Tandem dienyne ring-closing metathesis of alkynyl silaketals for the formation of bicyclic siloxanes

Article abstract of DOI:10.1016/j.jorganchem.2005.04.037

The tandem dienyne ring-closing metathesis of alkynyl silaketals containing two tethered olefins was achieved with second generation Grubbs NHC-ruthenium carbene complex to provide bicyclic siloxanes in good to excellent yield. Silaketals synthesized from

Full text of DOI:10.1016/j.jorganchem.2005.04.037

Journal of Organometallic Chemistry 690 (2005) 5508–5516
www.elsevier.com/locate/jorganchem
Tandem dienyne ring-closing metathesis of alkynyl silaketals
for the formation of bicyclic siloxanes
*
Jonathan B. Grimm, Ryan D. Otte, Daesung Lee
Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, WI 53706, USA
Received 31 March 2005; received in revised form 20 April 2005; accepted 25 April 2005
Available online 15 June 2005
Abstract
The tandem dienyne ring-closing metathesis of alkynyl silaketals containing two tethered olefins was achieved with second gen-
eration Grubbs NHC-ruthenium carbene complex to provide bicyclic siloxanes in good to excellent yield. Silaketals synthesized
from homoallylic or bishomoallylic alcohols via base-catalyzed alcoholysis of trialkynylsilanes were well tolerated in the metathesis
process and generated ring systems of various sizes. Removal of the silicon tether was achieved through protodesilylation with fluo-
ride to afford stereochemically defined (1E,3Z)-dienes.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Ruthenium carbene complexes; N-Heterocyclic carbenes; Enyne metathesis; Silicon tethering; 1,3-Dienes
1. Introduction
the use of very reactive NHC-ruthenium carbene cata-
lysts, subsequently becoming one of the most powerful
and practical methods for the formation of 1,3-dienes
from alkenes and alkynes [2,5]. Depending on the mode
of bond formation between the reacting alkene and al-
kyne, either a 1,2- or 1,3-substituted 1,3-diene (5 or 6)
can be formed (Scheme 1). There is, however, no mech-
anism within enyne metathesis for regio- and stereose-
lective generation of 1,4-substituted 1,3-dienes (7).
Given that 1,4-substitution is more frequently found in
many natural products and synthetic intermediates [6],
this limitation of the scope of enyne ring-closing metath-
esis (RCM) is significant. Conceptually, this restriction
can be eliminated if the temporary tether strategy is inte-
grated into enyne metathesis, as demonstrated in
Scheme 1. This process combines an enyne RCM and
a diene RCM to generate a bicyclic structure (8) via tan-
dem dienyne metathesis [5c,7]. Removal of the tethered X
group would then provide a 1,4-substituted E,Z-diene 7.
In choosing a tethering technology (‘‘X’’) for use in
this application, the silyloxy functionality presents itself
as an ideal choice. The silyl ether is one of the most
Olefin metathesis has emerged as one of the most
powerful carbon–carbon bond-forming methods in the
past decade, exerting a profound impact in synthetic
chemistry [1]. The growing field of research in metathesis
processes has been driven by the development of well-
defined ruthenium [2], molybdenum [1d,3], and tung-
sten-based [1d,4] catalyst systems that show excellent
reactivity and a wide range of functional group toler-
ance. Among these catalysts, ruthenium-based Grubbs
and Hoveyda-type complexes 1–3 are most relevant for
the metathesis between alkenes and alkynes (enyne
metathesis) to generate 1,3-dienes [5] (see Fig. 1).
Recently, enyne metathesis has gained growing atten-
tion due to its inherently tandem bond-forming nature,
allowing the synthesis of multiple conjugated bonds
and rings per synthetic step. The limited substrate scope
of enyne metathesis has been significantly broadened by
*
Corresponding author. Tel.: +1 608 265 8431; fax: +1 608 265 4534.
E-mail address: dlee@chem.wisc.edu (D. Lee).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.037

J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
5509
R1
R1
OH
R3
O
O
R3
O
N
N
Cl
O
2R
Si
R3
2R
OH
2R
Si
protodesilylate
RCM
N
N
Cl
Cy3P
Ru
O
Cl
Cl
R4
12
Ru
R4
11
Ru
R4
Cl
Ph
Cl
Ph
PCy3
PCy3
10
1
2
3
Scheme 2. Tandem dienyne RCM of alkynyl silaketals.
Fig. 1. Transition metal carbene catalysts for metathesis.
efficient tool to synthesize cyclic structures possessing a
1,4-disubstituted 1,3-diene. Protodesilylation would also
provide straightforward access to dienes 12 free of the
silicon tether. This process could be extremely useful
to the synthetic organic chemist, as it meets many
important goals simultaneously, including the synthesis
of stereochemically defined alkenes, a rapid increase in
molecular complexity, and annulation to form function-
alized rings of various sizes.
2
3
enyne RCM
and/or
1
1
5
6
4
4
-X
X
X
tether
removal
dienyne
RCM
1
7
8
9
Scheme 1. Generation of disubstituted 1,3-dienes by ring-closing
metathesis.
2. Results and discussion
The silaketal substrates necessary for the exploration
of this methodology (Table 1) were synthesized through
the reaction of trialkynylsilanes with alcohols in the
presence of a catalytic amount of base. The specific tri-
alkynylsilanes – 14a and 14b – were chosen on the basis
of their moderate reactivity and stability, as well as the
high volatility of the alkyne byproduct of the alcoholysis
reaction. A series of silylation reactions was performed
with silanes 14a and 14b and a variety of secondary
alcohols to generate silaketals 15a–15j (Table 1). The
widely used protecting [8] and tethering [9] groups in
modern synthetic chemistry due to the ease of its forma-
tion and removal; temporary silicon tethering (TST)
with the silyl ether has become increasingly widespread
in recent years. An extensive variety of transformations
has been achieved with silyl ether and silaketal tethers,
including Diels–Alder reactions [10], 1,3-dipolar cyc-
loadditions of nitronates and nitrones [11], Heck cou-
plings [12], silylformylation–allylsilylations [13], alkyne
cyclocarbonylations [14], and metal-mediated cycliza-
tions [15]. Various research groups have pursued the
use of silicon tethering in diene ring-closing metathesis
[16], but there are far fewer examples of silicon-tethered
enyne RCM [17]. Previous work has also demonstrated
the viability and utility of dienyne RCM in all-carbon
systems [7,18], as well as those containing nitrogen [19]
and phosphorus [20] ‘‘tethers.’’ To broaden the scope
of this strategy, we recently developed an efficient silicon
tethering approach to form silaketal-based dienyne
RCM substrates and briefly demonstrated the tandem
metathesis of this new substrate class to form fused bicy-
cles [21]. Herein we report the further exploration of the
silicon-tethered dienyne RCM process and its applica-
tion to the stereochemically controlled synthesis of 1,3-
dienes.
Table 1
Symmetrical alkynyl silaketals from trialkynylsilane
This study has been undertaken to investigate the
dienyne ring-closing metathesis of alkynyl silaketals 10
with ruthenium carbene catalysts (1–3) to form bicyclic
siloxanes represented by 11 (Scheme 2). Application of
our previous work on the base-catalyzed alcoholysis of
alkynylsilanes allows for efficient access to the unusual
silaketal substrates [21]. The union of the silicon tether
with the capacity of dienyne metathesis to form multiple
carbon–carbon bonds in tandem fashion provides an

5510
J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
optimal conditions involved the use of a catalytic
amount of NaH (10 mol%) in hexanes. In nearly all
cases, employment of two equivalents of alcohol cleanly
afforded the desired alkynyl silaketal with only trace
amounts of the trialkoxysilane, which was easily re-
moved by chromatography. The transformation was
generally complete in a few minutes at room tempera-
ture for 14a; silane 14b required an elevated reaction
temperature (50 °C) for 1–2 h to achieve complete con-
version. Because the secondary alcohols employed in
the formation of silaketals 15a–15e and 15j were race-
mic, these alkynyl silaketals were obtained as diastereo-
meric mixtures (the two meso compounds RS and SR
and the enantiomeric pair RR|SS). However, the 1H
and 13C NMR spectra of these silaketals showed only
small distinctions (if any) between the diastereomeric
resonances. Only single sets of signals were seen in the
NMR spectra of 15f–15i, as they were derived from
the corresponding enantiomerically pure secondary
alcohols [22].
With the necessary silaketals in hand, we examined
their reactivity in enyne ring-closing metathesis. When
silaketals 15a–15j were treated with a catalytic amount
of second generation Grubbs catalyst (2), the bicyclic sil-
oxanes 16a–16j were universally provided in moderate
to excellent yields in 3–4 h (Table 2). Small amounts
of monocyclized material were isolated in most cases,
and the desired product was purified by flash chroma-
tography without difficulty. Cyclization of silaketals
15a–15f proceeded easily at reflux (40 °C) in CH2Cl2
(2–3 mM), whereas reaction of those with disubstituted
alkenes (15g–15j) required a higher reaction tempera-
ture (110 °C in toluene). These results indicated that
the tandem enyne metathesis was amenable to terminal
as well as internal double bonds. Although the major-
ity of the siloxane products contain [5.4.0] ring systems,
Table 2
Tandem dienyne RCM of alkynyl silaketals and protodesilylation of siloxane products
a
cP, cyclopentyl.
All reactions performed at 2–3 mM in the indicated solvent with 7.5 mol% of catalyst 2.
b

J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
5511
silaketals with longer tethers were well tolerated (e.g.,
16j, a [6.5.0] bicycle).
HO
14a (1 eq)
NaH (10 mol %)
OMe
OMe
OH
Ph
cP
O
NaH (10 mol %)
Si
Ph
hexanes, rt
80%
hexanes, 50 ˚C
99%
The siloxanes 16f–16i derived from homochiral
silaketals were formed as nearly 1:1 mixtures of diaste-
reomers, where the two diastereomeric products were of-
ten separable on chromatography and allowed precise
and unambiguous characterization of the bicycles. These
results clearly indicated that the RCM reaction was not
stereoselective for a particular configuration at silicon.
Diastereomeric olefin resonances consistent with this
lack of selectivity were also present in the spectra of
Ph
Ph
19
18
Ph
cP
cP
cP
Ph
O
Ph
Ph
Ph
Ph
O
O
O
O
O
Si
Si
Si
2 (7.5 mol %)
not
CH2Cl2, 40 ˚C
78%
MeO
MeO
not
OMe
21
22
20
Ph Ph
HO
Ph
Ph
OH
OH
OH
TBAF
1
16a–16e and 16j, although the H and 13C NMR were
THF, 60 ˚C
77%
more difficult to fully analyze due to the presence, in
each case, of four nearly inseparable diastereomers.
Critical to the utility of a tethered RCM strategy is
the removal of the temporary tether subsequent to the
metathesis step. Excision of the silicon moiety from
the bicyclic siloxanes was easily achieved through pro-
todesilylation. Treatment of siloxanes 16a–16j with an
excess of TBAF in THF at reflux [13] yielded diols
17a–17j in moderate to good yield after only 30–45 min
(Table 2). Protodesilylation proceeded with retention of
double bond configuration. This was evidenced by the
coupling constants observed for the disubstituted alkene
moieties, which were characteristic in magnitude of trans
olefins (e.g., J = 15.8 Hz for 17f). Notably, diols that
existed as mixtures of two diastereomers (17a–17f, 17j)
MeO
MeO
24
23
Scheme 3. Selective tandem dienyne RCM of an unsymmetrical
silaketal.
HO
OMe
OMe
14a (1 eq)
NaH (10 mol %)
cP
O
OH
Si
NaH (10 mol %)
hexanes, rt
73%
hexanes, rt
85%
26
25
cP
cP
cP
O
O
O
O
O
O
Si
Si
Si
2 (7.5 mol %)
+
CH2Cl2, 40 ˚C
81%
OMe
OMe
MeO
28
27
29
1
behaved in the H and 13C NMR as essentially single
compounds. This fortuity allowed for unequivocal char-
acterization of the final products, particularly in those
cases where it was not possible to fully characterize the
siloxane intermediates (16a–16f, 16j).
HO
HO
OH
OH
TBAF
+
THF, 60 ˚C
81%
MeO
OMe
30
31
The tandem enyne RCM of symmetrical silaketals es-
chews a significant issue that becomes relevant when
considering the same process in unsymmetrical silake-
tals: chemoselectivity (‘‘group selectivity’’). In order to
begin to explore this matter, we synthesized two distinct,
unsymmetrical silaketals (20 and 27), each containing
two alkenes with tethers of different length and substitu-
tion (Schemes 3 and 4). Silane 14a was first reacted with
a single equivalent of tertiary alcohol 18 or 25 and cat-
alytic NaH to give silyl ethers 19 and 26 in excellent
yield (formation of the former required elevated temper-
ature). The alcoholysis reaction was repeated a second
time with either 3-buten-1-ol or 4-penten-1-ol (1 equiva-
lent) to accrue the desired silaketals 20 and 27 in 73–80%
yield. Exposure of 20 to ruthenium catalyst 2 in CH2Cl2
(3 mM) at 40 °C led to rapid formation of a single prod-
uct 21 in 78% yield. This surprising result was further
corroborated by protodesilylation with TBAF to afford
only the diol 23 (77%). Unambiguous assignment of the
product structures shown in Scheme 3 was achieved
through a homonuclear decoupling experiment. When
the vinyl proton of the trisubstituted double bond in
23 was radiated, loss of doublet splitting for the indi-
cated methylene protons adjacent to the tertiary alkoxy
Scheme 4. Nonselective tandem dienyne RCM of an unsymmetrical
silaketal.
carbon was observed. This required that the siloxane
RCM product was the [5.5.0] bicycle, which must result
from initial enyne metathesis of the longer, less substi-
tuted tether of 20. Unexpectedly, RCM of 27 (Scheme
4) did not possess the same selectivity, and a 1.5:1 mix-
ture of the two possible siloxanes 28 and 29 was isolated
(the identities of the major and minor products were not
clear from the spectral data). Although protodesilyla-
tion of this mixture proceeded without incident to yield
30 and 31, separation of the two diols was not possible.
The reasons for the surprising selectivity seen in the
RCM of 20 – and lack of such group selectivity in the
case of 27 – are not yet clear, and work is ongoing to ob-
tain more insight into the observed selectivity.
3. Conclusion
In summary, we have developed an efficient tandem
dienyne ring-closing metathesis of alkynyl silaketals to

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J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
generate bicyclic siloxanes in the presence of Grubbs
NHC-ruthenium catalyst. Removal of the silicon tether
through protodesilylation allows for the generation of
stereochemically defined 1,4-substituted 1,3-dienes. Fur-
ther exploration of the derivatization and elaboration of
these siloxanes – as well as the selectivity of the dienyne
RCM of unsymmetrical alkynyl silaketals – is in
progress.
27.4, 27.1, 25.13, 25.08, 24.9, 22.8, 14.2; HRMS (ESI)
calc. for C27H48O3Si [M + Na]+ 471.3270, found
471.3292.
15c (74%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.30–7.12 (10H, m), 5.94–5.77 (2H, m), 5.13–4.99 (4H,
m), 4.16–4.07 (4H, m), 3.363 (0.75H, s), 3.356 (1.50H, s),
3.351 (0.75H, s), 2.85–2.57 (4H, m), 2.48–2.30 (4H, m),
1.92–1.73 (6H, m), 1.69–1.46 (6H, m), 1.15–1.00 (1H,
m); 13C NMR (CDCl3, 75.4 MHz) d 142.7, 134.9,
128.6, 128.5, 125.8, 117.3, 101.8, 86.62, 86.56, 86.49,
72.7, 60.3, 57.7, 41.7, 41.6, 38.3, 31.9, 27.4, 27.2, 24.9;
HRMS (ESI) calc. for C33H44O3Si [M + Na]+
539.2957, found 539.2983.
4. Experimental
4.1. General information
15d (79%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 5.95–5.76 (2 H, m), 5.09–4.97 (4H, m), 3.99 (2H, pen-
tet, J = 5.6 Hz), 2.39–2.22 (4H, m), 1.89 (3H, s), 1.82–
1.68 (2H, m), 1.63–1.19 (22H, m), 1.06–0.94 (1H, m),
0.93–0.84 (6H, m); 13C NMR (CDCl3, 75.4 MHz) d
135.6, 116.6, 103.0, 79.2, 72.9, 41.8, 41.7, 36.6, 36.4,
32.2, 27.5, 27.2, 25.1, 22.9, 14.3, 4.9; HRMS (ESI) calc.
for C26H46O2Si [M + Na]+ 441.3165, found 441.3157.
15e (82%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.34–7.09 (10H, m), 5.96–5.76 (2H, m), 5.13–4.98 (4H,
m), 4.17–4.02 (2H, m), 2.86–2.71 (2H, m), 2.70–2.56
(2H, m), 2.48–2.29 (4H, m), 1.88 (3H, s), 1.87–1.71
(6H, m), 1.68–1.45 (6H, m), 1.10–0.96 (1H, m); 13C
NMR (CDCl3, 75.4 MHz) d 142.9, 135.2, 128.5, 125.8,
117.1, 103.5, 79.1, 72.5, 41.8, 38.4, 31.9, 27.5, 27.2,
25.1, 4.9; HRMS (ESI) calc. for C32H42O2Si
[M + Na]+ 509.2852, found 509.2856.
Hexanes, dichloromethane (CaH2), and THF (so-
dium/benzophenone) were freshly distilled prior to use.
Reactions were monitored by thin layer chromatogra-
phy (TLC) on precoated TLC glass plates (silica gel 60
˚
F
254, 250 lm thickness). Silica gel (60 A porosity, 32–
63 lm particle size) was used for flash chromatography.
1H and 13C NMR were obtained on 250 and 300 MHz
spectrometers; chemical shifts (d) are reported relative
to an internal standard of tetramethylsilane (TMS).
High resolution mass spectra were obtained using
Leu5-enkephalin or erythromycin as a lock mass on an
ESI-TOF spectrometer equipped with Z-spray and a
reflectron.
4.2. Synthesis of symmetrical silaketals 15a–15j
The following procedure for 15a is representative. Si-
lane 14a (200 mg, 0.66 mmol) and 4-penten-2-ol
(114 mg, 1.32 mmol) were dissolved in hexanes (4 mL).
Sodium hydride (60% dispersion in mineral oil, 5 mg,
0.13 mmol) was added, and the reaction was stirred at
room temperature for 20 min. The solution was then fil-
tered through a short plug of Celite, and evaporation of
the solvent afforded a yellow residue. Flash chromatog-
raphy of the crude product on silica gel with 99:1 to 97:3
hexanes:ether yielded 156 mg (70%) of 15a as a colorless
oil. 1H NMR (CDCl3, 300 MHz) d 5.83 (2H, ddt,
J = 17.2, 10.2, 7.1 Hz), 5.10–5.00 (4H, m), 4.23–4.10
(2H, m), 4.13 (2H, s), 3.39 (3H, s), 2.39–2.15 (4H, m),
1.85–1.70 (2H, m), 1.67–1.43 (6H, m), 1.21 (6H, d,
J = 6.4 Hz) 1.11–0.97 (1H, m); 13C NMR (CDCl3,
75.4 MHz) d 135.3, 116.9, 101.4, 101.3, 101.2, 86.4,
86.3, 86.2, 69.5, 69.4, 60.4, 57.6, 43.9, 27.3, 27.2, 27.1,
24.7, 24.6, 23.0; HRMS (ESI) calc. for C19H32O3Si
[M + Na]+ 359.2024, found 359.2018.
15f (67%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.37–7.22 (10H, m), 5.93–5.75 (2H, m), 5.13–4.98 (4H,
m), 4.53–4.49 (4H, m), 4.22 (2H, sextet, J = 5.6 Hz), 4.07
(2H, s), 3.55–3.39 (4H, m), 3.35 (3H, s), 2.51–2.23 (4H,
m), 1.82–1.67 (2H, m), 1.65–1.39 (6H, m), 1.14–0.99
(1H, m); 13C NMR (CDCl3, 75.4 MHz) d 138.7, 134.8,
128.4, 127.7, 127.6, 117.31, 117.26, 101.7, 86.0, 73.6,
73.5, 73.4, 72.0, 71.9, 60.3, 57.6, 38.9, 27.3, 27.2, 27.1,
24.6; HRMS (ESI) calc. for C33H44O5Si [M + Na]+
571.2856, found 571.2866.
15g (81%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 5.53–5.35 (4H, m), 4.13 (2H, s), 4.00–3.89 (2H, m),
3.39 (3H, s), 2.30–2.14 (4H, m), 1.83–1.17 (30H, m),
1.10–0.96 (1H, m), 0.94–0.83 (6H, m); 13C NMR
(CDCl3, 75.4 MHz) d 127.8, 127.22, 127.15, 101.1,
87.0, 73.55, 73.51, 60.4, 57.6, 40.5, 40.4, 36.6, 36.4,
32.21, 32.18, 27.4, 27.2, 25.2, 25.1, 25.0, 22.9, 18.2,
14.2; HRMS (ESI) calc. for C29H52O3Si [M + Na]+
499.3583, found 499.3562.
15b (71%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 5.85 (2H, ddt, J = 17.2, 10.4, 7.2 Hz), 5.10–4.98 (4H,
m), 4.12 (2H, s), 4.01 (2H, pentet, J = 5.5 Hz), 3.39
(3H, s), 2.36–2.25 (4H, m), 1.84–1.69 (2H, m), 1.66–
1.19 (22H, m), 1.10–0.97 (1H, m), 0.94–0.83 (6H, m);
13C NMR (CDCl3, 75.4 MHz) d 135.4, 116.81, 116.77,
101.3, 86.8, 73.1, 60.4, 57.6, 41.7, 41.6, 36.5, 36.4, 32.1,
15h (81%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.31–7.11 (10H, m), 5.56–5.38 (4H, m), 4.11 (2H, s),
4.10–4.01 (2H, m), 3.36 (3H, s), 2.85–2.71 (2H, ddd,
J = 13.8, 10.2, 6.4 Hz), 2.69–2.56 (2H, m), 2.40–2.22
(4H, m), 1.91–1.47 (18H, m), 1.14–1.00 (1H, m); 13C
NMR (CDCl3, 75.4 MHz) d 142.9, 128.6, 128.5, 127.7,
127.40, 127.38, 125.8, 101.6, 86.8, 73.2, 60.4, 57.6,

J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
5513
40.6, 40.4, 38.34, 38.27, 31.9, 27.4, 27.2, 25.0, 18.2;
HRMS (ESI) calc. for C35H48O3Si [M + Na]+
567.3270, found 567.3276.
ABX, mA = 1198.8, JAX = 1.1, mB = 1183.5, JBX = 0.9,
JAB = 11.8 Hz), 3.33 (3H, s), 2.49 (1H, dt, J = 17.7,
4.1 Hz), 2.41–2.33 (2H, m), 2.24 (1H, ddd, J = 17.7,
5.5, 4.6 Hz), 1.82–1.20 (24H, m), 1.11–0.97 (1H, m),
0.93–0.84 (6H, m); 13C NMR (CDCl3, 75.4 MHz) dia-
stereomer 1: d 142.6, 138.1, 137.1, 125.3, 76.1, 73.9,
72.0, 57.8, 37.9, 36.8, 36.5, 32.1, 32.0, 27.4, 27.1, 27.0,
26.0, 25.8, 25.1, 24.4, 22.9, 22.8, 14.2; diastereomer 2:
d 141.8, 135.5, 134.4, 126.8, 77.4, 72.2, 71.4, 57.9, 40.1,
38.5, 37.1, 35.1, 32.1, 27.6, 27.3, 27.0, 26.8, 26.1, 25.7,
24.7, 22.9, 22.8, 14.3; HRMS (ESI) calc. for C25H44O3Si
[M + Na]+ 443.2957, found 443.2945.
16h (88%, colorless oil). 1H NMR (CDCl3, 300 MHz)
diastereomer 1: d 7.31–7.10 (10H, m), 6.73 (1H, t,
J = 5.1 Hz), 5.63 (1H, t, J = 6.9 Hz), 4.08–3.91 (4H,
m), 3.31 (3H, s), 2.88–2.62 (4H, m), 2.52–2.28 (2H, m),
2.25–2.17 (2H, m), 2.04–1.32 (12H, m), 1.13–0.99 (1H,
m); diastereomer 2: d 7.31–7.11 (10H, m), 6.70 (1H, t,
J = 4.5 Hz), 5.49 (1H, t, J = 4.5 Hz), 4.24–4.06 (2H,
m), 4.01–3.88 (2H, AB of ABX, mA = 1190.2,
JAX = 1.1, mB = 1176.1, JBX = 1.0, JAB = 12.0 Hz), 3.32
(3H, s), 2.94–2.58 (4H, m), 2.49–2.19 (4H, m), 2.12–
1.97 (1H, m), 1.93–1.38 (11H, m), 1.14–1.00 (1H, m);
13C NMR (CDCl3, 75.4 MHz) diastereomer 1: d 142.7,
142.6, 142.5, 138.6, 137.1, 128.8, 128.7, 128.4, 125.8,
124.8, 75.9, 73.2, 70.9, 57.9, 39.7, 39.5, 36.8, 36.7, 32.3,
31.5, 27.5, 27.1, 27.03, 26.99, 24.4; diastereomer 2: d
142.6, 142.2, 135.2, 134.6, 128.9, 128.6, 128.5, 128.3,
126.1, 125.9, 125.8, 77.3, 71.3, 70.5, 57.9, 40.14, 40.08,
38.8, 35.6, 32.3, 32.1, 27.7, 27.4, 27.0, 26.8, 24.9; HRMS
(ESI) calc. for C31H40O3Si [M + Na]+ 511.2644, found
511.2654.
15i (74%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.37–7.08 (10H, m), 5.50–5.12 (4H, m), 4.95 (1H, dd,
J = 6.8, 6.0 Hz), 4.68 (1H, t, J = 6.4 Hz), 4.00 (2H, s),
3.25 (3H, s), 2.55–2.15 (4H, m), 1.71–1.33 (14H, m),
1.02–0.89 (1H, m); 13C NMR (CDCl3, 75.4 MHz) d
144.3, 144.2, 128.1, 127.9, 127.8, 127.6, 127.4, 127.2,
127.1, 127.0, 126.4, 126.3, 101.8, 86.1, 75.9, 75.6, 60.2,
57.5, 43.7, 43.5, 27.1, 24.5, 18.2, 18.1; HRMS (ESI) calc.
for C31H40O3Si [M + Na]+ 511.2644, found 511.2631.
15j (66%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 5.49–5.32 (4H, m), 4.16–4.00 (2H, m), 2.12–1.92 (8H,
m), 1.89 (3H, s), 1.82–1.69 (2H, m), 1.67–1.41 (10H,
m), 1.40–1.23 (12H, m), 1.23–1.15 (6H, m), 1.05–0.93
(1H, m), 0.93–0.83 (6H, m); 13C NMR (CDCl3,
75.4 MHz) d 130.6, 130.2, 102.7, 79.0, 69.2, 39.5, 32.8,
31.6, 29.6, 29.0, 27.5, 27.2, 24.9, 23.5, 22.8, 14.3, 4.9;
HRMS (EI) calc. for C30H54O2Si [M]+ 474.3893, found
474.3879.
4.3. Dienyne RCM of alkynyl silaketals to generate
siloxanes 16a–16j, 21, 28, 29
The following procedure for 16f is representative.
Silaketal 15f (100 mg, 0.180 mmol) was dissolved in
dry CH2Cl2 (90 mL) in a 200 mL round bottom flask
equipped with a reflux condenser. Nitrogen was bubbled
through the solution for 20 min before adding catalyst 2
(12 mg, 0.014 mmol) in CH2Cl2 (5 mL). The reaction
was then stirred at reflux until complete by TLC
(approximately 4 h). The solvent was removed to yield
a brown residue that was flash chromatographed on sil-
ica with 95:5 to 85:15 hexanes:ether, affording 78 mg
(83%) of 16f as a colorless, viscous oil (diastereomers
not separable). 1H NMR (CDCl3, 250 MHz) d 7.37–
7.22 (10H, m), 6.81–6.72 (1H, m), 5.68 (0.5H, t,
J = 7.0 Hz), 5.59–5.52 (0.5H, m), 4.61–4.50 (4H, m),
4.37–4.25 (1H, m), 4.22–4.10 (1H, m), 4.08–3.90 (2H,
m), 3.64–3.34 (4H, m), 3.320 (1.5H, s), 3.316 (1.5H, s),
2.67–2.15 (4H, m), 1.85–1.33 (8H, m), 1.14–0.96 (1H,
m); 13C NMR (CDCl3, 75.4 MHz) d 143.0, 142.1,
140.8, 139.0, 138.62, 138.55, 136.9, 134.7, 134.4, 128.5,
127.9, 127.8, 127.7, 126.2, 124.7, 75.8, 74.6, 74.5, 74.3,
73.6, 73.5, 73.2, 72.8, 71.3, 70.7, 70.4, 58.0, 57.9, 37.0,
33.8, 33.6, 32.1, 27.5, 27.4, 27.2, 27.1, 27.0, 26.8, 24.6,
24.2; HRMS (ESI) calc. for C31H40O5Si [M + Na]+
543.2543, found 543.2558.
16i (75%, colorless oil). 1H NMR (CDCl3, 300 MHz)
diastereomer 1: d 7.46–7.17 (10H, m), 6.81 (1H, dd,
J = 6.6, 2.8 Hz), 5.81 (1H, t, J = 7.3 Hz), 5.17 (1H, d,
J = 8.1 Hz), 5.06 (1H, dd, J = 10.2, 3.0 Hz), 4.05 (2H,
s), 3.37 (3H, s), 2.88–2.75 (1H, m), 2.53–2.32 (3H, m),
1.91–1.75 (2H, m), 1.71–1.46 (6H, m), 1.31–1.16 (1H,
m); diastereomer 2: d 7.41–7.11 (10H, m), 6.90 (1H, t,
J = 4.4 Hz), 5.51 (1H, t, J = 4.3 Hz), 5.27 (1H, t,
J = 5.1 Hz), 4.87 (1H, dd, J = 9.8, 2.0 Hz), 4.09–3.94
(2H, AB of ABX, mA = 1213.4, JAX = 1.0, mB = 1191.6,
JBX = 1.0, JAB = 11.7 Hz), 3.36 (3H, s), 2.78 (2H, t,
J = 4.9 Hz), 2.63–2.39 (2H, m), 1.95–1.78 (2H, m),
1.70–1.45 (6H, m), 1.28–1.14 (1H, m); 13C NMR
(CDCl3, 75.4 MHz) diastereomer 1: d 145.0, 144.4,
143.5, 140.0, 137.7, 128.4, 128.3, 127.4, 127.1, 125.7,
125.6, 124.1, 76.3, 75.7, 74.2, 58.2, 39.8, 39.5, 27.5,
27.3, 27.1, 24.2; diastereomer 2: d 144.8, 143.4, 142.7,
135.5, 134.3, 128.3, 128.1, 127.0, 126.3, 126.1, 125.7,
77.4, 73.1, 72.0, 58.1, 43.4, 35.9, 27.7, 27.4, 27.1, 26.9,
24.7; HRMS (ESI) calc. for C27H32O3Si [M + Na]+
455.2018, found 455.2032.
16g (96%, colorless oil). 1H NMR (CDCl3, 300 MHz)
diastereomer 1: d 6.77 (1H, dd, J = 6.9, 2.5 Hz), 5.64
(1H, t, J = 6.7 Hz), 4.11–3.88 (4H, m), 3.32 (3H, s),
2.50–2.31 (2H, m), 2.30–2.06 (2H, m), 1.81–1.19 (24H,
m), 1.10–0.95 (1H, m), 0.94–0.82 (6H, m); diastereomer
2: d 6.75 (1H, dd, J = 5.5, 3.5 Hz), 5.52 (1H, t,
J = 4.5 Hz), 4.17–4.03 (2H, m), 4.03–3.91 (2H, AB of
1
21 (78%, colorless oil). H NMR (CDCl3, 300 MHz)
d 7.55–7.42 (4H, m), 7.32–7.11 (6H, m), 6.52 (1H, t,
J = 7.1 Hz), 5.44 (1H, t, J = 7.7 Hz), 3.97 (1H, td,

5514
J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
J = 11.4, 4.5 Hz), 3.91 (2H, s), 3.75 (1H, ddd, J = 11.6,
6.2, 2.2 Hz), 3.25–3.14 (5H, m), 2.55–2.40 (1H, m),
2.33–2.20 (1H, m), 1.99–1.42 (10H, m), 1.28–1.10 (1H,
m); 13C NMR (CDCl3, 75.4 MHz) d 148.8, 146.7,
143.8, 141.7, 140.8, 128.1, 127.9, 126.6, 126.5, 126.1,
125.6, 120.9, 80.5, 76.0, 61.3, 57.8, 40.9, 28.4, 27.73,
27.66, 27.2, 27.1, 26.5, 25.5; HRMS (ESI) calc. for
C28H34O3Si [M + Na]+ 469.2175, found 469.2154.
41.5, 37.1, 35.6, 32.1, 25.6, 22.8, 21.0, 14.2; HRMS
(ESI) calc. for C19H36O2Si [M + Na]+ 319.2613, found
319.2617.
17e (65%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.34–7.12 (10H, m), 6.52 (1H, d, J = 15.8 Hz), 5.70
(1H, dt, J = 15.8, 7.5 Hz), 5.33 (1H, t, J = 7.7 Hz),
3.74–3.58 (2H, m), 2.88–2.74 (2H, m), 2.74–2.60 (2H,
m), 2.45–2.19 (4H, m), 2.03 (2H, bs), 1.89–1.70 (7H,
m); 13C NMR (CDCl3, 75.4 MHz) d 142.2, 135.0,
130.5, 128.6, 126.9, 126.0, 125.0, 71.1, 70.6, 41.6, 38.7,
38.6, 35.7, 32.3, 21.0; HRMS (ESI) calc. for C25H32O2Si
[M + Na]+ 387.2300, found 387.2299.
4.4. Protodesilylation to form diols 17a–17j, 23, 30, 31
The following procedure for 17a is representative.
Siloxane 16a (104 mg, 0.34 mmol) was taken up in dry
THF (3 mL), and TBAF (1.0 M in THF, 1.35 mL,
1.35 mmol) was added. The solution was stirred at reflux
for 45 min. It was then diluted with EtOAc and satu-
rated NH4Cl. The organic layer was removed, and the
aqueous layer was extracted twice more with EtOAc.
The combined organic layers were dried over MgSO4
and evaporated to give a yellow oil. Flash chromatogra-
phy of the crude product on silica with 1:1 to 1:0
EtOAc:hexanes yielded diene 17a (49 mg, 68%) as a
thick, colorless oil. 1H NMR (CDCl3, 300 MHz) d
6.38 (1H, dd, J = 15.8, 1.0 Hz), 5.85 (1H, dt, J = 15.8,
7.4 Hz), 5.60 (1H, t, J = 7.7 Hz), 4.06 (2H, s), 3.93–
3.80 (2H, m), 3.32 (3H, s), 2.46–2.18 (6H, m), 1.22
(3H, d, J = 5.9 Hz), 1.20 (3H, d, J = 6.4 Hz); 13C
NMR (CDCl3, 75.4 MHz) d 135.1, 128.0, 127.8, 127.7,
75.2, 67.8, 67.4, 57.8, 43.5, 37.3, 23.2, 23.0; HRMS
(ESI) calc. for C12H22O3 [M + Na]+ 237.1467, found
237.1479.
17b (89%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 6.38 (1H, d, J = 15.9 Hz), 5.86 (1H, dt, J = 15.9,
7.4 Hz), 5.61 (1H, t, J = 7.7 Hz), 4.07 (2H, s), 3.73–
3.60 (2H, m), 3.32 (3H, s), 2.42–2.31 (3H, m), 2.27–
2.14 (1H, m), 1.96 (2H, bs), 1.54–1.21 (16H, m),
0.96–0.83 (6H, m); 13C NMR (CDCl3, 75.4 MHz) d
135.1, 128.1, 127.85, 127.79, 75.2, 71.8, 71.2, 57.8, 41.8,
37.2, 37.1, 35.6, 32.0, 25.5, 22.8, 14.1; HRMS (ESI) calc.
for C20H38O3 [M + Na]+ 349.2719, found 349.2714.
17c (78%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.32–7.14 (10H, m), 6.38 (1H, d, J = 15.8 Hz), 5.84
(1H, dt, J = 15.8, 7.1 Hz), 5.59 (1H, t, J = 7.6 Hz),
4.04 (2H, s), 3.74–3.62 (2H, m), 3.30 (3H, s), 2.87–2.74
(2H, m), 2.74–2.61 (2H, m), 2.44–2.32 (3H, m), 2.31–
2.18 (1H, m), 2.06 (1H, bs), 1.98 (1H, bs), 1.86–1.72
(4H, m); 13C NMR (CDCl3, 75.4 MHz) d 142.3, 142.1,
135.3, 128.6, 127.9, 127.6, 126.01, 125.97, 75.1, 71.0,
70.5, 57.9, 41.9, 38.7, 35.7, 32.2; HRMS (ESI) calc. for
C26H34O3 [M + Na]+ 417.2406, found 417.2422.
17f (68%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.40–7.27 (10H, m), 6.36 (1H, d, J = 15.8 Hz), 5.85
(1H, dt, J = 15.8, 7.6 Hz), 5.58 (1H, t, J = 7.5 Hz),
4.56 (2H, s), 4.55 (2H, s), 4.03 (2H, s), 3.95–3.84 (2H,
m), 3.54–3.47 (2H, m), 3.38 (2H, dd, J = 9.5, 7.3 Hz),
3.28 (3H, s), 2.46 (2H, bs), 2.40 (2H, t, J = 7.1 Hz),
2.33 (2H, t, J = 6.6 Hz); 13C NMR (CDCl3, 75.4 MHz)
d 138.14, 138.07, 134.9, 128.6, 127.9, 127.4, 126.8,
75.0, 74.1, 74.0, 73.5, 70.4, 70.2, 57.8, 37.8, 31.6; HRMS
(ESI) calc. for C26H34O5 [M + Na]+ 449.2304, found
449.2315.
17g (87%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 6.38 (1H, d, J = 15.8 Hz), 5.86 (1H, dt, J = 15.8,
7.3 Hz), 5.61 (1H, t, J = 7.6 Hz), 4.07 (2H, s), 3.73–
3.60 (2H, m), 3.32 (3H, s), 2.42–2.31 (3H, m), 2.27–
2.14 (1H, m), 1.86 (1H, bs), 1.77 (1H, bs), 1.54–1.21
(16H, m), 0.96–0.83 (6H, m); 13C NMR (CDCl3,
75.4 MHz) d 135.2, 128.1, 127.8, 75.2, 71.8, 71.3, 57.8,
41.8, 37.2, 37.1, 35.6, 32.0, 25.6, 22.8, 14.2; HRMS
(ESI) calc. for C20H38O3 [M + Na]+ 349.2719, found
349.2709.
17h (83%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.32–7.14 (10H, m), 6.38 (1H, d, J = 15.8 Hz), 5.84
(1H, dt, J = 15.8, 7.4 Hz), 5.59 (1H, t, J = 7.6 Hz),
4.04 (2H, s), 3.74–3.62 (2H, m), 3.30 (3H, s), 2.87–2.74
(2H, m), 2.74–2.61 (2H, m), 2.44–2.32 (3H, m), 2.31–
2.11 (3H, m), 1.86–1.72 (4H, m); 13C NMR (CDCl3,
75.4 MHz) d 142.2, 142.1, 135.3, 128.6, 127.9, 127.6,
125.99, 125.97, 75.1, 70.9, 70.5, 57.9, 41.9, 38.7, 35.7,
32.2; HRMS (ESI) calc. for C26H34O3 [M + Na]+
417.2406, found 417.2415.
17i (88%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 7.39–7.22 (10H, m), 6.34 (1H, d, J = 15.7 Hz), 5.81
(1H, dt, J = 15.7, 7.4 Hz), 5.57 (1H, d, J = 7.6 Hz),
4.72 (2H, dd, J = 7.6, 5.4 Hz), 3.99 (2H, s), 3.22 (3H,
s), 2.75–2.46 (4H, m), 2.28 (2H, bs); 13C NMR (CDCl3,
75.4 MHz) d 144.1, 135.3, 128.6, 128.5, 128.0, 127.8,
127.6, 127.5, 126.0, 75.0, 74.1, 73.7, 57.8, 43.7, 37.5;
HRMS (ESI) calc. for C22H26O3 [M + Na]+ 361.1780,
found 361.1782.
17d (70%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 6.52 (1H, d, J = 15.4 Hz), 5.72 (1H, dt, J = 15.4,
7.5 Hz), 5.35 (1H, t, J = 7.7 Hz), 3.72–3.56 (2H, m),
2.43–2.15 (4H, m), 1.86 (3H, s), 1.80 (2H, bs), 1.54–
1.22 (16H, m), 0.95–0.83 (6H, m); 13C NMR (CDCl3,
75.4 MHz) d 134.8, 130.4, 127.1, 125.3, 72.0, 71.4,
17j (50%, colorless oil). 1H NMR (CDCl3, 300 MHz)
d 6.54–6.45 (1H, m), 5.76–5.62 (1H, m), 5.27 (1H, t,
J = 7.5 Hz), 3.90–3.74 (2H, m), 2.35–2.14 (4H, m), 1.80
(3H, s), 1.66 (2H, bs), 1.62–1.44 (4H, m), 1.24–1.15

J.B. Grimm et al. / Journal of Organometallic Chemistry 690 (2005) 5508–5516
5515
(6H, m); 13C NMR (CDCl3, 75.4 MHz) d 132.5, 130.3,
128.5, 127.7, 67.9, 67.7, 39.1, 29.8, 23.7, 20.8; HRMS
(ESI) calc. for C13H24O2Si [M + Na]+ 235.1674, found
235.1682.
1H NMR (CDCl3, 250 MHz) d 7.39–7.14 (10H, m),
5.86–5.60 (2H, m), 5.03–4.88 (4H, m), 4.01 (2H, s),
3.52–3.19 (4H, m), 3.33 (3H, s), 2.06–1.94 (2H, m),
1.80–1.40 (10H, m), 1.00–0.81 (1H, m); 13C NMR
(CDCl3, 75.4 MHz) d 146.9, 146.7, 138.6, 134.2, 127.8,
127.3, 127.2, 126.93, 126.89, 117.8, 114.6, 101.1, 86.7,
81.7, 63.1, 60.3, 57.6, 45.6, 31.6, 30.1, 27.5, 27.20,
27.15, 25.4; HRMS (ESI) calc. for C30H38O3Si
[M + Na]+ 497.2488, found 497.2469.
1
23 (77%, colorless oil). H NMR (CDCl3, 300 MHz)
d 7.49–7.39 (4H, m), 7.36–7.17 (6H, m), 6.36 (1H, d,
J = 15.7), 5.89 (1H, dt, J = 15.7, 7), 5.42 (1H, t,
J = 7.4), 3.96 (2H, s), 3.62 (2H, t, J = 6.4), 3.20 (2H, d,
J = 7.4), 3.15 (3H, s), 2.68 (1H, s), 2.21 (2H, q, J = 7),
1.75 (1H, bs), 1.66 (2H, pentet, J = 7.0); 13C NMR
(CDCl3, 75.4 MHz) d HRMS (ESI) calc. for C23H28O3
[M + Na]+ 375.1953, found 375.1936. 146.8, 136.7,
132.5, 128.4, 127.1, 126.2, 125.7, 125.1, 78.1, 75.2,
62.5, 57.6, 40.1, 32.3, 30.1.
1
27 (73%, colorless oil). H NMR (CDCl3, 300 MHz)
d 5.842 (1H, ddt, J = 16.9, 10.2, 6.6 Hz), 5.838 (1H, ddt,
J = 17.2, 10.2, 6.9 Hz), 5.14–4.89 (4H, m), 4.13 (2H, m),
4.13 (2H, s), 3.89–3.75 (2H, m), 3.39 (3H, s), 2.39–2.30
(2H, m), 2.20–2.09 (2H, m), 1.83–1.68 (2H, m), 1.67–
1.42 (6H, m), 1.34 (3H, s), 1.33 (3H, s), 1.08–0.94 (1H,
m); 13C NMR (CDCl3, 75.4 MHz) d 139.5, 135.4,
116.6, 114.0, 100.8, 87.6, 75.4, 63.0, 60.4, 57.6, 43.7,
37.2, 29.7, 29.6, 28.9, 27.4, 27.3, 27.1, 25.3; HRMS
(ESI) calc. for C20H34O3Si [M + Na]+ 373.2159, found
373.2175.
4.5. Synthesis of silyl ethers 19 and 26
The following procedure for 19 is representative. Si-
lane 14a (500 mg, 1.64 mmol) and alcohol 18 (368 mg,
1.64 mmol) were combined in hexanes (10 mL), and so-
dium hydride (60% dispersion in mineral oil, 6 mg,
0.16 mmol) was added. The reaction was stirred at
50 °C until complete by TLC (approximately 2.5 h).
The solution was then filtered through Celite. Evapora-
tion of the solvent gave a yellow oil that was chromato-
graphed on silica gel with 95:5 to 90:10 hexanes:ether to
obtain 750 mg (99%) of a colorless oil. 1H NMR
(CDCl3, 300 MHz) d 7.39–7.15 (10 H, m), 5.72 (1H,
ddt, J = 17.1, 10.3, 6.8 Hz), 5.03–4.89 (2H, m), 3.99
(4H, s), 3.35 (2H, d, J = 6.8 Hz), 3.31 (6H, s), 1.85–
1.70 (2H, m), 1.67–1.45 (6H, m), 1.12–0.98 (1H, m);
13C NMR (CDCl3, 75.4 MHz) d 146.3, 134.0, 127.8,
127.4, 127.0, 118.0, 102.0, 87.3, 82.4, 60.3, 57.7, 45.2,
27.25, 27.22, 26.5; HRMS (ESI) calc. for C29H34O3Si
[M + Na]+ 481.2175, found 481.2186.
Acknowledgments
We thank WARF, NSF (CHE-0401783), and NIH
(R01 CA106673-01) for generous financial support of
this work, as well as NSF and NIH for support of
NMR and mass spectrometry instrumentation. J.B.G.
thanks NSF for a predoctoral fellowship.
References
[1] For reviews on olefin metathesis, see: (a) R.H. Grubbs, S. Chang,
Tetrahedron 54 (1998) 4413–4450;
(b) A. Furstner, Angew. Chem., Int. Ed. 39 (2000) 3012–3043;
¨
1
26 (85%, colorless oil). H NMR (CDCl3, 300 MHz)
(c) S.J. Connon, S. Blechert, Angew. Chem., Int. Ed. 42 (2003)
1900–1923;
d 5.84 (1H, ddt, J = 16.9, 10.2, 6.6 Hz), 5.03 (1H, ddt,
J = 16.9, 2.0, 1.7 Hz), 4.94 (1H, ddt, J = 10.2, 2.0,
1.1 Hz), 4.13 (4H, s), 3.39 (6H, s), 2.20–2.09 (2H, m),
1.87–1.72 (2H, m), 1.70–1.45 (8H, m), 1.38 (6H, s),
1.19–1.04 (1H, m); 13C NMR (CDCl3, 75.4 MHz) d
139.5, 114.1, 101.6, 88.6, 76.3, 60.5, 57.7, 43.4, 29.4,
28.8, 27.2, 26.5; HRMS (ESI) calc. for C20H32O3Si
[M + Na]+ 371.2018, found 371.2022.
(d) R.R. Schrock, A.H. Hoveyda, Angew. Chem., Int. Ed. 42
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