Alkali-metal-catalyzed addition of primary and secondary phosphines to carbodiimides. A general and efficient route to substituted phosphaguanidines

Article abstract of DOI:10.1039/b609198a

Organo alkali metal compounds such as ⁿBuLi and (Me ₃Si)₂NK act as excellent catalyst precursors for the addition of phosphine P-H bonds to carbodiimides, offering a general and atom-economical route to substituted phospha

Full text of DOI:10.1039/b609198a

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COMMUNICATION
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Alkali-metal-catalyzed addition of primary and secondary phosphines to
carbodiimides. A general and efficient route to substituted
phosphaguanidines{
Wen-Xiong Zhang, Masayoshi Nishiura and Zhaomin Hou*
Received (in Cambridge, UK) 29th June 2006, Accepted 18th July 2006
First published as an Advance Article on the web 2nd August 2006
DOI: 10.1039/b609198a
n
Organo alkali metal compounds such as BuLi and
readily available alkali metal compounds such as (Me
Si) NM
3 2
(
Me Si)
3
2
NK act as excellent catalyst precursors for the addition
(M 5 Li, Na, K) and RLi (R 5 n-Bu, CH SiMe ) can be used as
3
2
excellent catalyst precursors. Aromatic C–Br and C–Cl bonds
survived the reaction conditions. The alkali metal phosphaguani-
dinate species has been confirmed to be a true catalyst species.
As a control experiment, N,N9-diisopropylcarbodiimide was
of phosphine P–H bonds to carbodiimides, offering a general
and atom-economical route to substituted phosphaguanidines,
with excellent tolerability to aromatic C–Br and C–Cl bonds.
Metal-catalyzed C–P bond formation reactions by P–H bond
activation are among the most important transformations in
heated with diphenylphosphine in C D Cl at 140 uC, but no
6 5
reaction was observed in 12 h (Table 1, entry 1). In contrast,
1,2
synthetic organic chemistry. Catalytic addition of phosphine
P–H bonds across the C–N double bond of carbodiimides
R9NLCLNR9) could be, in principle, a straightforward and atom-
economical route to substituted phosphaguanidines
addition of a small amount of an organo alkali metal compound
R
2
3 2
such as (Me Si) NM (M 5 Li, Na, K) or RLi (R 5 n-Bu,
(
CH SiMe ) resulted in rapid reaction to give the corresponding
2
3
phosphaguanidine 1a at room temperature (Table 1).{ THF
seemed to be a better solvent than benzene or toluene (Table 1,
R9NLC(PR )(NHR9), a class of heteroatom-containing com-
2
pounds which may be used as building blocks for organic
3
synthesis and as unique ligands for various metal complexes.
entries 2–4). Among (Me
increased as the metal size becomes larger (K . Na . Li) (Table 1,
entries 2, 7, 8). Thus, in the presence of 1 mol% of (Me Si) NK,
PH with PrNLCLNPr yield 1a quantitatively at
3 2
Si) NM (M 5 Li, Na, K), the activity
However, such catalytic process has been hardly explored. The
synthesis of neutral phosphaguanidines was first reported in
3
2
i
i
the reaction of Ph
2
4,5
1980.
room temperature in less than 5 min (Table 1, entries 9–11).
Addition of diphenylphosphine to an N,N9-diaryl
NLCLNC Me-p
was reported to occur at elevated temperatures to afford the
substituted carbodiimide such as p-MeC
6
H
4
6 4
H
Table 2 summarizes some representative results of the
(Me
diimides having various substituents. In the presence of 1 mol% of
Me Si) NK, the reaction of Ph PH with carbodiimides having
Si) NM-catalyzed reactions between phosphines and carbo-
3 2
corresponding N,N9-diaryl substituted phosphaguanidine
4
(
p-MeC H NLC(PPh )(NHC H Me-p). However, N,N9-dialkyl
3
2
2
6
4
2
6
4
N,N9-diaryl, N-aryl-N9-alkyl, and N,N9-dialkyl substituents was
or N-alky-N9-aryl substituted phosphaguanidines could not be
obtained in this way because the less electrophilic N,N9-dialkyl or
N-alky-N9-aryl carbodiimides could not accept nucleophilic
completed at room temperature within 5 min to yield the
attack of
a
phosphine. Silylated phosphaguanidines
)}{N(SiMe )R9} could be obtained by insertion
Table 1 Alkali-metal-catalyzed addition of diphenylphosphine to
N,N9-diisopropylcarbodiimide
a
R9NLC{PR(SiMe
3
3
of a carbodiimide into one of the P–Si bonds of bis-silylated
phosphines, but depending on the nitrogen substituents, silyl-
migration could occur to give the phosphaalkene derivatives
5
RPLC{N(SiMe )R9} . Very recently, Coles and coworkers
3
2
reported the synthesis of N,N9-dialkyl substituted phosphaguani-
dines R9NLC(PPh )(NHR9) (R9 5 Cy, i-Pr) by nucleophilic
addition of LiPPh (generated in-situ from Ph PH and n-BuLi) to
2 2
b
2
Temp Time Conv
(uC)
Entry Cat. (mol%)
Solvent
(min) (%)
a stoichiometric amount of R9NLCLNR9, followed by proto-
nolysis of the resultant lithium phosphaguanidinates
1
2
3
4
5
6
7
8
9
—
(Me
(Me
3
(Me Si) NLi (3)
n-BuLi (3)
(Me
(Me
(Me Si) NK (3)
3
(Me Si) NK (1)
3
3
(Me Si) NK (1)
C
C
6
D
D
5
Cl
140
r.t.
r.t.
720
40
15
45
40
40
10
,5
,5
,5
,5
1a (0)
3
Si)
Si)
2
NLi (3)
NLi (3)
6
6
1a (.99)
1a (.99)
1a (.99)
1a (.99)
1a (.99)
1a (.99)
1a (.99)
1a (.99)
1a (.99)
1a (.99)
3a
[Ph PC(NR9 )Li] with [HNEt ][Cl]. We report here a novel
2 2 3
3
2
THF-d
toluene-d8 r.t.
8
catalytic synthesis of phosphaguanidines by alkali-metal-catalyzed
6–8
addition of phosphines to carbodiimides.
2
C
6
C
6
C
6
D
6
D
6
D
6
6
6
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
The commercially
3
Si)CH
2
Li (3)
Si) NNa (3)
3 2
Organometallic Chemistry Laboratory, RIKEN (The Institute of
Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama
C D
2
6
2
C D
6
3
51-0198, Japan. E-mail: houz@riken.jp; Fax: 81 48 462 4665;
Tel: 81 48 467 9393
Electronic supplementary information (ESI) available: experimental
10
11
a
THF-d8
toluene-d8 r.t.
2
(Me Si) NK (1)
3
2
{
Conditions:
diphenylphosphine, 0.60
mmol; diisopropyl-
31
carbodiimide, 0.60 mmol. Conversion determined by P NMR.
details, X-ray data for 2, and scanned NMR spectra of all new products.
See DOI: 10.1039/b609198a
b
3
812 | Chem. Commun., 2006, 3812–3814
This journal is ß The Royal Society of Chemistry 2006

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a
Table 2 Catalytic addition of phosphines to carbodiimides
Scheme 1 Catalytic
addition
of
diphenylphosphine
to
N,N9-diisopropylcarbodiimide under a solvent-free condition.
b
M
(mol%) Time (%)
Yield
2
Entry R R PH
3
1
R, R
products could be isolated in excellent yields by a single
recrystallization.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
Ph
Ph
Ph
Ph
Ph
(2-MeC
(3-MeC
(4-MeC
(3,5-Me
(4-MeOC
(4-ClC
(3,5-Cl
(4-BrC
PhEtPH
PhEtPH
PhEtPH
(i-Bu)
(i-Bu)
(i-Bu)
2
2
2
2
2
PH
PH
PH
PH
PH
i-Pr
Cy
K (1)
K (1)
t-Bu, Et K (1)
5 min 1a (99)
5 min 1b (99)
5 min 1c (98)
5 min 1d (99)
5 min 1e (99)
5 min 1f (99)
5 min 1g (99)
5 min 1h (99)
5 min 1i (98)
5 min 1j (99)
5 min 1k (98)
5 min 1l (98)
5 min 1m (97)
The present catalytic reaction could also be carried out under a
solvent-free condition on a larger preparative scale, demonstrating
well its practical usefulness. For example, addition of 1 mol% of
Ph, Cy
p-tolyl
i-Pr
i-Pr
i-Pr
K (1)
K (1)
K (1)
K (1)
K (1)
K (1)
K (1)
K (1)
K (1)
K (1)
K (3)
Li (3)
Li (3)
K (3)
Li (3)
Li (3)
K (1)
K (1)
6
6
6
H
H
H
4
4
4
)
)
)
2
2
2
PH
PH
PH
i
i
(
Me
3
Si) NK to Ph
2
2
PH (50 mmol) and PrNLCLNPr (50 mmol) at
i i
room temperature yielded PrNLC(PPh )(NH Pr) (1a) quantita-
2
2
C
6
H
3
)
2
PH i-Pr
tively in 30 min (Scheme 1).
1
1
1
1
1
1
1
1
1
1
2
2
a
6
4
H
)
4
)
2
PH
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
In a 1 : 1 : 1 reaction of (Me
CyNLCLNCy, the potassium phosphaguanidinate complex
[Ph PC(NCy) K(OEt )] (2) was isolated quantitatively from a
3 2 2
Si) NK, Ph PH, and
6
H
2
PH
PH
PH
2
C
6
H
3
)
2
6
H
4
)
2
2
2
2 2
4 h
24 h
8 h
12 h
24 h
48 h
1 h
1n (96)
1n (0)
1n (90)
1o (95)
1o (0)
1o (25)
1p (97)
1q (80)
diethyl ether solution and confirmed by an X-ray diffraction
analysis.§ There is a crystallographic inversion centre in 2.
Complex 2 adopts an ‘‘inverse sandwich’’ dimeric structure, in
which the two K atoms are bridged by two coplanar guanidinate
c
2
2
2
PH
PH
PH
d
2
2
units in a m-g ,g -fashion through the nitrogen atoms (Fig. 1).
PhPH
CyPH
2
e
This coordination mode is in contrast with that observed in a
2
1 h
3
b
lithium analogue, and as far as we are aware, has not been
9
reported previously for a phosphaguanidinate unit. Addition of
Conditions: phosphine, 2.02 mmol; carbodiimide, 2.00 mmol;
catalyst, 0.02 mmol; solvent, mL, unless otherwise noted.
5
b
c
d
Isolated yield. Toluene, 110 uC. THF, 110 uC; conversion
2
molar equiv. of Ph PH to a THF solution of 2 yielded 1b and
2
3
1
e
i
i
2
determined by P NMR. ( PrNLCNH Pr) (PCy) (15%, determined
31
2
KPPh quantitatively.
by P NMR) was also observed.
A possible mechanism for the present catalytic reaction could be
proposed as shown in Scheme 2. The acid–base reaction between
(Me Si) NK and a phosphine P–H bond should yield straightfor-
corresponding substituted phosphaguanidines quantitatively
Table 2, entries 1–5). A wide range of diarylphosphines could
3
2
(
wardly a phosphide species such as A. Nucleophilic addition of the
phosphide species to a carbodiimide would afford the phospha-
guanidinate species B, which on abstraction of a proton from
another molecule of phosphine would yield the phosphaguanidine
product 1 and regenerate the phosphide A. The isolation of 2 and
its reaction with Ph PH to give 1b and KPPh strongly support
be used as the nucleophiles. The reaction was not affected by either
electron-withdrawing or -donating substituents or their positions
at the phenyl ring of the phosphines (Table 2, entries 6–13).
Aromatic C–Cl (entries 11 and 12) and C–Br (entry 13) bonds
survived the catalytic conditions to yield selectively the corre-
sponding halogen-substituted phosphaguanidines 1k–m, a new
class of phosphaguanidine building blocks that could be useful for
construction of further larger phosphaguanidine derivatives. It is
also noteworthy that such halogen-tolerance was also observed
even when n-BuLi was used as a catalyst. This is in sharp contrast
2
2
this mechanism.
In summary, we have demonstrated that alkali metal
compounds such as (Me Si) NK can act as an excellent catalyst
3
2
precursor for the addition of various phosphine P–H bonds to
carbodiimides, which offers the first general and atom-economical
6 4 2
with the stoichiometric reactions between (4-XC H ) PH/n-BuLi
i
i
and PrNLCLNPr, which yielded a mixture of the X-containing
phosphaguanidines and the X-free phosphaguanidines (X 5 Cl,
3
Br) after protonolysis with [HNEt ][Cl]. The reaction of an
alkyl/aryl phosphine such as ethylphenylphosphine (entry 14) or a
dialkyl phosphine such as diisobutylphosphine (entry 17) with
i
i
PrNLCLNPr required a longer time for completion, probably
owing to the weaker acidity of these phosphines compared to that
i
i
of diaryl phosphines. The reaction of PhPH
2
with PrNLCLNPr
i
afforded selectively the mono-addition product PrNL
C(PHPh)(NH Pr) (1p) (entry 20), while in the case of CyPH ,
2
i
i
i
the bis-addition product ( PrNLCNH Pr) (PCy) was also obtained
2
Fig. 1 ORTEP drawing of 2 (ellipsoids drawn at the 50% probability
˚
level; hydrogen atoms omitted for clarity). Selected bond length (A): K(1)–
as
a minor product (15%) in addition to the mono-
i
i
addition product PrNLC(PHCy)(NH Pr) (1q, 80%) (entry 21).
In most of the above reactions, the resulting phosphaguanidine
N(1) 2.891(2), K(1)–N(2) 2.759(2), K(1)–N(19) 2.857(2), K(1)–N(29)
2.788(2), N(1)–C(1) 1.331(3), N(2)–C(1) 1.319(3). 9 5 2x, 2y, 2z.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 3812–3814 | 3813

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21
§
T 5 173(1) K, Monoclinic, space group P2(1)/c, a 5 11.1009(12),
84 2 4 2 2 w
Cryatal data for 2: C58H K N O P , M 5 1009.43 g mol ,
˚
b 5 19.529(2), c 5 14.3917(15) A, a 5 90, b 5 112.539(2), c 5 90u,
3
23
21
,
˚
V 5 2881.6(5) A , Z 5 2, rcalcd 5 1.163 Mg m , m 5 0.263 mm
reflections collected: 14823, independent reflections: 5091 (Rint 5 0.0259),
Final R indices [I . 2sI]: R 5 0.0539, wR 5 0.1626, R indices (all data):
5 0.0689, wR 5 0.1716. The unique coordinated ether molecule has its
CH groups disordered equally over two sites (only one of which is shown
1
2
R
1
2
2
in Fig. 1). No allowance was made for the ether H atoms. CCDC 606651.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b609198a
1
2
Review: O. Delacroix and A. C. Gaumont, Curr. Org. Chem., 2005, 9,
851–1882.
(a) A. M. Kawaoka and T. J. Marks, J. Am. Chem. Soc., 2005, 127,
311–6324; (b) M. R. Douglass, C. L. Stern and T. J. Marks, J. Am.
1
Scheme 2 A possible mechanism of catalytic addition of phosphines to
carbodiimides.
6
Chem. Soc., 2001, 123, 10221–10238; (c) A. D. Sadow and A. Togni,
J. Am. Chem. Soc., 2005, 127, 17012–17024; (d) H. Ohmiya, H. Yorimitsu
and K. Oshima, Angew. Chem., Int. Ed., 2005, 44, 2368–2370; (e)
M. O. Shulyupin, M. A. Kazankova and I. P. Beletskaya, Org. Lett.,
route to substituted phosphaguanidines, with excellent tolerability
to aromatic carbon–halogen bonds. In addition, this catalytic
process is very clean and can also be carried out under solvent-free
conditions, showing high potential of practical use.
2002, 4, 761–763.
3
(a) J. Grundy, M. P. Coles and P. B. Hitchcock, Dalton Trans., 2003,
2573–2577; (b) M. P. Coles and P. B. Hitchcock, Chem. Commun., 2002,
This work was partly supported by RIKEN’s Ecomolecular
Science Research Program and by the Natural Science Foundation
of China (20328201).
2794–2795; (c) N. E. Mansfield, M. P. Coles and P. B. Hitchcock, Dalton
Trans., 2005, 2833–2841; (d) J. Grundy, M. P. Coles, A. G. Avent and
P. B. Hitchcock, Chem. Commun., 2004, 2410–2411; (e) N. E. Mansfield,
M. P. Coles and P. B. Hitchcock, Dalton Trans., 2006, 2052–2054; (f)
M. P. Coles, Dalton Trans., 2006, 985–1001; (g) N. E. Mansfield,
M. P. Coles, A. G. Avent and P. B. Hitchcock, Organometallics, 2006, 25,
2470–2474; (h) D. H. M. W. Thewissen, H. P. M. M. Ambrosius,
H. L. M. van Gaal and J. J. Steggerda, J. Organomet. Chem., 1980, 192,
101–113.
4 (a) D. H. M. W. Thewissen and H. P. M. M. Ambrosius, Recl. Trav.
Chim. Pays-Bas, 1980, 99, 344–346; (b) H. P. M. M. Ambrosius,
A. H. I. M. van der Linden and J. J. Steggerda, J. Organomet. Chem.,
1980, 204, 211–220.
Notes and references
{
A typical procedure for the preparation of phosphaguanidines 1 by use of
Me Si) NK as a catalyst. Under a dry and oxygen-free argon atmosphere,
a THF solution (3 mL) of diphenylphosphine (376 mg, 2.02 mmol) was
added to a THF solution (2 mL) of (Me Si) NK (4 mg, 0.02 mmol) in a
(
3
2
3
2
Schlenk tube. Then N,N9-diisopropylcarbodiimide (252 mg, 2.00 mmol)
was added to the above reaction mixture. After 5 min of stirring, the
solvent was removed under reduced pressure. The residue was extracted
with hexane and filtered to give a clean solution. After removal of the
solvent under vacuum, the residue was recrystallized in hexane to provide a
colorless solid 1a. It should be noted that this type of phosphaguanidine is
very sensitive to oxygen, and must be stored under an inert atmosphere. IR
5 K. Issleib, H. Schmidt and H. Meyer, J. Organomet. Chem., 1980, 192,
33–39.
6
For examples of alkali-metal-catalyzed reactions, see: (a) T. Harada,
K. Mizunashi and K. Muramatsu, Chem. Commun., 2006, 638–639; (b)
T. Harada, K. Muramatsu, T. Fujiwara, H. Kataoka and A. Oku, Org.
Lett., 2005, 7, 779–781; (c) N. Yamagiwa, H. Qin, S. Matsunaga and
M. Shibasaki, J. Am. Chem. Soc., 2005, 127, 13419–13417; (d) M. Itoh,
K. Inoue, J. Ishikawa and K. Iwata, J. Organomet. Chem., 2001, 629,
(
Nujol): n 5 3431 (N–H), 1599 (C5N), 1462, 1377, 1173, 1026, 743,
21 1
6
96 cm ; H NMR (300 MHz, C
CH(CH ), 1.23 (d, J 5 6.3 Hz, 6H, CH(CH
NH), 4.28–4.43 (m, 2H, CH), 7.03–7.05 (m, 6H, C
6 6
D ): d 5 0.94 (d, J 5 6.6 Hz, 6H,
3
3
)
2
3
)
2
), 3.63 (d, J 5 6.3 Hz, 1H,
), 7.42–7.47 (m, 4H,
6 5 6 6
C H ); C NMR (75 MHz, C D ): d 5 22.5, 25.3, 42.9, 52.2 (d,
3 3
6 5
H
13
1
–6; (e) J. Ishikawa and M. Itoh, J. Catal., 1999, 185, 454–461; (f)
L. Assadourian and G. Gau, Appl. Organomet. Chem., 1991, 5, 167–172;
g) H. Prines and H. E. Eschinazi, J. Am. Chem. Soc., 1956, 78,
178–1180.
2
J
PC 5 35.3 Hz), 129.0 (d, JPC 5 6.8 Hz), 129.3, 134.3 (d, JPC 5 19.8 Hz),
1 1 31 1
1
(
3
35.5 (d,
160 MHz, C
13.1834; Found 313.1853.
Isolation of the potassium phosphaguanidinate [Ph
2). Under a dry and oxygen-free argon atmosphere, a THF solution (3 mL)
of diphenylphosphine (372 mg, 2.00 mmol) was added to a THF solution
5 mL) of (Me Si) NK (399 mg, 2.00 mmol) in a Schlenk tube. Then
J
PC 5 13.7 Hz), 152.4 (d,
JPC 5 31.6 Hz); P{ H} NMR
(
1
+
6
D
6
): d 5 218.5; HRMS Calcd for [M + H]
19 26 2
C H N P
7
8
For examples of rare-earth-metal-catalyzed nucleophilic addition to
carbodiimides, see: (a) W.-X. Zhang, M. Nishiura and Z. Hou, J. Am.
Chem. Soc., 2005, 127, 16788–16789; (b) W.-X. Zhang, M. Nishiura and
Z. Hou, Synlett, 2006, 8, 1213–1216.
For examples of transition-metal-promoted nucleophilic addition to
carbodiimides, see: (a) J. Vicente, J. A. Abad, M.-J. L o´ pez-S a´ ez and
P. G. Jones, Organometallics, 2006, 25, 1851–1853; (b) F. Montilla,
A. Pastor and A. Galindo, J. Organomet. Chem., 2004, 689, 993–996; (c)
T.-G. Ong, G. P. A. Yap and D. S. Richeson, J. Am. Chem. Soc., 2003,
2 2 2 2
PC(NCy) K(OEt )]
(
(
3
2
N,N9-dicyclohexylcarbodiimide (413 mg, 2.00 mmol) was added to the
above reaction mixture. After 1 h of stirring, the solvent was removed
under reduced pressure. The residue was extracted with ether and filtered to
give a clean solution. The solution volume was reduced under vacuum to
precipitate 2 as light yellow crystalline powder (969 mg, 0.96 mmol, 96%
yield). Single crystals of 2 suitable for X-ray analysis were grown in ether at
room temperature overnight. IR (Nujol): n 5 2122, 1582, 1471, 1377, 1339,
125, 8100–8101.
2
2
9
For examples of amidinate and guanidinate complexes having a m-g ,g -
structure, see: (a) C. Knapp, E. Lork, P. G. Watson and R. Mews, Inorg.
Chem., 2002, 41, 2014–2025; (b) G. R. Giesbrecht, A. Shafir and
J. Arnold, J. Chem. Soc., Dalton Trans., 1999, 3601–3604; (c)
P. B. Hitchcock, M. F. Lappert and M. Layh, J. Chem. Soc., Dalton
Trans., 1998, 3113–3117; (d) P. B. Hitchcock, M. F. Lappert and
D.-S. Liu, J. Organomet. Chem., 1995, 488, 241–248; (e) M. S. Eisen and
M. Kapon, J. Chem. Soc., Dalton Trans., 1994, 3507–3510; (f) D. Stalke,
M. Wedler and F. T. Edelmann, J. Organomet. Chem., 1992, 431,
C1–C5.
2
1 1
1
1
076, 979, 740 cm ; H NMR (400 MHz, C
2H, (CH CH O), 1.21–1.86 (br, 40H, CH
CH(Cy)), 3.28 (q, J 5 7.2 Hz, 8H, (CH CH O), 6.76–6.80 (m, 4H, C
.03–7.09 (m, 8H, C ), 7.65 (br, 8H, C
): d 5 15.8, 25.0, 26.0, 35.5, 55.8, 66.0, 129.0 (d, JPC 5 6.6 Hz), 129.3,
6
D
6
): d 5 1.01 (t, J 5 7.2 Hz,
(Cy)), 3.11–3.16 (m, 4H,
),
6 5
H ); C NMR (100 MHz,
3
2
)
2
2
3
2
)
2
6 5
H
1
3
7
6
H
5
3
6 6
C D
1
2
1
J
34.3 (d,
J
PC 5 18.9 Hz), 135.9 (d,
J
PC 5 16.5 Hz), 139.8 (d,
PC 5 34.6 Hz); P{ H} NMR (160 MHz, C ): d 5 214.8, 218.0,
20.6; Anal. Calcd for C58 : C, 69.01; H, 8.39; N, 5.55;
Found: C, 68.96; H, 8.18; N, 5.38%.
1
31
1
6 6
D
2
84 2 4 2 2
H K N O P
3
814 | Chem. Commun., 2006, 3812–3814
This journal is ß The Royal Society of Chemistry 2006