Cyano-Schmittel Cyclization through Base-Induced Propargyl-Allenyl Isomerization: Highly Modular Synthesis of Pyridine-Fused Aromatic Derivatives

Article abstract of DOI:10.1002/chem.201503374

The cyano-Schmittel cyclization of i situ-generated cyano-allenes has been carried out. The DFT calculation results suggest that the diradical pathway plays a major role in this cyclization. The reactions can be conveniently performed in a one-pot manner through cascade Sonogashira coupling of terminal cyano-ynes with organic halides, followed by base-promoted propargyl-allenyl isomerization/cyclization, leading to an efficient access to pyridine-fused polycyclic architectures. In particular, a large variety of aryl or heteroaryl rings such as furans, thiophenes and pyridines can be incorporated into the follow-up cyano-Diels-Alder reactions, highlighting the great synthetic utility of this chemistry.

Full text of DOI:10.1002/chem.201503374

DOI: 10.1002/chem.201503374
Full Paper
&
Cyclization
Cyano-Schmittel Cyclization through Base-Induced Propargyl-
Allenyl Isomerization: Highly Modular Synthesis of Pyridine-Fused
Aromatic Derivatives
[
a]
Xu You, Xin Xie, Haoyi Chen, Yuxue Li,* and Yuanhong Liu*
Abstract: The cyano-Schmittel cyclization of in situ-generat-
ed cyano-allenes has been carried out. The DFT calculation
results suggest that the diradical pathway plays a major role
in this cyclization. The reactions can be conveniently per-
formed in a one-pot manner through cascade Sonogashira
coupling of terminal cyano-ynes with organic halides, fol-
lowed by base-promoted propargyl-allenyl isomerization/
cyclization, leading to an efficient access to pyridine-fused
polycyclic architectures. In particular, a large variety of aryl
or heteroaryl rings such as furans, thiophenes and pyridines
can be incorporated into the follow-up cyano-Diels–Alder re-
actions, highlighting the great synthetic utility of this
chemistry.
Introduction
[1]
Thermal cycloaromatizations of enediynes and enyne-al-
[
2,3]
lenes
have attracted much attention over the last two de-
cades due to their fundamentally intriguing reaction mecha-
nism and potential medical applications, such as in the design
[4]
of antitumor antibiotic agents. Theoretical and experimental
studies revealed that these reactions proceed mainly via diradi-
[
5]
cal intermediates. Two different processes, namely, Myers–
2
7 [2]
2
6 [3]
Saito (C ÀC ) and Schmittel (C ÀC ) cyclizations, are usually
involved in enyne–allene systems, which depend on the nature
of the substituents on alkyne terminus (Scheme 1). The syn-
thetic potential of these thermal enyne–allene cyclizations has
been extended to their hetero-analogues. Most of the studies
focused on heteroatom substitution on the allene moiety, such
[
6a,b]
[6c,d]
as enyne–ketenes,
enyne–carbodiimides,
enyne–keten-
Scheme 1. Thermal cyclization of enyne–allenes and cyano-allenes.
[
6e]
[6f]
imines and enyne–isocyanates, whereas little attention has
been paid to modification of the alkyne moiety. It is expected
that if the alkyne moiety in enyne–allenes is replaced by a ni-
trile functionality, cycloaromatization of the resulting cyano-al-
lenes might also be a feasible transformation; however, such
reactions are quite rare. The development of such transforma-
tions would be of great interest for the synthesis of polyfused
heterocycles through CÀC and CÀN bond formation. Earlier re-
ports indicated that thermal reactions of (Z)-2,4,5-hexatrieneni-
triles failed to provide the desired pyridine products even at
[
7]
high reaction temperatures of 150–2608C. A cyano Myers-
type cyclization of the in situ-formed cyano-allenyl sulfone to
isoquinoline has been reported; however, only one example
[
8]
[9a]
was described with <20% yield. Wang et al. reported that
cycloaddition of cyano and arylallenes formed in situ could be
used for the synthesis of the parent ring cores of camptothe-
cin; however, the reaction was restricted to 2(1H)-pyridone-
[
9a,b]
bridged system.
Recently, Danheiser et al. found that ther-
molysis of cyanodiynes at 140–2008C could afford substituted
pyridines, which involves an intramolecular Diels–Alder reac-
tion between in situ-generated vinylallene and nonconjugated
[
a] X. You, X. Xie, H. Chen, Y. Li, Y. Liu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
[
9c]
cyano group. In this paper, we report a successful example
of Schmittel-type cyclization of cyano-involved aza-enyne-al-
lenes. Our work disclosed that thermal cycloaddition of cyano-
allenes could be a common and general reaction pattern,
which may become an ideal and important protocol for the
construction of pyridine rings. Remarkably, it is also interesting
3
45 Lingling Lu, Shanghai 200032 (P. R. China)
Fax: (+86)021-64166128
E-mail: yhliu@mail.sioc.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503374.
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to find that not only aryl groups but also heteroaryl groups
such as furans, thiophenes, and pyridines can be incorporated
into the follow-up cyano-Diels–Alder reactions successfully to
furnish the fused products, which greatly expands the scope
to 2a in 93% yield (entry 7). The reaction could also be per-
formed in 1,2-dichloroethane (DCE) or THF, providing 2a in
95% and 74% yields, respectively (entries 9 and 10). Interest-
ingly, the reaction of 1a with 10 mol% DBU at room tempera-
ture for 40 min afforded the cyano-allene 3a, which was gener-
ated through base-catalyzed propargyl-allenyl isomeriza-
[10]
and the synthetic utility of this chemistry. It is noted that
there is no report for cyano-Diels–Alder reactions in which
a heteroaryl ring acts as a part of the diene.
[
9a,b,12]
tion
in 98% yield (entry 11). Compound 3a was relatively
stable at room temperature, and could be isolated in a pure
form. KOtBu could also induce the isomerization of 1a to 3a
efficiently at À788C (entry 12). These experiments indicated
that the formation of cyano-allene 3a was the initial step in
this cascade reaction, and possibly 3a was an intermediate
leading to 2a through formal [4+2] cycloaddition. Without the
base, the reaction did not take place even after heating the
mixture at 1808C for 6 h, implying that the reaction proceeded
exclusively through the allenic intermediate. It is known that
the cyano group rarely participates in the electrocyclization re-
actions as an enophile or dienophile such as in Diels–Alder re-
Results and Discussion
In the course of our research on transition-metal-catalyzed
transformation of functionalized alkynes, we occasionally
found that o-(cyano)phenyl propargyl ether 1a cyclized
smoothly in the presence of a base only without the need of
metal catalysts. The product was identified as indeno[1,2-
b]quinoline 2a (Table 1). The results indicated that a cycloaddi-
[
9,13]
Table 1. Formation of indenoquinoline 2a and cyano-allene 3a catalyzed
actions.
There are few reports of the [4+2] cycloaddition of
by base.
nitriles with 1,3-dienes, which usually require harsh condi-
[
13a,b]
tions
or are restricted to activated nitriles bearing electron-
[
13c,d]
withdrawing groups.
Thus, this new finding may serve as
an attractive synthetic strategy for the synthesis of heterocy-
cles from unactivated nitriles.
During the study of the generality and scope of the reaction,
we became interested in developing a more direct route to in-
denoquinolines from readily available precursors of terminal
alkyne 4 and organic halides through a cascade Sonogashira
coupling/cycloaddition process shown in Scheme 2, which
[
a]
[a]
Entry
Base
Solvent
Temp
8C]
Time
Yield of
Yield
[
2a [%]
of 3a [%]
1
2
3
4
5
6
7
8
9
Cs
KOtBu
CO
DABCO
2
CO
3
toluene
toluene
toluene
toluene
toluene
80
80
80
80
80
80
80
50
80
80
RT
À78
180
18 h
8 h
6 h
18 h
18 h
12 h
6 h
18 h
5 h
92
82
–
–
–
–
–
–
–
–
–
–
98
95
–
K
2
3
–(98)
25 (75)
6 (93)
98
Et
–
3
N
Et
3
N
DBU
DBU
DBU
DBU
DBU
KOtBu
–
toluene
toluene
DCE
THF
toluene
THF
93
31
95
74
–
–
1
0
9 h
1
1
40 min
10 min
6 h
1
1
2
3
Scheme 2. One-pot synthesis of indenoquinolines from terminal alkyne 4
and organic halides.
toluene
–(100)
[
a] Yields are of isolated products. The yields of the recovered 1a are
shown in parentheses.
would greatly enhance the efficiency of the synthesis by avoid-
ing the isolation of internal alkyne 1.
After detailed optimization of the catalysts and solvents, we
were delighted to find that the use of 2 mol% of [Pd(PPh ) ]
tion involving phenyl-yne and cyano moiety was efficiently
achieved, although the process might be accompanied with
a temporary disruption of aromaticity. The unique reaction
3
4
and 5 mol% of CuI with Et N as the solvent gave very good re-
3
sults. The results are summarized in Table 2. The present
method could be applied successfully to a wide variety of aryl/
heteroaryl halides and alkenyl halides. Aryl iodides, when bear-
ing electron-withdrawing or electron-donating groups on the
aryl rings, all reacted well with 4, providing the cyclized prod-
ucts 2b–2l in good to excellent yields. The functionalities such
as p-Br, p-Cl, p-CF , p-CO Et, p-CN, p-Me, p-MeO, o-F, and o-CF
[11]
mode and potential bioactivity of indenoquinolines prompt-
ed us to investigate this reaction in detail under various condi-
tions. The results are summarized in Table 1. For example, heat-
ing a solution of 1a in toluene in the presence of 10 mol%
Cs CO or KOtBu afforded 2a in 92% and 82% yields, respec-
2
3
tively (Table 1, entries 1 and 2). However, the use of K CO or
2
3
3
2
3
DABCO as the base either could not afford the desired prod-
were well tolerated during the reaction, leading to 2b–2h and
2j–2k in 65–94% yields. Aryl bromide bearing a vinyl group
on the aryl ring was also compatible, furnishing 2i in 47%
yield, while the vinyl group remained intact. The sterically de-
manding substrates, such as 1-iodo-3,5-dimethylbenzene, also
worked well to furnish 2l in 81% yield. In this case, a longer
uct, or gave 2a in low yield (entries 3 and 4). A catalytic
amount of Et N appeared to be less effective (entry 5); howev-
3
er, when Et N was used as the solvent, 2a could be formed
3
quantitively (entry 6). 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)
also proved to be an efficient catalyst for this reaction, leading
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[
14]
materials. Our method could be readily extended to furanyl
or thienyl halides, leading to furan or thiophene-fused prod-
ucts 2o–2s in 55–81% yields. When 2-iodothiophene or 3-bro-
mothiophene were employed, the reaction proceeded smooth-
ly to give 2p and 2q with the anti or syn-fused thiophene
rings in 81% and 55% yields, respectively. The results further
demonstrated the diversity and flexibility of this method, be-
cause different types of polycyclic heterocycles could be easily
constructed just by changing the halide position in heteroaryl
halide substrates. Interestingly, the use of 2,5-diiodothiophene
gave 2s, featuring seven-fused carbo- and heterocyclic rings,
in 57% yield, which might find utilities in materials science. In-
terestingly, 2-pyridyl iodide also turned out to be an effective
substrate for this reaction to give 2t in 65% yield, in which the
pyridyl group was successfully embedded in a naphthyridine
framework. It should be noted that the pyridine ring rarely un-
dergoes the [4+2] cycloaddition reaction due to its low reacti-
Table 2. One-pot synthesis of polycycles 2 from terminal alkyne 4 and or-
ganic halides.
[a]
[a]
Organic
halide
Product
Organic
halide
Product
[
10a,12d]
vity.
The reaction of 3-iodopyridine with 4 afforded a mix-
ture of two regioisomers 2u and 2u’ in 59% and 14% yields,
respectively, indicating that both of the C-2 and C-4 position
of 3-iodopyridine are reactive during the reaction. The alkenyl
bromide (E)-bromostyrene was well-accommodated, leading to
5
H-indeno[1,2-b]pyridine 2v in 65% yield, along with 30% of
desilylated product. The structures of indenoquinolines were
unambiguously confirmed by X-ray crystallography of 2a, 2b,
[
15]
2
n, and 2t.
We next explored the possibility of the cascade reactions be-
tween linear-type cyano-ynes such as 4-oxa-1-cyano-6-yne 5
with organic halides (Scheme 3). However, under similar condi-
[
a] Yields are of isolated products. [b] The reaction was carried out in
Et N/toluene using 2.2 equiv of 4, 5 mol% [Pd(PPh ] and 5 mol% CuI.
c] Desilylated product was also obtained in 30% yield.
3
3 4
)
[
reaction time of 40 h was required. However, the use of 2-
iodo-1,3,5-trimethylbenzene only afforded a complex mixture
at 808C. 1-Bromo- or 1-iodo-naphthalene were also suitable for
this reaction, and in the latter case, the desired product 2m
was obtained in higher yield of 82%, possibly due to the
higher reactivity of iodide in the Sonogashira reaction. In the
case of 2-bromonaphthalene, two regioisomers might be
formed by cyclization through its C-1 or C-3 position. However,
only one regioisomer 2n was obtained in 66% yield, indicating
that CÀN bond formation occurred selectively at the C-1 posi-
tion of the naphthyl substituent.
Scheme 3. One-pot synthesis of polycycles 6 from 4-oxa-1-cyano-6-yne 5
and organic halides.Yields are of isolated products. [a] (E)-b-Bromostyrene
was used. [b] The second step was carried out at 808C for 5 h.
tions to the one-pot synthesis of 2, only the product derived
from Sonogashira coupling was observed. After many efforts,
we found that if the reaction was carried out in a stepwise
manner by first achieving Pd-catalyzed Sonogashira reaction in
Et N/CH CN, followed by addition of 2 equiv of DBU, the de-
3
3
Heteroacenes bearing furan or thiophene rings constitute
one of the most common classes of small-molecular electronic
sired quinoline product 6 could be formed in good to excel-
lent yields. The reaction also showed excellent scope with re-
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spect to aryl/heteroaryl halides, as well as alkenyl bromides.
Varying the substituent at the propargylic position with an
alkyl group led to the formation of only the coupled product
proceed through deprotonation and formation of a contact
[
12c]
ion pair followed by reprotonation.
The use of [D ]1a bear-
5
ing five deuterium atoms on the aromatic ring led to [D ]2a in
5
7
, and no desired cyclization product was observed, possibly
99% yield, in which one of the deuterium atoms moved to the
C-1 position on the five-membered ring (Scheme 4, [Eq. (4)]). In
due to the fact that the allenic intermediate could not be
formed efficiently in this case. The results indicate that the
presence of an aryl substituent at the propargylic position is
crucial for successful transformation.
1
this case, the deuterium incorporation of D decreased to 78%,
possibly due to the benzylic D/H exchange between the cy-
clized product and water in the reaction mixture. Indeed, addi-
To understand the reaction mechanism, various control ex-
periments were carried out. We first investigated the thermal
behavior of the isolated cyano-allene 3a. Heating a solution of
tion of 5.0 equiv D O and 10 mol% DBU to non-deuterated 2a
2
and stirring at 808C for 5 h resulted in an incorporation of deu-
terium at the C1-position on the five-membered ring. The ab-
sence of deuterium at C3 indicates that the subsequent 1,5-H
shift process may not involve a double 1,3-H shift.
3
a in toluene at 808C for 6 h afforded the desired indenoqui-
noline 2a with a lower yield of 83% (compared to the results
shown in Table 1, entry 7). The lower yield for 2a may result
from the instability of 3a. In the presence of 10 mol% DBU,
a similar reaction rate and yield to 2a were observed
To disclose the reaction mechanism for the transformation
[
16]
of 1 to 2 catalyzed by DBU, density functional theory (DFT)
[
17]
studies have been performed with GAUSSIAN09 program
[
18]
(
Scheme 4, [Eq. (1)]), indicating that DBU did not play a role in
using the (U)M06 and the 6-31+G** basis set. Harmonic vi-
bration frequency calculations were carried out under 353.15 K
and the optimized structures are all shown to be either
minima (with no imaginary frequency) or transition states (with
one imaginary frequency). Substrate 1a was used in the calcu-
lation models. For a reaction that the number of molecules of
the reactant and the product are not equal, corrections were
made to the calculated free energies based on “the theory of
[
19]
free volume”, that is, under 353.15 K, for two-to-one (or one-
À1
to-two) reactions, a correction of À3.0 (or 3.0) kcalmol was
made.
As shown in Scheme 5A, the isomerization of substrate 1a
to allene 3a occurs through 1,3-proton transfer assisted by the
À1
DBU catalyst. Over transition state TS-DP1 (14.5 kcalmol ), the
propargyl proton of 1a is abstracted by the nitrogen atom of
the DBU catalyst, leading to the ion pair INT1. In the anion
part of INT1, the negative charges are distributed on the
whole conjugated system, as indicated by the increased nega-
[
20]
tive NPA charges. Interestingly, the alkynyl group in INT1 re-
mains linear, and the bond angle C C C is 176.4 degree, which
a
b c
is close to that of 1a. Subsequently, over a small barrier of
À1
0
.2 kcalmol the protonation of the alkynyl carbon (TS-PT1,
À1
1
1.4 kcalmol ) yields the allene isomer 3a. The isomerization
of 1a to allene 3a is easy, the overall reaction barrier is
4.5 kcal, and the allene isomer is more thermodynamically
Scheme 4. Control experiments.
1
[
21]
À1
stable by 6.7 kcalmol . These results are in good accord-
ance with the experimental observations that the isomerization
can be fulfilled under room temperature within 40 min, and
the allene isomer can be isolated in high yield (98%; see
Table 1, entry 11).
accelerating the cycloaddition reaction. We next performed
isotope-labeling experiments. Treatment of [D ]1a with a deu-
terium labeled at the propargylic position with a catalytic
1
amount of DBU at 808C provided [D ]2a in 94% yield without
1
a significant loss of deuterium, and in which deuterium was lo-
The cyclization of the allene isomer 3a may occur through
a concerted closed-shell pathway (Path-a) or the stepwise
open shell singlet-diradical (SD) pathway (Path-b) yielding the
cated at the C-3 position (Scheme 4, [Eq. (2)]). Reaction of 1a
in a mixed solvent of toluene/CD OD afforded the deuterated
3
À1
product [D ]2a in 67% yield (Scheme 4, [Eq. (3)]). The incorpo-
cyclized intermediate INT3. Path-b is only 0.8 kcalmol lower
2
ration of a deuterium at C-1 might be caused by H/D exchange
of the benzylic proton in cyclized product under basic condi-
tions (see below). The slight deuterium label at C3 indicated
that H/D exchange of the propargylic proton in the substrate
was involved only to a small extent during the isomerization
process. The above results indicate that a fast 1,3-H shift in ini-
tial propargyl–allenyl isomerization process occurs, which may
than Path-a. This small energy difference is consistent with the
[
5i]
former DFT study on enyne–allenes. The diradical pathway
plays a major role. However, these two mechanisms are con-
[
5i]
comitant, especially under higher reaction temperatures. The
cyclization step is the rate-determining step of the whole reac-
tion. The spin density distribution shows that the intermediate
INT2-SD is a typical singlet diradical (Scheme 5B).
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À1
Scheme 5. The reaction pathways of substrate 1a. The selected bond lengths are in angstroms, and the relative free energies (DG, 353.15 K) are in kcalmol
.
The numbers with minus sign in (A) are the NPA charges on the nearby bonds (sum of the charges on two bonded atoms), and the numbers with minus and
plus signs in INT2-SD are spin densities on the nearby carbon or nitrogen atoms. Calculated at the (U)M06/6-31+G**/SDD level.
The barriers of the deprotonation of INT3 to INT4, and the
The barriers of deprotonation of 2a to INT4, and reprotona-
tion of INT4 to 2a are 20.3 and 3.4 kcalmol , respectively, in-
À1
subsequent protonation of INT4 to 2a, are only 3.8 and
À1
3
.4 kcalmol , respectively. These small barriers show that even
dicating that this process is very fast and reversible under the
experimental conditions. This result is consistent with the ob-
served H/D exchange in the isotope-labeling experiment of
non-deuterated 2a. In addition, the [4+2] cycloaddition of the
a very weak base/acid can act as an efficient proton shuttle in
the last 1,5-proton-transfer process. This may account for the
fact that allene can react without the DBU catalyst (Scheme 4,
[
22]
[
Eq. (1)]).
allene anion species was ruled out safely.
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To find out the origin of the high reactivity of 1a, the reac-
tion pathways of a model substrate 1a’ without the OTBS
group (2-(3-phenylpropa-1,2-dienyl)benzonitrile) were calculat-
Typical procedure for palladium-catalyzed tandem coupling/
cyclization reaction of terminal alkyne 4 with organic hal-
ides
[22]
ed.
The results show that: 1) the OTBS group has small
o-(Cyano)phenyl propargyl ether 4 (91.4 mL, 0.33 mmol), iodoben-
effect on the first deprotonation step; 2) however, without the
OTBS group the barrier of the cyclization step increases in
zene (33.6 mL, 0.3 mmol), Et N (2.0 mL), [Pd(PPh3)4] (6.9 mg,
3
0
.006 mmol), and CuI (2.9 mg, 0.015 mmol) were added to a seala-
À1
large scale from 23.8 to 30.0 kcalmol . Therefore, the OTBS
ble Schlenk tube under argon. Then the tube was sealed and the
reaction mixture was gradually warmed up to 808C and stirred at
the same temperature until the reaction was complete, as moni-
tored by TLC. After cooling down to room temperature, the reac-
group has a large effect on the reactivity of the substrate. Fur-
ther analysis shows that the p-electronic donation of the OTBS
group can stabilize the highly polarized transition structures,
[23]
tion mixture was quenched by saturated NH Cl solution and ex-
4
consequently decreasing the cyclization barriers.
tracted with ethyl acetate. The combined organic extracts were
washed with water and brine, and dried over Na SO . The solvent
2
4
was evaporated under the reduced pressure and the residue was
purified by column chromatography on silica gel (eluent: petrole-
um ether/ethyl acetate=30/1) to afford the product 2a in 93%
yield (97 mg).
Conclusion
In summary, we have disclosed a successful example of cyano-
Schmittel cyclization of cyano-allenes. The DFT calculation re-
sults are consistent with the experiments, and suggest that the
diradical pathway plays a major role in this cyclization. The re-
actions can be conveniently performed in a one-pot manner
through cascade Sonogashira coupling of terminal cyano-ynes
with organic halides, followed by base-promoted propargyl-al-
lenyl isomerization/cyclization, leading to an efficient access to
pyridine-fused polycyclic architectures. In particular, a large va-
riety of aryl or heteroaryl rings such as furans, thiophenes, and
pyridines can be incorporated into the follow-up cyano-Diels–
Alder reactions, highlighting the great synthetic utility of this
chemistry. Further investigations into the reaction mechanism
and its synthetic applications are currently in progress in our
laboratory.
Acknowledgements
We thank the National Natural Science Foundation of China
(
Grant Nos. 21125210, 21372249, 21421091), Chinese Academy
of Science and the Major State Basic Research Development
Program (Grant No. 2011CB808700) for financial support.
Keywords: alkynes
·
cyano-Schmittel
cyclization
·
indenoquinoline · nitriles · propargyl-allenyl isomerization
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Typical procedure for DBU-promoted isomerization/
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In a nitrogen-filled glove box, to an oven-dried screw-cap vial
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(
(
volume: 4.0 mL) were added 1a (104.3 mg, 0.3 mmol), toluene
1.0 mL), and DBU (4.5 mL, 0.03 mmol). Then the vial was taken out-
side the dry box and stirred at 808C in an oil-bath until the reac-
tion was complete, as monitored by TLC. After the mixture was
cooled down to room temperature, the solvent was evaporated
under the reduced pressure and the residue was purified by
column chromatography on silica gel to afford 2a in 93% yield
1
(
(
97 mg) as a white solid. H NMR (400 MHz, Me Si, CDCl ): d=0.18
4
3
s, 3H), 0.20 (s, 3H), 0.99 (s, 9H), 5.83 (s, 1H), 7.47–7.53 (m, 3H),
7
1
.62 (d, J=7.2 Hz, 1H), 7.69 (t, J=7.6 Hz, 1H), 7.81 (d, J=7.6 Hz,
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3
H), 8.10 (s, 1H), 8.16–8.19 ppm (m, 2H); C NMR (100 MHz, Me Si,
4
CDCl ): d=À3.76, À3.69, 18.07, 25.78, 73.01, 121.73, 125.26, 125.91,
3
1
1
2
27.49, 128.21, 129.10, 129.25, 129.35, 130.51, 131.55, 137.84,
39.01, 148.01, 148.78, 159.99 ppm; IR (film): n˜ =3057, 2953, 2928,
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2236; e) M. Schmittel, J. P. Steffen, M. . W. ngel, B. Engels, C. Lennartz,
À1
1104, 1067, 1004, 919, 859, 836, 770, 755, 737, 713, 688, 666 cm .
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+
HRMS (ESI): m/z calcd for C H NOSi [M+H] : 348.1778; found:
3
2
2
26
48.1792.
Chem. Eur. J. 2015, 21, 18699 – 18705
www.chemeurj.org
18704
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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[
[
Received: August 25, 2015
1
413685 (6a) contain the supplementary crystallographic data for this
Published online on November 12, 2015
Chem. Eur. J. 2015, 21, 18699 – 18705
www.chemeurj.org
18705
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim