Synthesis of azasilacyclopentenes and silanols: Via Huisgen cycloaddition-initiated C-H bond insertion cascades

Article abstract of DOI:10.1039/c7sc03130k

An unusual transition metal-free cascade reaction of alkynyl carbonazidates was discovered to form azasilacyclopentenes. Mild thermolysis afforded the products via a series of cyclizations, rearrangements, and an α-silyl C-H bond insertion (rather than the more common Wolff rearrangement, 1,2-shift, or β-silyl C-H insertion) to form silacyclopropanes. A mechanistic proposal for the sequence was informed by control experiments and the characterization of reaction intermediates. The substrate scope and post-cascade transformations were also explored.

Full text of DOI:10.1039/c7sc03130k

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Synthesis of azasilacyclopentenes and silanols via
Huisgen cycloaddition-initiated C–H bond
insertion cascades†
Cite this: DOI: 10.1039/c7sc03130k
Jiun-Le Shih, ‡ Santa Jansone-Popova, ‡ Christopher Huynh
and Jeremy A. May
*
An unusual transition metal-free cascade reaction of alkynyl carbonazidates was discovered to form
azasilacyclopentenes. Mild thermolysis afforded the products via a series of cyclizations, rearrangements,
and an a-silyl C–H bond insertion (rather than the more common Wolff rearrangement, 1,2-shift, or b-
silyl C–H insertion) to form silacyclopropanes. A mechanistic proposal for the sequence was informed by
control experiments and the characterization of reaction intermediates. The substrate scope and post-
cascade transformations were also explored.
Received 17th July 2017
Accepted 21st August 2017
DOI: 10.1039/c7sc03130k
rsc.li/chemical-science
14,15
cascade reactions is still rare.
An investigation to generate
Introduction
intermediate 12 and use that highly reactive species and its high
1
Carbenes have been employed in cyclopropanations, B u¨ chner potential for complex and interesting transformations has
2
3
4
ring expansion, ylide formation, X–H bond insertion, and unveiled surprising and novel reactivity. Starting with carbo-
5
especially C–H bond insertions, which have provided a power- nazidates like 5, carbene 12 was accessed via a metal-free
ful tool for synthetic chemistry. Cascade reactions using highly Huisgen cyclization and dediazatization. From this reactive
6
reactive carbenes allow the efficient synthesis of complex intermediate, C–H bond insertion, bond fragmentations, and
7
molecules. Our research group recently reported the formation pseudopericyclic rearrangements have been identied, and
of bridged polycycles via carbene/alkyne cascade reactions mechanistic intermediates isolated. Reactivity patterns suggest
8
terminated in C–H bond insertion (Scheme 1). We were also the intermediacy of imino-conjugated triplet carbenes.
interested in utilizing this cascade reaction to synthesize
nitrogen-containing polycyclic compounds like 7. Instead,
a complex sequence with an unusual a-silyl C–H bond insertion Results and discussion
formed azasilacyclopentene 8 from carbonazidate 5.
9
The initial goal was to establish a nitrene/alkyne cascade reac-
tion to synthesize nitrogen-containing polycycles like 7.
Nitrenes have been used for the aziridination of olens, the
10
C–H amination of alkanes, and the synthesis of bioactive
11
molecules. Pioneering work from Blakey showed nitrene/
alkyne cascades proceeded through an endo-cyclization with
the alkyne to the postulated vinyl cation/metalloenamine 10a,
which potentially could rearrange to the a-iminometallo-
12
carbene 10b or react directly (Scheme 2). Importantly, metal-
locarbene 12 could not be accessed via that approach, and a-
diazoimines are difficult to synthesize. Carbene 12 could be
directly produced from a transition metal catalyzed triazole ring
13
opening. The incorporation of C–H bond insertions in these
Scheme 1 Carbene- and nitrene-based cascade reactions.
Department of Chemistry, University of Houston, Fleming Building Room 112, 3585
Cullen Blvd, Houston, TX 77204-5003, USA. E-mail: jmay@uh.edu
†
Electronic supplementary information (ESI) available: Full experimental details
and compound characterizations, including X-ray diffraction data, are provided.
CCDC 1477662–1477666 and 1477728. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c7sc03130k
‡
Current affiliation: Chemical Science Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37831-6119, UAS.
Scheme 2 Nitrene and triazole carbene formation.
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2 2 2
Treating carbamate 13 with Rh (esp) and PhI(OAc) resulted in
the formation of multiple products: the predicted bridged
16
tricycle 14, propellane 15, tetrahydropyranone 16, and silyl
16
vinyl nitrile 17 (Scheme 3). A control experiment revealed that
acetic acid, produced during the carbamate oxidation, slowly
promoted the rearrangement of 14 to propellane 15 in situ. This
exploratory reaction demonstrated a proof-of-principle for
a nitrene/alkyne cascade that results in C–H insertion to
generate a C–C bond; however, controlling product selectivity
was problematic. Consequently, other nitrene precursors were
sought to achieve a more selective outcome.
Scheme 5 Avoiding a-silyl C–H bond insertion.
was heated again to form 26–28, indicating that it is a viable
reaction intermediate and that the vinyl nitrile and ketone
products are derived from it. Enyne 24, however, was not
observed, demonstrating that it likely formed by elimination
from 23.
17
Carbonazidate 18 allowed an improved yield (31%) of
tricycle 14 without a catalyst (Scheme 4A). Surprisingly, silanol
19 was also isolated, which was thought to be the result of silica
These results have been incorporated in a proposed mech-
anism for the cascade reaction (Scheme 6). Identication of a-
diazo imine 25 suggests that a Huisgen cycloaddition gener-
gel-mediated hydrolysis of a labile product during purication
since it was not observed in the NMR of the crude reaction
material. Dimethyl carbonazidate 20a was prepared to mini-
mize overlapping NMR signals in order to identify the imme-
22
1
3,23
ated triazole 30, followed by ring opening to 31.
carbene 32 (for R t-BuSi) could be directly produced via
Si ¼ Ph
dinitrogen extrusion. Silanol 28 would be produced from 32 by
The a-imino
1
13
diate product of the reaction. Aer H and C NMR analysis, its
3
2
18
structure was determined to be azasilacyclopentene 21a
Scheme 4B). The NMR signals of 21a (vinyl proton: 4.36 ppm;
enamine carbons: 108.3 and 150.2 ppm) correlated well to those
1
4b
1
,2-phenyl migration (see 33 to 34, Path A) and hydration.
(
When R Si ¼ i-Pr Si, loss of N would form carbene 35, which
3
3
2
1
9
15
could proceed through either a transannular ring insertion into
of oxasilacyclopentene 22. In addition, N NMR analysis of
a
1
5
the ether-activated methylene (H , see Path B), or insertion into
N-enriched azasilacyclopentene 21b (110.5 ppm, no signals
from 305 to 375 ppm) suggested that the nitrogen was involved
in an enamine motif, corroborating the H and C NMR data.
Hydrolysis of 21a to the silanol proved to be facile with a small
amount of water.
b
20
the isopropyl C–H bond (H , see Path C). The former leads to
1
13
the bridged tricycles 36 and 14, which can rearrange in the
presence of acid to propellane 15 through an elimination/
8
addition sequence. The latter pathway would form silacyclo-
propane 38. Silacyclopropanes are usually formed through
The products 19 and 21 appeared to be formed via C–H bond
insertion at one of the isopropyl groups. In order to bias product
formation toward the bridged tricycle by removing all a-silyl
C–H bonds, tert-butyldiphenyl silylacetylene carbonazidate 23
was prepared (Scheme 5). The reaction produced ve
compounds, 24–28. The a-diazo oxazolone 25 and the silanol 28
were crystallized and analyzed by X-ray diffraction to conrm
24
silylene transfer, but there are a few cases of formation via
intramolecular C–H bond insertion, though these intermedi-
25
ates were unstable and were not isolated. Likewise, interme-
26
diate 38 was not stable, but rearranged to azasilacyclopentene
9, which could be observed and characterized spectroscopi-
3
cally (vide supra). Exposure to water rapidly hydrolyzed the weak
N–Si bond to give silanol 19.
21
the structures shown. In a control experiment, diazoimine 25
The isolation of ketone 26 and vinyl nitrile 27 suggested
either a 6-endo reaction of a nitrene derived from 29 to form
zwitterion 41, or else 41 would be derived from carbenes 32 or
10–15
35 (Path D).
Since 26 and 27 were formed from heating
diazoimine 25 (Scheme 5), it is likely that the latter route was
operative. Therefore, a carbene electrocyclization of 32 (likely
pseudopericyclic) would allow the highly strained azirine 40 to
26
Scheme 3 Cascade reaction of a carbamate.
be transiently formed. Either of the C–N bonds could frag-
ment, with an internal fragmentation forming the ring
expanded product 41. A retro-hetero Diels–Alder reaction allows
for decarboxylation (Path D(a)), and 1,2-silyl migration in zwit-
terion 42 would give the vinyl nitriles 17 or 27. Alternatively, C–C
bond fragmentation would provide ketones 16 or 26 (Path D(b)).
These pathways account for all the observed products and
intermediates.
Only a few examples of intramolecular C–H bond insertion to
form silacyclopropanes have been reported, and in those cases
these intermediates were also eeting and inferred from
25
downstream products rather than observed directly. For
simple silicon-substituted carbenes like 44, the triplet state is
27,28
signicantly stabilized relative to the singlet state.
1,2-
Scheme 4 Cascade reaction of carbonazidates.
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Scheme 6 Proposed mechanism.
Rearrangements to silenes are the most common outcomes for
these compounds, though cyclopropanations and C–H inser- Scheme 7, the carbenes 32 and 35 are triplet carbenes and
It appears that unlike the other acyl silyl carbenes in
29
35
tions have also been observed in suitable substrates (see 47 and therefore exhibit diradical character. The C–C bond formation
1). Intramolecular cyclopropanations appear to be possible would then proceed via hydrogen atom abstraction from the
with silyl carbenes of either a triplet or singlet nature if pendant carbon attached to the silicon in 71 to generate an a-silyl
5
36
olens are present (47 and 49, respectively). On the other hand, radical 72 (Scheme 8), which would be stabilized by back-
the presence of a carbonyl induces more singlet-like reac- bonding from silicon d-orbitals. Then, the silacyclopropane 73
37
28,30
tivity,
reactions. Intramolecular C–H bond insertions may also occur
for these cases, but the preference for insertion is typically at the nazidates was examined (Table 2). Varying the alkyl groups at R
including Wolff rearrangements and X–H insertion would be formed from radical combination.
31
Next, a series of acyclic, cyclic, and heterocyclic carbo-
1
32
b- or g-position relative to silicon (see 60). A single report
exists for imine-adjacent silyl carbene formation, which is also
derived from a triazole precursor (61). In this case, the sole
product was a 1,2-silyl shi to give ketene imine 63. Based on
these precedents, products 36 (intramolecular C–H bond
insertion) or 66 (1,2-shi: see 34, Scheme 6) would have been
expected to dominate this reaction, but instead outcomes
derived from silacyclopropane formation (see Schemes 3 and
34
Table 1 Variation of the silyl alkyl group
4A) dominated when C–H bond insertion was possible and the
formation of 66 and derivatives thereof were not observed.
The unanticipated formation of the azasilacyclopentenes
from the reaction cascade prompted further investigation. The
acyclic carbonazidate 20a was selected to bias the reaction for
azasilacyclopentene formation by eliminating competitive C–H
insertion in an appended ring (Table 1, entries 1–5). A lower
yield of 21a resulted from the use of a halogenated solvent,
dichloroethane, but the use of hexanes resulted in fewer
byproducts and a slightly higher yield (entry 3). However, the
14
use of Rh
and 5).
2 2
(esp) or Cp*RuCl(cod) was deleterious (entries 4
We were curious if C–H bond insertion would take place at
different positions on larger silyl alkanes, or the a-position was
preferred. However, all of the alkyl groups only generated the
azasilacyclopentenes corresponding to insertion into the C–H
19b
bond alpha to silicon (entries 6–9). These results contradict
literature examples of singlet or metal carbene insertion into
alkylsilanes, which show insertion primarily at the methylene or
33
methine beta to silicon, presumably due to the “b-silicon
effect”. In the examples in Table 1, none of the available b, g, or
d-C–H bonds reacted. High regioselectivity was shown even in
the isobutyl substrate 69f, where the electron-rich b-methine
was untouched.
a
1
Yield based on H NMR peak integration relative to methyl-4-nitro-
b
c
benzoate. Isolated yields (average of 2 trials). 1 mol% Rh
2
(esp)
2
.
d
2 mol% Cp*RuCl(cod).
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34
Table 2 Propargylic variation in the substrates
a
1
Yield based on H NMR peak integration relative to methyl-4-nitro-
b
c
ꢀ
benzoate. Isolated yields (average of 2 trials). Hexanes, 90 C.
d
ꢀ
e
ꢀ
Isopropyl acetate, 100 C. Hexanes, 100 C.
Scheme 9 Reactions of azasilacyclopentene.
Scheme 7 Typical outcomes for silyl-substituted carbenes.
to greater conformational exibility to react with the activated
C–H bonds compared to the 5-membered rings (entries 11 and
12). For the 3-methyl cyclohexane 74k, the reaction produced
a 1 : 1 ratio of azasilacyclopentene 75k and bridged tricycle 77.
For the tosyl piperidine 74l, the major isolated product was the
40
silanol 76l, but some bridged bicycle was observed.
Preliminary tests were made for transformations of the
azasilacyclopentene products. Methyl silyl ether 79 is obtained
in good yield by ring opening of 21a in methanol. Treatment of
1
6
2
1a with CuBr
also produced 80 with CuBr
hydrolyzed by adventitious water before spirocycle formation
Scheme 9).
2
generated spiro-hemiaminal 80. Silanol 69a
Scheme 8 Proposed triplet carbene/diradical pathway.
2
, suggesting that 21a is rst
(
2
and R of 74 does not signicantly affect the formation of the
azasilacyclopentene. Although an allyl group could potentially
undergo cyclopropanation with an intermediate silyl carbe-
Conclusions
32a,38
ne,
only the azasilacyclopentene 75e is seen (entry 5). Cyclic
substrates generally provided spiro-oxazolones (entry 6–10). A novel approach for the synthesis of azasilacyclopentenes via
Bridged polycycles were not observed, even for pyrrolidine 74i a Huisgen cycloaddition-initiated cascade reaction terminating
and thiolane 74j, where the heteroatoms could activate a-C–H in an a-silyl C–H bond insertion has been discovered. Control
39
bonds for carbene insertion. Two substrates with six- experiments suggest a likely mechanism with triplet silylcar-
membered rings provided a different result, presumably due bene intermediates undergoing C–H bond insertion at the
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carbon attached to silicon to form transient silacyclopropanes.
Further reaction of the azasilacyclopentene showed the forma-
tion of interesting products. Ongoing efforts are seeking selec-
tive synthesis of bridged polycycles and vinyl nitriles from the
same precursors.
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Conflicts of interest
There are no conicts of interest to declare.
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The authors thank the Welch Foundation (grant E-1744) and the
NSF (grant CHE-1352439) for funding. J.-L. S. is grateful to CBIP
(UH) and the Studying Abroad Scholarship of Taiwan for
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9
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