Asymmetric hydrocyanation of α,β-unsaturated ketones into β-cyano ketones with the [Ru(phgly)2(binap)]/C6H 5OLi catalyst system

Article abstract of DOI:10.1002/anie.201100939

Enantioselective conjugate addition of HCN to α,β-unsaturated ketones catalyzed by the combined system of [Ru{(S)-phgly}₂{(S)- binap}] and C₆H₅OLi has afforded β-cyano ketones in high yield (see scheme). No detectable amount of the corresponding 1,2-adduct was produced and tert-C₄H₉OCH₃ was the solvent of choice. The cyanation was conducted with a substrate-to-catalyst molar ratio in the range of 200:1-1000:1 at -20-0°C. Copyright

Full text of DOI:10.1002/anie.201100939

DOI: 10.1002/anie.201100939
Asymmetric Hydrocyanation
Asymmetric Hydrocyanation of a,b-Unsaturated Ketones into b-Cyano
Ketones with the [Ru(phgly)2(binap)]/C6H5OLi Catalyst System**
Nobuhito Kurono, Noriyuki Nii, Yusuke Sakaguchi, Masato Uemura, and Takeshi Ohkuma*
Catalytic asymmetric hydrocyanation of a,b-unsaturated
ketones into the corresponding chiral b-cyano ketones is a
challenging scientific endeavor. Four major hurdles must be
cleared before this reaction can be realized: 1) use of HCN as
a cyanide source,[1] 2) high 1,4-addition selectivity over 1,2-
addition, 3) sufficient enantioface selectivity and, 4) high
catalytic activity (low catalyst loading). Recently, Shibasaki
and co-workers reported pioneering studies on asymmetric
1,4-addition of cyanide to the conjugate enones catalyzed by
chiral Gd and Sr compounds.[2–5] A wide range of 1,4-adducts
were obtained in high enantiomeric excess (ee), but two
equivalents of a tert-C4H9(CH3)2SiCN/2,6-dimethylphenol
system were required as a cyanide source to achieve the
best catalyst performance.[6] Furthermore, the substrate-to-
Scheme 1. Asymmetric hydrocyanation of 1-phenyl-2-buten-1-one (1a)
with 3 and C6H5OLi.
catalyst molar ratio (S/C) of 10–200 in these reactions was
relatively low.[2]
Our research group recently reported the asymmetric
Table 1: Asymmetric hydrocyanation of 1-phenyl-2-buten-1-one (1a).[a]
cyanation of aldehydes and a-keto esters catalyzed by our
Entry 1a/3a/PhOLi Solvent
T [8C] t [h] Yield [%][b] ee [%][b]
original [Ru(phgly)2(binap)]/Li salt systems.[7,8] The corre-
sponding cyanated products were obtained in high ee. The
spectroscopic analysis suggested that the bimetallic species
[Li·Ru(phgly)2(binap)]+ acted as a chiral Lewis acidic cata-
lyst. Herein, we describe the efficient asymmetric conjugate
addition of HCN to a,b-unsaturated ketones catalyzed by the
combined system of [Ru(phgly)2(binap)] and C6H5OLi. The
reaction was carried out with an S/C of 200–1000 at ꢀ20–08C
to afford the b-cyano ketones in up to 98% ee.
1-Phenyl-2-buten-1-one (1a) was selected as a typical
enone substrate to optimize the reaction conditions
(Scheme 1 and Table 1). The [Ru{(S)-phgly}2{(S)-binap}]
((S,S,S)-3a) complex was prepared according to the method
described in our previous report.[7] The reaction of 1a
(1.0 mmol) and HCN prepared by mixing (CH3)3SiCN
(1.5 mmol) and CH3OH (1.5 mmol) was conducted in tert-
C4H9OCH3 (6 mL) with (S,S,S)-3a (20 mm in THF, 2.0 mmol,
1
2
3
4
5
6
7
8
500:1:1
500:0:1
500:1:0
500:1:0.5
500:1:2
500:1:1
500:1:1
500:1:1
500:1:1
500:1:1
500:1:1
500:1:1
1000:1:1
2000:1:1
500:1:1
tBuOMe
tBuOMe
tBuOMe
tBuOMe
tBuOMe
Et2O
25
25
25
25
25
25
25
25
25
0
1
1
1
1
1
1
1
1
1
89
35
<1
53
>99
80
21
<1
44
99
89
–
n.d.
90
82
83
56
n.d.
64
[c]
n-hexane
CH2Cl2
9
toluene
tBuOMe
tBuOMe
tBuOMe
tBuOMe
tBuOMe
10
11[d]
12[e]
13
14
15
5
93
0
5
99
93
0
5
98
90
0
5
98
90
0
5
18
<1
96
n.d.
97
tBuOMe ꢀ20
[a] Unless otherwise stated, the reactions were carried out using 1a
(1.0 mmol) and HCN (1.5 mmol) in solvent (6 mL) with (S,S,S)-3a
(20 mm in THF) and C6H5OLi (20 mm in THF) in the ratio given in the
Table. HCN was prepared in situ from (CH3)3SiCN and CH3OH in a 1:1
ratio. [b] Data for (S)-2a were determined by GC on a chiral stationary
phase. [c] A racemic product was obtained. [d] Isolated HCN was used.
[e] 3b was used as a catalyst. n.d.=not determined.
[*] Dr. N. Kurono, N. Nii, Y. Sakaguchi, M. Uemura, Prof. Dr. T. Ohkuma
Division of Chemical Process Engineering
Faculty of Engineering, Hokkaido University
Sapporo, Hokkaido 060-8628 (Japan)
Fax: (+81)11-706-6598
E-mail: ohkuma@eng.hokudai.ac.jp
[**] This work was supported by Grants-in-Aid from the Innovation Plaza
Hokkaido in JST (No. 01-B07) and JSPS (No. 21350048). N.K. is the
grateful recipient of a fellowship from the Global COE Program,
“Catalysis as the Basis for Innovation in Materials Science” from
MEXT (Japan). M.U. is also the grateful recipient of a JSPS Research
Fellowship for Young Scientists. phgly=phenylglycinate,
S/C = 500) and C6H5OLi (20 mm in THF, 2.0 mmol) at 258C
for 1 hour and gave (R)-3-cyano-1-phenyl-1-butanone ((R)-
2a) in 89% yield and 89% ee (Table 1, entry 1). Notably, no
1,2-addition product was observed. The reaction catalyzed
only by C6H5OLi gave racemic 2a in 35% yield (Table 1,
entry 2). No conversion was observed in the reaction with 3a
in the absence of C6H5OLi (Table 1, entry 3). The use of a 3a/
C6H5OLi system in a 1:0.5 or 1:2 ratio afforded 2a in 53%
binap=2,2’-bis(diphenylphosphanyl)-1,1’-binaphthyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100939.
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Table 2: Asymmetric hydrocyanation of a,b-unsaturated ketones 1.[a]
yield and 90% ee, and > 99% yield and 82% ee, respectively
(Table 1, entries 4 and 5). These observations suggested that
3a and C6H5OLi smoothly formed the 1:1 bimetallic species,
[Li·Ru(phgly)2(binap)]+,[7,8] and the chiral species had a
higher reactivity than that of achiral C6H5OLi alone. The
solvent of choice was tert-C4H9OCH3, while (C2H5)2O gave a
slightly less satisfactory result (Table 1, entry 6). The yield
and enantioselectivity were significantly decreased in less
polar solvents (Table 1, entries 7–9). The cyanation at 08C
was completed in 5 hours and afforded the adduct in 93% ee
(Table 1, entry 10). The same result was obtained by using the
isolated HCN prepared according to the method described in
the literature,[9] despite the very low catalyst loading (S/C =
500; Table 1, entry 11). This high reactivity at the low catalyst
loading is the important point of difference between Shiba-
sakiꢀs system and the present system.[2,6] We chose the HCN
formed in situ for use in this study for operational and safety
reasons. The [Ru(phgly)2(tol-binap)] (3b) exhibited a similar
efficiency (Table 1, entry 12).[10] The high catalytic activity of
the 3a/C6H5OLi system allowed us to conduct the cyanation
with an S/C of 1000 at 08C (Table 1, entry 13). The reaction
with an S/C of 2000 did not proceed (Table 1, entry 14). The
excellent ee value of 97% was achieved in the reaction at
ꢀ208C, although the reaction took longer to achieve com-
pletion (Table 1, entry 15).[11]
Thus, we selected the reaction conditions using 1a and
1.5 equivalents of HCN prepared from (CH3)3SiCN and
CH3OH in tert-C4H9OCH3 with 3a and C6H5OLi (3a/
C6H5OLi = 1:1) at an S/C of 500 at 08C or ꢀ208C when a
higher ee value for the product was required (see the
Experimental Section). The 1,4-adduct 2a was quantitatively
isolated by column chromatography on silica gel for a
reaction carried out on a 3 mmol scale (Table 2, entries 1
and 2). Phenyl ketones (R1 = C6H5 in Scheme 2) with alkyl
substituents at the b position (R2), 1b–1g, reacted with HCN
catalyzed by the 3a/C6H5OLi system under the standard
conditions and afforded the corresponding b-cyano ketones in
90–96% ee (Table 2, entries 3–6, 9, and 10). Although the
reactivity of 1c and 1d with long alkyl chains was somewhat
lower, the enones substituted by secondary and tertiary alkyl
groups, 1e–1g, showed reactivity comparable to that of the
methyl-substituted ketone 1a. The cyanation of 1e with an
S/C of 1000 at 08C or with an S/C of 500 at ꢀ208C proceeded
smoothly and gave 2e in 92% ee and 98% ee, respectively
(Table 2, entries 7 and 8). Chalcone (1h), a b-phenyl enone,
was much less reactive than the corresponding b-alkyl
substrates, but the cyanation product 2h was obtained in
92% ee and 88% yield in the reaction with an S/C of 200 for
47 hours (Table 2, entry 11).[12]
Entry
1
S/C[b]
T [8C]
t [h]
Yield [%][c]
ee [%][d]
1
2
3
4
5
6
7
8
1a
1a
1b
1c
1d
1e
1e
1e
1 f
1g
1h
1i
1j
1k
1l
1m
1m
1m
1n
1o
1p
1q
1r
500
500
500
500
500
500
1000
500
500
500
200
500
500
200
500
500
1000
500
500
200
500
500
500
200[f]
0
5
24
3
18
18
3
5
18
5
5
47
5
5
12
3
2a, 99
2a, 98
2b, 98
2c, 94
2d, 97
2e, 99
2e, 98
2e, 98
2 f, 98
2g, 99
2h, 88
2i, 96
94
97
95
93
90
96
92
98
95
96
92
82
95
98
95
96
94
98
95
91
93
95
95
93
ꢀ20
0
0
0
0
0
ꢀ20
9
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0
0
0
0
0
0
0
0
2j, 97
2k, 96
2l, 96
2
5
2m, 97
2m, 96
2m, 98
2n, 97
2o, 99
2p, 99
2q, 98
2r, 97[e]
2s, 80[g]
ꢀ20
12
3
12
12
3
0
0
0
0
0
5
24
1s
0
[a] Unless otherwise stated, reactions were conducted using
1
(3.0 mmol) and HCN (4.5 mmol) in tert-C4H9OCH3 (18 mL) with a
solid (S,S,S)-3a and C6H5OLi (60 mm in THF). 3a/C6H5OLi=1:1. HCN
was prepared in situ from (CH3)3SiCN and CH3OH in a 1:1 ratio.
[b] Substrate-to-catalyst (3a) molar ratio. [c] Yield of isolated 2.
[d] Determined by GC or HPLC on a chiral stationary phase. [e] Con-
taminated by about 1% of an unidentified compound. [f] 3b was used as
a catalyst. [g] The yield determined by GC methods was 99%.
Scheme 2. Asymmetric hydrocyanation of a,b-unsaturated ketones 1 to
the b-cyano ketones 2.
A series of substituted phenyl ketones, 1i–1o, was applied
to the asymmetric hydrocyanation. The 2’-Cl phenyl ketone 1i
was treated with 3a under the typical reaction conditions and
gave 2i in 82% ee (Table 2, entry 12). A high enantioselec-
tivity of 95% was achieved in the cyanation of the 3’-Cl
substrate 1j (Table 2, entry 13). The phenyl ketones with an
electron-donating CH3O group at the 3’- or 4’-position, 1k
and 1o, showed lower reactivity (Table 2, entries 14 and 20).
However, an excellent ee value of 98% was observed in the
reaction of 1k. The cyanation of substrates with an electron-
withdrawing Cl, CF3, or CH3OCO group at the 4’-position, 1l–
1n, was faster than the reaction of unsubstituted ketone 1a
with maintaining high enantioselectivity (Table 2, entries 15,
16, and 19). The 4’-CF3 ketone 1m smoothly reacted with an
S/C of 1000 at 08C or with an S/C of 500 at ꢀ208C and
afforded 2m in 94% ee and 98% ee, respectively (Table 2,
entries 17 and 18).
The cyanation of 2’-naphthyl ketone 1p under the
standard conditions was completed in 12 hours and gave 2p
in 93% ee (Table 2, entry 21). The 2’-furyl and 2’-thienyl
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Angew. Chem. Int. Ed. 2011, 50, 5541 –5544

Table 4: Sequential hydrocyanation of different substrates.[a]
ketones, 1q and 1r, were converted into the 1,4-adducts, 2q
and 2r, with 95% ee in both cases (Table 2, entries 22 and 23).
The reaction of 3-hepten-2-one (1s), an aliphatic enone, with
the 3a/C6H5OLi system was slow, but the cyanation with an
S/C of 200 at 08C for 24 hours afforded 2s in 99% yield and
93% ee when 3b was used instead of 3a (Table 2, entry 24).[11]
The chiral Gd and Sr catalysts reported by Shibasaki exhibit
wider applicability to the reaction of aliphatic and b,b-
disubstituted enones.[2]
The 3b/C6H5OLi catalyst was applied to the regioselective
cyanation of a dialkenyl ketone. When cyclohexenyl pentenyl
ketone 4 was subjected to the cyanation conditions, the
monocyanated product 5 (at the pentenyl group) was
obtained in 96% ee (Scheme 3). The regioselectivity was
estimated to be greater than 99%.
Run number[b]
1
t [h]
Yield [%][c]
ee [%][d]
1
2
3
4
5
1n
1p
1j
3
12
5
3
12
96
96
98
95
99
93
90
96
95
96
1l
1k[e]
[a] Reactions were conducted using 1 (8.2 mmol) and HCN (11.8 mmol)
in tert-C4H9OCH3 (48 mL) at 08C with a solid (S,S,S)-3a and C6H5OLi
(50 mm in THF). 1/3a/C6H5OLi=500:1:1 (initial). HCN was prepared in
situ from (CH3)3SiCN and CH3OH in a 1:1 ratio. [b] Number of times the
catalyst was used. [c] Yield of isolated 2. [d] Determined by GC or HPLC
on a chiral stationary phase. [e] Reaction using 1k (3.2 mmol) and HCN
(4.8 mmol) in tert-C4H9OCH3 (19 mL).
original [Ru(phgly)2(binap)]/C6H5OLi system. The reaction
was carried out with an S/C in the range of 200–1000 at
ꢀ208C!08C. A series of aryl-, hetero-aryl-, and alkyl-
substituted enones was converted into the 1,4-addition
products in up to 98% ee without formation of a detectable
amount of the 1,2-adducts. The reaction of cyclohexenyl
pentenyl ketone afforded the monocyanated product at the
less-hindered site in high regio- and enantioselectivity. The
robust [Ru(phgly)2(binap)] complex can be reused with
addition of fresh C6H5OLi without loss of the stereoselectiv-
ity. We hope these findings will contribute to the progress of
synthetic organic chemistry.
Scheme 3. Regioselective cyanation of dienone 4.
The Ru complex 3a was so robust that it was recovered by
column chromatography on silica gel from the reaction
mixture in the open air, and was reusable as a cyanation
catalyst with the addition of fresh C6H5OLi. As shown in
Table 3, 3a could be used five times in the cyanation of 1a
Experimental Section
The typical procedure for the hydrocyanation of 1-phenyl-2-buten-1-
one (1a): Caution: (CH3)3SiCN and HCN that is formed in situ must
be used in a well-ventilated fume hood owing to their high toxicity.
Ruthenium complex (S,S,S)-3a (6.2 mg, 6.1 mmol)[7,8] was placed in a
50 mL Schlenk flask, and the air present in this apparatus was
replaced by argon. Anhydrous CH3OH (146 mg, 4.6 mmol) was
added to this flask, and the mixture was cooled to 08C. Then
(CH3)3SiCN (445 mg, 4.5 mmol) was added in a dropwise manner,
and the mixture was stirred for 15 min. To the solution containing
HCN, anhydrous tert-C4H9OCH3 (18 mL) and C6H5OLi (60 mm in
THF, 100 mL, 6.0 mmol) were added at 08C, and the mixture was
stirred for 30 min. Then 1a (447 mg, 3.1 mmol) was added to this
solution in a dropwise manner over 5 min, and the reaction mixture
was stirred for 5 h. After the solvent and the volatile compounds were
evaporated under reduced pressure, the residue was purified by
column chromatography on silica gel to give (S)-2a (colorless oil,
Table 3: Recycled use of 3a in the hydrocyanation of 1a.[a]
Run number[b]
Conversion [%][c]
Yield [%][d]
ee [%][c]
1
2
3
4
5
>99
>99
>99
>99
>99
97
93
99
99
99
93
94
93
93
92
[a] Reactions were conducted using 1a (10 mmol) and HCN (15 mmol)
in tert-C4H9OCH3 (60 mL) at 08C for 5 h with a solid (S,S,S)-3a and
C6H5OLi (50 mm in THF). 1a/3a/C6H5OLi=500:1:1 (initial). HCN was
prepared in situ from (CH3)3SiCN and CH3OH in a 1:1 ratio. [b] Number
of times the catalyst was used. [c] Determined by GC on a chiral
stationary phase. [d] Yield of isolated 2a.
24
531 mg, 99% yield, 94% ee). ½aꢁD ¼ꢀ6.7 degcm3 gꢀ1 dmꢀ1 (c =
25
1.07 gcmꢀ3, CHCl3); literature[2a] ½aꢁD ¼ꢀ6.2 degcm3 gꢀ1 dmꢀ1 (c =
0.6 gcmꢀ3, CHCl3), 88% ee (absolute configuration was unreported);
1H NMR (400 MHz, CDCl3): d = 1.43 (d, 3H, J = 6.8 Hz, CH3), 3.23
(dd, 1H, J = 6.5, 17.0 Hz, CHH), 3.31–3.40 (m, 1H, CHCN), 3.43 (dd,
1H, J = 6.2, 17.0 Hz, CHH), 7.48–7.51 (m, 2H, aromatic H), 7.60–7.63
(m, 1H, aromatic H), 7.95–7.97 ppm (m, 2H, aromatic H); 13C NMR
(67.7 MHz, CDCl3): d = 17.8 (CH3), 20.5 (CH), 42.2 (CH2), 122.6 (C),
128.0 (CH), 128.8 (CH), 133.8 (CH), 135.8 (C), 195.1 ppm (C); HRMS
(ESI): m/z calcd for C11H11ClNO: 208.05292 [M+Cl]ꢀ; found:
208.05292. The ee value of 2a was determined by GC on a chiral
stationary phase using an InertCap CHIRAMIX column (0.25 mm ꢁ
30 m, depth of film = 0.25 mm, GL Science); carrier gas: helium
(217 kPa); column temp: 1708C heating to 1798C at a rate of
0.58Cminꢀ1; injection temp; 2508C; retention time (tR) of (R)-2a:
17.5 min (3.1%), tR of (S)-2a: 16.5 min (96.9%). The ee value was not
changed by purification with column chromatography. The absolute
with an initial S/C of 500 at 08C with maintaining high
enantioselectivity. The total turnover number was about 2500.
The notable robustness and reusability of 3a make it suitable
for practical use.
In addition, different a,b-unsaturated ketones 1 were
cyanated sequentially with this catalyst-reuse procedure.
Table 4 lists the results. The catalyst efficiency and enantio-
selectivity for all runs were comparable to those of the regular
single-run reactions shown in Table 2.
In summary, we have reported here the efficient enantio-
selective conjugate addition of HCN into the a,b-unsaturated
ketones to afford the b-cyano ketones catalyzed by our
Angew. Chem. Int. Ed. 2011, 50, 5541 –5544
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5543

Communications
configuration was determined after conversion to 2-methyl-4-oxo-4-
[4] For cyanation of a,b-unsaturated N-acylpyrroles, see: a) T. Mita,
K. Sasaki, M. Kanai, M. Shibasaki, J. Am. Chem. Soc. 2005, 127,
514 – 515; b) I. Fujimori, T. Mita, K. Maki, M. Shiro, A. Sato, S.
Furusho, M. Kanai, M. Shibasaki, Tetrahedron 2007, 63, 5820 –
5831.
26
phenylbutanoic acid in 55% ee. ½aꢁD ¼ꢀ18.4 degcm3 gꢀ1 dmꢀ1 (c =
20
0.592 gcmꢀ3, CHCl3); literature[13] ½aꢁD ¼ꢀ32.5 degcm3 gꢀ1 dmꢀ1 (c =
0.69 gcmꢀ3, CHCl3) for the S enantiomer.
[5] For cyanation of other activated alkenes using acetone cyano-
hydrin or ethyl cyanoformate as a cyanide source, see: a) L.
Bernardi, F. Fini, M. Fochi, A. Ricci, Synlett 2008, 1857 – 1861;
b) J. Wang, W. Li, Y. Liu, Y. Chu, L. Lin, X. Liu, X. Feng, Org.
Lett. 2010, 12, 1280 – 1283.
[6] Use of pure HCN as a cyanide source gave low yield and
enantioselectivity in the reaction with the Gd catalyst. The Sr
catalyst is expected to be labile in the presence of a large excess
of HCN; for details see reference [2].
Received: February 7, 2011
Revised: March 29, 2011
Published online: May 3, 2011
Keywords: asymmetric catalysis · hydrocyanation · lithium ·
.
ruthenium · a,b-unsaturated ketones
[1] HCN is a toxic compound, but it is utilized in industrial processes
for the production of useful chemical compounds, such as a-
hydroxy acids, a-amino acids, and methacrylates; for example,
see: P. Poechlauer, W. Skranc, M. Wubbolts in Asymmetric
Catalysis on Industrial Scale: Challenge, Approaches and Sol-
utions (Eds.: H. U. Blaser, E. Schmidt), Wiley-VCH, Weinheim,
2004, pp. 151 – 164.
[2] a) Y. Tanaka, M. Kanai, M. Shibasaki, J. Am. Chem. Soc. 2008,
130, 6072 – 6073; b) Y. Tanaka, M. Kanai, M. Shiabasaki, J. Am.
Chem. Soc. 2010, 132, 8862 – 8863.
[3] For cyanation of a,b-unsaturated imides, see: a) G. M. Sammis,
E. N. Jacobsen, J. Am. Chem. Soc. 2003, 125, 4442 – 4443;
b) G. M. Sammis, H. Danjo, E. N. Jacobsen, J. Am. Chem. Soc.
2004, 126, 9928 – 9929; c) C. Mazet, E. N. Jacobsen, Angew.
Chem. 2008, 120, 1786 – 1789; Angew. Chem. Int. Ed. 2008, 47,
1762 – 1765; d) N. Madhavan, M. Weck, Adv. Synth. Catal. 2008,
350, 419 – 425.
[7] a) N. Kurono, K. Arai, M. Uemura, T. Ohkuma, Angew. Chem.
2008, 120, 6745 – 6748; Angew. Chem. Int. Ed. 2008, 47, 6643 –
6646; b) N. Kurono, M. Uemura, T. Ohkuma, Eur. J. Org. Chem.
2010, 1455 – 1459.
[8] N. Kurono, T. Yoshikawa, M. Yamasaki, T. Ohkuma, Org. Lett.
2011, 13, 1254 – 1257.
[9] O. Glemser in Handbook of Preparative Inorganic Chemistry,
2nd ed. (Eds.: G. Brauer, R. F. Riley), Academic Press, New
York, 1963, pp. 658 – 660.
[10] Tol-binap = 2,2’-bis(di-4-tolylphosphanyl)-1,1’-binaphthyl.
[11] The b-amino ketones 2a in 88% ee and 2s in 92% ee were
obtained by Shibasakiꢀs Gd system (see reference [2a]).
[12] The conjugate addition of (CH3)3SiCN to 1h catalyzed by a
chiral sodium phosphate with an S/C of 5 gave 2h in 71% ee.
See: J. Yang, S. Wu, F.-X. Chen, Synlett 2010, 2725 – 2728.
[13] R. V. Hoffman, H.-O. Kim, J. Org. Chem. 1995, 60, 5107 – 5113.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5541 –5544