Highly Activated Second-Generation Grubbs-Hoveyda Catalyst Driven by Intramolecular Steric Strain

Article abstract of DOI:10.1055/s-0035-1562468

Various Grubbs-Hoveyda second-generation catalysts activated by intramolecular steric strain were prepared. The variant bearing a 9-anthracenyl group in the ligand moiety exhibited the highest catalytic activity. The new anthracenyl-type-activated catalyst was used in a ring-closing metathesis reaction to effectively provide a 4-substituted product effectively.

Full text of DOI:10.1055/s-0035-1562468

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
Y. Kobayashi et al.
Letter
Synlett
Highly Activated Second-Generation Grubbs–Hoveyda Catalyst
Driven by Intramolecular Steric Strain
Yuki Kobayashia
Hiroki Miyazakia
Sae Inukaia
Chika Takagia
Reo Makinoa
Kento Shimowakia
Rina Igarashia
Yuya Sugiyamaa
Shuichi Nakamurab
Masato Matsugi*a
a Faculty of Agriculture, Meijo University, 1-501 Shiogamaguchi,
Tempaku-ku, Nagoya 468-8502, Japan
matsugi@meijo-u.ac.jp
b Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso, Showa-ku, Nagoya 466-8555, Japan
We would like to dedicate this letter to Prof. Takayuki Shioiri on his 80th birthday.
Received: 21.04.2016
Accepted after revision: 08.06.2016
Published online: 13.07.2016
Herein, we report a novel activation method for the sec-
ond-generation GH catalyst by the immobilization of the
molecular conformation via the intramolecular steric strain
in the ligand moiety. We then applied the catalyst with the
highest activity to the catalysis of the ring-closing metathe-
sis (RCM) reactions of tetrasubstituted olefins, which are
usually difficult to achieve using second-generation GH cat-
alysts.
DOI: 10.1055/s-0035-1562468; Art ID: st-2016-u0283-l
Abstract Various Grubbs–Hoveyda second-generation catalysts acti-
vated by intramolecular steric strain were prepared. The variant bearing
a 9-anthracenyl group in the ligand moiety exhibited the highest cata-
lytic activity. The new anthracenyl-type-activated catalyst was used in a
ring-closing metathesis reaction to effectively provide a 4-substituted
product effectively.
We hypothesized that if an extensively spread aromatic
structure was introduced in the 3-position of the 2-iso-
propoxy styrene ligand, an intramolecular steric hindrance
would exist between the aromatic ring of the ligand moiety
and the methyl of the alkoxy group (Figure 2). This steric
hindrance would influence the coordination environment
between the lone electron pair on the isopropoxy oxygen
and ruthenium and therefore the activity of the catalyst.
Actually, it has been known that the catalyst 1b shows
higher activity than GH second-generation catalyst 1a.7
The catalysts prepared for this study are shown in Fig-
ure 2, and the synthetic routes for these catalysts are shown
in Scheme 1. All new catalysts (1c–f)8 and the known cata-
Key words catalyst, metathesis, activation, ligand, steric strain
The second-generation Grubbs–Hoveyda (GH) catalyst1
(1a) is one of the most frequently used catalysts in metath-
esis reactions in synthetic organic chemistry (Figure 1).2 To
date, many modifications of 1a have been conducted to im-
prove its catalytic activity, and some pioneering catalysts
have been reported.3 Optimization of the N-heterocyclic
carbene (NHC) ligand4 and the aromatic bidentate ligand
(2-isopropoxy styrene)5 as well as the immobilization of 1a
onto polymers6 have been investigated, and a variety of at-
tractive catalysts have been discovered.
MsN
Cl
NMs
1b7: Ar = Ph, Ar' = H
1c: Ar = 1-naphthyl, Ar' = H
1d: Ar = 2-methoxy-1-naphthyl, Ar' = H
1e: Ar = 9-phenanthrenyl, Ar' = H
1f: Ar = 9-anthracenyl, Ar' = H
1g: Ar = H, Ar' = 9-anthracenyl
MsN
Cl
NMs
Ru
Cl
Ru
Cl
iPrO
Ar'
2
5
iPrO
3
Ar
steric hindrance
1a
Figure 1 Second-generation Grubbs–Hoveyda catalyst
Figure 2 Second-generation Grubbs–Hoveyda catalysts expected to
offer conformation control via intramolecular steric hindrance
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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Y. Kobayashi et al.
Letter
Synlett
Table 1 Chemical Shifts of the Methyl Group on the Isopropyl Func-
tion in CDCl3 at 23 °C
RCM reactions using diethyl 2-allyl-2-(2-methylal-
lyl)malonate11 (8) were examined to confirm the relative
catalytic activities of the prepared catalysts. The reactions
were conducted with CDCl3 as the solvent to allow NMR
monitoring of the reaction aliquots. At certain reaction
times, the conversion (%) of the reaction was determined by
Catalyst
δ (ppm)
Catalyst
δ (ppm)
1a
1b
1c
1.27
0.81
0.75
1d
1e
1f
0.79
0.57
0.77
1
recording H NMR spectra of reaction mixtures and calcu-
lating the relative integrals of the corresponding olefinic
protons of product 9. Interestingly, a strong difference in
catalytic activity was observed between 1a and catalysts
1b–f (Figure 3). The result clearly indicates that the intro-
duction of a polycyclic aromatic moiety at the 3-position on
the aromatic ligand is an effective means of achieving sig-
nificant activation of second-generation GH catalysts. Im-
portantly, we found that the introduction of a 9-anthrace-
nyl functional group led to a strong acceleration of RCM ca-
talysis. Indeed, 1f showed the highest activity among the
catalysts examined. The catalytic activity was extremely
low when the 5-position-substituted catalyst 1g was used.
lyst 1b7 were prepared from 3-bromo-2-hydroxybenzalde-
hyde (2) in moderate yields via alkylation, Wittig reaction,
Suzuki–Miyaura coupling, and successive ligand exchange
with second-generation Grubbs catalyst in the presence of
CuCl(I).9 The 5-position-substituted catalyst 1g was also
prepared from 5-bromo-2-hydroxybenzaldehyde 3 via a
similar route.10
R
OHC
HO
a–c
iPrO
X' or Ar'
X'
X or Ar
X
4: X = Br, X' = H, R = CH=CH2
5: Ar = Ph, X' = H, R = CHO
2: X = Br, X' = H
3: X = H, X' = Br
6: X = H, Ar' = 9-anthracenyl, R = CHO
f
d, e
iPrO
Ar'
1b–g
Ar
7b–f: Ar = aromatics, Ar' = H
7g: Ar = H, Ar' = 9-anthracenyl
Scheme 1 Synthesis of second-generation Grubbs–Hoveyda catalysts
1b–f bearing a biaryl ligand structure. Reagents and conditons: a) i-PrI
(5.0 equvi), K2CO3 (8.0 equiv), DMF, 60 °C, 2 h; 99%; b) [Ph3PMe]Br (2.3
equiv), NaHMDS (4.3 equiv), dry THF, –78 °C to 0 °C, 3 h; 4 quant.; c)
ArB(OH)2 (1.1 equiv), K3PO4 (5.1 equiv), S-Phos (0.125 equiv), Pd(OAc)2
(0.05 equiv), THF–H2O (2:1 v/v); reflux, 20 h; 5, 97%; 6, 45%; d)
ArB(OH)2 (1.1 equiv), K3PO4 (5.0 equiv), S-Phos (0.125 equiv), Pd(OAc)2
(0.05 equiv), THF–H2O (2:1 v/v); reflux, 3–22 h; 7c, quant.; 7d, 67%; 7e,
60%; 7f, quant.; e) NaOt-Bu (3.0–7.5 equiv), [Ph3PMe]Br (3.0–7.5
equiv), dry Et2O, r.t., 1 h; 7b, 55%; 7g, 27%; f) Grubbs II catalyst (1.0
equiv), CuCl(I) (2.0 equiv), dry CH2Cl2, 30 °C, 3 h; 1b, 10%; 1c, 35%; 1d,
35%; 1e, 40%, 1f, 49%; 1g, 67%.
Figure 3 RCM reactions of diethyl 2-allyl-2-(2-methylallyl)malonate 8
in CDCl3 at 23 °C
These findings led us to hypothesize that catalyst 1f
could effectively catalyze RCM reactions of tetrasubstituted
olefins, which are usually difficult to achieve using second-
generation GH catalysts. We compared the catalytic activi-
ties of second-generation GH catalyst 1a and catalyst 1f in
an RCM reaction using 4-methyl-N,N-bis(2-methylal-
lyl)benzenesulfonamide4a 10 as a substrate (Table 2).12 As
expected, catalyst 1f showed a higher catalytic activity than
the original catalyst 1a. The reaction employing 1f afforded
the tetrasubstituted RCM product 11 in 75% yield after 24
hours, whereas 1a afforded the product in 5% yield under
same reaction conditions (Table 2, entries 1, 2). Important-
ly, these results show that a dramatic improvement in acti-
1
As shown in Table 1, the H NMR spectra of 1b–f indi-
cated that the chemical shifts of the methyl group on the
isopropoxy moiety are quite different from that of the orig-
inal second-generation GH catalyst 1a (δ = 0.57–0.81 ppm
vs. δ = 1.27 ppm). This finding suggests that all the isoprop-
oxy groups of 1b–f experience quite different environ-
ments due to the shielding effect. The catalysts bearing bi-
cyclic or tricyclic aromatic function showed higher field
shifts than monocyclic 1b, and a strong shielding effect was
observed in the catalyst 1e, the ligand that has an angularly
distributed π-conjugated system.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
Y. Kobayashi et al.
Letter
Synlett
vation can be achieved by simply introducing an anthrace-
nyl functional group to the ligand. Interestingly, accelerated
reactivity and increased yield were achieved when the RCM
reaction of 10 was conducted in a fluorous solvent (Table 2,
entries 4 and 5).12 Among the solvents studied, perfluoro-
toluene (PFT) exhibited the highest reactivity (Table 2, en-
try 7).13 Although benzotrifluoride (BTF) was a more effec-
tive solvent than toluene, the activation achieved was infe-
rior that achieved using PFT. We found that 5 mol% of the
catalyst was sufficient to complete the RCM reaction when
0.1 M of the reactant was used in PFT. A satisfactory conver-
sion of 97% was obtained in this case (Table 2, entry 10).
Figure 4 Conformation of 1f based on B3LYP/LANL2DZ calculations as
implemented in Gaussian 03
We speculate the possibility of the presence of an intra-
molecular CH–π interaction18 between isopropyl group and
aromatic moiety. It is an intermolecular interaction be-
tween a CH moiety and a π plane, which was proposed by
Nishio et al. in 1977.19 The calculation data of the shorter
distance than the sum of the individual van der Waals radii
strongly supports the existence of this interaction.
Most importantly, as shown in Table 3, the maximum
UV-Vis absorption wavelengths of catalyst 1f were red-
shifted relative to those of the 5-position-substituted iso-
mer 1g (λmax = 394.6, 373.8, 356.4 nm and λmax = 390.8,
370.8, 353.2 nm, respectively).18c Meanwhile, the maximum
UV-Vis absorption wavelengths of the bidentate ligand pre-
cursors of 1f and 1g were almost the same (see Supporting
Information). These UV-Vis data support an orbital interac-
tion, and the red shift is evidence of a CH–π interaction in
1f.20 Although more definitive evidence of the interaction
could be obtained by X-ray crystallographic analysis of the
catalyst 1f, we were unable to obtain suitable single crys-
tals.
Table 2 Ring-Closing Metathesis Reactions of 4-Methyl-N,N-bis(2-
methylallyl) Benzenesulfonamide 11
catalyst
Ts
(10 mol%)
Ts
N
N
solvent (0.05 M)
10
11
Entry
Catalyst
Solvent
Temp (°C)
Time (h)
Conv (%)a
1
2
1a
1f
1f
1f
1f
1f
1f
1f
1f
1f
CH2Cl2
CH2Cl2
toluene
BTF
reflux
reflux
60
24
24
10
10
10
1
5
75
3
69
4
60
89
5
PFT
60
100
73
6
BTF
60
7
PFT
60
1
88
8b
9b
10b,c
PFT
60
2
56
PFT
60
24
2
53
PFT
60
97 (87)d
Table 3 UV-Vis Spectrum of 1f and the Regioisomer 1g in Chloroforma
a Determined by 1H NMR spectroscopy.
b The concentration of 1f was 5 mol%.
c Reactant concentration was 0.1 M.
d Isolated yield.
Molecular orbital calculations (B3LYP/LANL2DZ)14 of
catalyst 1f were conducted for conformational analysis. As
shown in Figure 4, the CH moiety of the methyl group was
found to be located close to the π-face of the anthracenyl
group. The closest interatomic distance between the CH
moiety and the aromatic π-plane was shorter (2.39 Å) than
the sum of the individual van der Waals radii.15 Consistent
with the MO calculation, the NOESY spectrum of the cata-
lyst 1f in CDCl3 indicate a small distance between the CH
moiety of the methyl group and the π moiety (see Support-
ing Information). The calculated bond angle14 (O–Ru–C)
was 78.5°. It was a fairly narrow angle compared with the
corresponding angle of the X-ray data16 of the original sec-
ond-generation GH catalyst 1a.17
Catalyst
λmax (nm)
1f
394.6
390.8
373.8
370.8
356.4
353.2
1g
a The solution concentration was 1.25·10–2 M.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
Y. Kobayashi et al.
Letter
Synlett
An activation mechanism considering the rate-deter-
mining step of RCM reactions using second-generation GH
catalysts has been reported by the Grubbs and Gladysz
groups.21 According to their report, the rate-determining
step is the release of the bidentate ligand on ruthenium,
and the reaction rate is thus influenced by the ligation abil-
ity of the original ligand in the molecule.22 Similarly, we at-
tribute the effective activation of the catalysts in part to the
decrease in the ligation ability on ruthenium. These biden-
tate ligands 7b–f would experience steric strain due to the
intramolecular CH–π interaction, which requires a highly
ordered conformation, and would thus release the ligand
more easily than the original second-generation GH cata-
lyst.
In summary, we have prepared modified second-gener-
ation GH catalysts activated by the remarkable intramolec-
ular steric strain. The catalyst 1f exhibited the highest cata-
lytic activity, and we have shown that catalyst 1f can be
used successfully in an RCM reaction to generate a 4-substi-
tuted product. We believe that catalyst 1f is one of the most
useful second-generation GH type catalysts because of its
ease of derivation from the original catalyst and superior
catalytic activity.
Organometallics 2010, 29, 775. (d) Leitgeb, A.; Abbas, M.;
Fischer, R. C.; Poater, A.; Cavallo, L.; Slugovc, C. Catal. Sci. Tech-
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Fukumoto, K.; Onoda, A.; Mizohata, E.; Inoue, T.; Bocola, M.;
Schwaneberg, U.; Hayashi, T.; Okuda, J. ACS Catal. 2015, 5, 7519.
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M.; Rivard, M.; Buschmann, N.; Jiricek, J.; Blechert, S. Tetrahe-
dron 2003, 59, 6545. (b) Dragutan, V.; Dragutan, I. Platinum Met.
Rev. 2005, 49, 33. (c) Barbasiewicz, M.; Bieniek, M.; Michrowska,
A.; Szadkowska, A.; Makal, A.; Woźniak, K.; Grela, K. Adv. Synth.
Catal. 2007, 349, 193. (d) Matsugi, M.; Kobayashi, Y.; Suzumura,
N.; Tsuchiya, Y.; Shioiri, T. J. Org. Chem. 2010, 75, 7905.
(e) Olszewski, T. K.; Bieniek, M.; Skowerski, K.; Grela, K. Synlett
2013, 24, 903.
(6) For selected examples, see: (a) Allen, D. P.; Wingerden, M. M. V.;
Grubbs, R. H. Org. Lett. 2009, 11, 1261. (b) Cheong, J. L.; Wong,
D.; Lee, S.-G.; Lim, J.; Lee, S. S. Chem. Commun. 2015, 51, 1042.
(c) Chen, S.-W.; Zhang, Z.-C.; Zhai, N.-N.; Zhong, C.-M.; Lee, S.-G.
Tetrahedron 2015, 71, 648. (d) Skowerski, K.; Białecki, J.;
Czarnocki, S. J.; Żukowska, K.; Grela, K. Beilstein J. Org. Chem.
2016, 12, 5.
(7) Wakamatsu, H.; Blechert, S. Angew. Chem. Int. Ed. 2002, 41,
2403.
(8) Characterization Data of Catalyst 1c
Green crystal, mp 169.0–171.5 °C. 1H NMR (270 MHz, CDCl3):
δ = 0.75 (dd, J = 52.1, 6.2 Hz, 6 H), 2.50 (br s, 18 H), 4.06–4.18
(m, 5 H), 5.30 (s, 1 H), 6.96–7.06 (m, 6 H), 7.34–7.44 (m, 5 H),
7.76–7.82 (m, 3 H), 16.72 (s, 1 H). 13C NMR (68 MHz, CDCl3): δ =
18.5, 20.6, 21.0, 51.4, 77.7, 122.7, 123.1, 125.1, 126.1, 126.4,
127.0, 127.4, 128.0, 128.3, 128.4, 129.3, 131.9, 133.0, 138.8,
147.7, 149.8, 211.3, 299.3. IR: 2918, 1558, 1476, 1420, 1255,
1193, 1032, 777, 733, 646 cm–1. HRMS (FAB+): m/z calcd for
Acknowledgment
This work was supported by JSPS KAKENHI Grant Number 26450145,
and Prof. Uozumi’s JST-ACCEL program.
C
41H45Cl2N2ORu: 754.1952; found: 754.1942.
Characterization Data of Catalyst 1d
Green crystal, mp 172.5–173.2 °C. 1H NMR (270 MHz, CDCl3):
δ = 0.67 (d, J = 6.21 Hz, 3 H), 0.90–0.93 (m, 3 H), 2.50 (br s, 18 H),
3.79 (s, 3 H), 4.15 (s, 4 H), 4.20–4.24 (m, 1 H), 6.93–7.04 (m, 4
H), 7.24–7.35 (m, 5 H), 7.48–7.54 (m, 2 H), 7.69–7.75 (m, 1 H),
7.82 (d, J = 9.18 Hz, 1 H), 16.64 (s, 1 H). 13C NMR (68 MHz,
CDCl3): δ = 18.5, 20.8, 21.1, 51.6, 56.1, 77.8, 112.6, 121.8, 122.7,
122.9, 133.5, 135.7, 138.7, 147.5, 150.6, 153.7, 211.9, 299.5.
HRMS (FAB+): m/z calcd for C42H47Cl2N2O2Ru: 784.2058; found:
784.2106.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1562468.
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References and Notes
(1) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H.
J. Am. Chem. Soc. 1999, 121, 791.
Characterization Data of Catalyst 1e
(2) (a) Fürstner, A. Alkene Metathesis in Organic Synthesis; Springer:
New York, 1998. (b) Grubbs, R. H. Handbook of Metathesis;
Wiley-VCH: Weinheim, 2003. (c) Wang, H.; Matsuhashi, H.;
Doan, B. D.; Goodman, S. N.; Ouyang, X.; Clark, W. M. Tetrahe-
dron 2009, 65, 6291. (d) Kong, J.; Chen, C.; Padros, J. B.; Cao, Y.;
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2012, 77, 3820.
(3) For selected reviews, see: (a) Fürstner, A. Angew. Chem. Int. Ed.
2000, 39, 3012. (b) Connon, S. J.; Blechert, S. Angew. Chem. Int.
Ed. 2003, 42, 1900. (c) Clavier, H.; Grela, K.; Kirschning, A.;
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3708. (e) Higman, C. S.; Lummiss, J. A. M.; Fogg, D. E. Angew.
Chem. Int. Ed. 2016, 55, 3552.
Green crystal, mp 163.7–165.0 °C. 1H NMR (270 MHz, CDCl3):
δ = 0.77 (dd, J = 62.6, 5.9 Hz, 6 H), 2.52 (br s, 18 H), 4.17–4.28
(m, 5 H), 6.95–7.03 (m, 6 H), 7.39–7.42 (m, 1 H), 7.50–8.71 (m, 8
H), 8.67 (d, J = 8.1 Hz, 2 H), 16.94 (s, 1 H). 13C NMR (68 MHz,
CDCl3): δ = 20.4, 21.1, 21.3, 52.4, 77.8, 122.5, 123.0, 123.3, 126.8,
126.9, 127.0, 127.5, 127.7, 128.8, 130.3, 131.1, 131.3, 134.7,
136.3, 138.8, 147.5, 149.9, 211.3, 298.9. IR: 2920, 1480, 1424,
1255, 1203, 1097, 921, 851, 745, 720 cm–1. HRMS (FAB+): m/z
calcd for C45H47Cl2N2ORu: 804.2187; found: 804.2195.
Characterization Data of Catalyst 1f
Green crystal, mp 172.3–173.7 °C. 1H NMR (270 MHz, CDCl3):
δ = 0.57 (d, J = 6.2 Hz, 6 H), 2.52 (br s, 18 H), 3.82-3.92 (m, 1 H),
4.17 (s, 4 H), 6.96–7.10 (m, 6 H), 7.33–7.81 (m, 6 H), 7.88 (d, J =
36.5 Hz, 2 H), 7.98 (d, J = 1.6 Hz, 2 H), 8.43 (s, 1 H), 16.77 (s, 1 H).
13C NMR (68 MHz, CDCl3): δ = 18.4, 21.1, 21.4, 51.4, 78.6, 123.0,
123.0, 125.3, 125.4, 126.4, 127.0, 127.5, 128.2, 129.4, 130.2,
131.0, 133.0, 135.9, 147.8, 150.8, 211.3, 298.7. IR: 2917, 1481,
(4) For selected examples, see: (a) Berlin, J. M.; Campbell, K.; Ritter,
T.; Funk, T. W.; Chlenov, A.; Grubbs, R. H. Org. Lett. 2007, 9,
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Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 5313. (c) Petit, L. V.;
Clavier, H.; Linden, A.; Blumentritt, S.; Nolan, S. P.; Dorta, R.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

E
Y. Kobayashi et al.
Letter
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1443, 1255, 1197, 1098, 1014, 925, 886, 851, 732 cm–1. HRMS
(FAB+): m/z calcd for C45H47Cl2N2ORu: 804.2187; found:
804.2167.
(14) For B3LYP, see: (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. For
LANL2DZ, see: (b) Dunning, T. H.; Hay, P. J. Modern Theoretical
Chemistry; Vol. 3; Plenum Press: New York, 1977. (c) Hay, P. J.;
Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (d) Hay, P. J.; Wadt, W.
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(9) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am.
Chem. Soc. 2000, 122, 8168.
(10) Characterization Data of Catalyst 1g
(15) Bondi, A. J. Phys. Chem. 1964, 68, 441.
Green crystal, mp 300 °C (dec.). 1H NMR (270 MHz, CDCl3): δ =
1.33 (d, J = 5.7 Hz, 6 H), 2.11–2.42 (br m, 18 H), 4.10 (s, 4 H),
4.92–5.01 (m, 1 H), 6.93–6.97 (m, 6 H), 7.28–7.50 (m, 6 H), 7.68
(d, J = 8.9 Hz, 2 H), 7.98 (d, J = 8.4 Hz, 2 H), 8.43 (s, 1 H), 16.64 (s,
1 H). 13C NMR (68 MHz, CDCl3): δ = 21.1, 21.4, 112.9, 125.1,
125.3, 125.5, 126.7, 127.3, 128.3, 129.5, 130.7, 131.4, 132.4,
132.6, 135.6, 139.0, 145.4, 152.0, 211.0, 296.4. HRMS (FAB+):
m/z calcd for C45H48Cl2N2ORu: 804.2162; found: 804.2187.
(11) Kobayashi, Y.; Inukai, S.; Kondo, N.; Watanabe, T.; Sugiyama, Y.;
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Chem. Soc. 2000, 122, 8168.
(17) The bond angle (O–Ru–C) of 1a was 79.0° (79.3° in X-ray) when
calculated in the same method.
(18) For selected reviews, see: (a) Oki, M. Acc. Chem. Res. 1990, 23
351. (b) Nishio, M.; Umezawa, Y.; Hirota, M.; Takeuchi, Y. Tetra-
hedron 1995, 51, 8665. (c) Nishio, M.; Umezawa, Y.; Hirota, M.
The CH/π Interaction; Wiley-VCH: Weinheim, 1998.
(19) Nishihata, K.; Nishio, M. Tetrahedron Lett. 1977, 18, 1041.
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