiv) in CH2Cl2
(7.00 mL) and the resulting mixture was heated to 40 ꢁC
in the dark. After 15 h, the black suspension was cooled to
CH3), 2.12 (d, J = 7.0 Hz, 3H, CH3CH). 13C NMR
(125.8 MHz, CDCl3, 20 ꢁC): d 137.9, 133.5, 131.1, 129.7,
129.6, 127.4, 125.6, 123.5, 120.9 (q, JCF = 320.2 Hz, CF3),
116.9, 116.1, 111.8, 61.4 (MeCHPh), 21.0 (CH3–pyr), 18.2
(CH3CHPh). FTIR (neat) cmꢀ1: 3106 (m), 1662 (w), 1562
(w), 1458 (w), 1259 (s). HRMS-ESI (m/z): calcd for
C16H17N2 [M+]: 237.1392, found: 237.1393.
23 ꢁC and was filtered through
a plug of Celite
(2.5 · 1.75 cm). The filter cake was rinsed with MeOH
(3 · 3 mL) and the filtrate was concentrated in vacuo to
yield a viscous red oil. Purification of the residue by flash
column chromatography (2.5 · 25 cm, 1% MeOH in
CH2Cl2 to 10% MeOH in CH2Cl2) afforded 12c as a red
oil. The product was triturated from CH2Cl2 (1 mL) by the
addition of Et2O (10 mL). Collection of the solid and
removal of the volatiles afforded imidazopyridinium salt
12c as an off-white solid (136 mg, 40%). Mp: 125–127 ꢁC.
TLC (10% MeOH in CH2Cl2), Rf: 0.48 (UV, KMnO4).
11. 1H NMR (500 MHz, C6D6, 20 ꢁC): d 7.01–7.15 (m, 5H,
PhH), 6.84 (d, J = 9.0 Hz, 1H, C8–H), 6.76 (s, 1H, C1–H),
6.33 (dd, J = 9.3 Hz, 6.3 Hz, 1H, C7–H), 5.87 (d,
J = 6.0 Hz, 1H, C6–H), 5.74 (q, J = 7.0 Hz, 1H, MeCH),
2.76 (s, 3H, pyr–CH3), 1.82 (d, J = 7.0 Hz, 3H, CH3CH).
1
12. Ketone 15c: H NMR (500 MHz, CDCl3, 20 ꢁC): d 8.10
(app d, J = 7.5 Hz, 2H, PhHo), 7.76–7.80 (m, 2H, C3–H
and C4–H), 7.60 (app t, J = 7.5 Hz, 1H, PhHp), 7.49 (app
t, J = 7.8 Hz, 2H, PhHm), 7.35 (dd, J = 6.8 Hz, 2.3 Hz,
1H, C5–H), 2.65 (s, 3H, CH3). 13C NMR (125.8 MHz,
CDCl3, 20 ꢁC): d 194.0, 157.8, 154.7, 137.1, 136.3, 133.0,
131.3, 128.2, 125.9, 121.8, 24.6.
20
½aꢁD +5 (c 0.505, CHCl3). The ee of 12c was determined to
be 98.3% by conversion to thiourea 17b and analysis by
HPLC (Chirapak AD-H, 100% hexanes to 30% isopropa-
nol in hexanes over 10 min, 2.0 mL/min, tmajor = 5.14 min,
1
13. Synthesis of 5-methyl-1-phenyl-2-(1-(S)-phenylethyl)-imid-
azo[1,5-a]pyridin-2-ium trifluoromethanesulfonate (12c):
(S)-a-Methylbenzylamine (14a, 107 lL, 0.837 mmol,
tminor = 4.75 min). H NMR (500 MHz, CDCl3, 20 ꢁC): d
10.07 (s, 1H, C3–H), 7.63, (app t, J = 7.8 Hz, 1H), 7.56 (t,
J = 7.8 Hz, 2H), 7.24–7.28 (m, 5H), 7.25 (d, J = 10.0 Hz,
1H, C8–H), 7.16 (m, 2H), 7.12 (dd, J = 9.5 Hz, 7.0 Hz,
1H, C7–H), 6.90 (dt, J = 6.5 Hz, 0.5 Hz, 1H, C6–H), 5.80
(q, J = 7.0 Hz, 1H, CHPh), 2.95 (s, 3H, pyr–CH3), 2.24 (d,
J = 7.5 Hz, 3H, CHCH3). 13C NMR (125.8 MHz, CDCl3,
20 ꢁC): d 138.4, 134.5, 131.2, 129.8, 129.5, 129.3, 129.1,
126.9, 125.9, 125.6, 124.9, 123.3, 120.9 (q, J = 320.5 Hz,
CF3), 117.1, 115.2, 60.3 (CH3CHPh), 21.7 (CH3–pyr), 18.3
(CH3CH). FTIR (neat) cmꢀ1: 3106 (m), 2989 (w), 1661
(m), 1554 (m), 1456 (s). HRMS-ESI (m/z): calcd for
C22H21N2 [M+]: 313.1699, found: 313.1692.
1.10 equiv),
p-toluenesulfonic
acid
monohydrate
(14.5 mg) and anhydrous sodium sulfate (1.00 g) were
added to a solution of ketone 15c (150 mg, 0.760 mmol,
1 equiv) in toluene (4.00 mL) and the resulting suspension
was heated to 110 ꢁC. After 24 h, the mixture was cooled
to an ambient temperature, and was concentrated in
vacuo. The resulting light orange residue was diluted with
hexanes (2.0 mL) and filtered. The filter cake was washed
with hexanes (3 · 2 mL) and the filtrate was concentrated
in vacuo to give the desired imine 13c (220 mg, 96%) as a
mixture of ketoimine isomers (47:53). 2,2-Dimethyl-
propionic acid chloromethyl ester (150 lL, 1.02 mmol,
14. Seo, H.; Kim, B. Y.; Lee, J. H.; Park, H.-J.; Son, S. U.;
Chung, Y. K. Organometallics 2003, 22, 4783.