Visible-Light-Induced Cycloaddition of α-Ketoacylsilanes with Imines: Facile Access to β-Lactams

Article abstract of DOI:10.1002/anie.202102451

We report the synthesis of β-lactams from α-ketoacylsilanes and imines, which proceeds via a formal [2+2] photochemical cycloaddition with in situ generation of siloxyketene. This mild and operationally simple reaction proceeds in an atom-economic fashion with broad substrate scope, including aldimines, ketimines, hydrazones, and fused nitrogen heterocycles, affording a variety of important β-lactams with satisfactory diastereoselectivities in most cases. This reaction also features good functional-group tolerance, facile scalability and product diversification. Experimental and computational studies suggest that α-ketoacylsilanes can serve as photochemical precursors by engaging in a 1,3 silicon shift to the distal carbonyl group.

Full text of DOI:10.1002/anie.202102451

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How to cite:
International Edition: doi.org/10.1002/anie.202102451
German Edition: doi.org/10.1002/ange.202102451
Photochemistry
Hot Paper
Visible-Light-Induced Cycloaddition of a-Ketoacylsilanes with Imines:
Facile Access to b-Lactams
Jian-Heng Ye, Peter Bellotti, Tiffany O. Paulisch, Constantin G. Daniliuc, and Frank Glorius*
Dedicated to Professor Peter Kꢀndig on the occasion of his 75th birthday
Abstract: We report the synthesis of b-lactams from a-
ketoacylsilanes and imines, which proceeds via a formal
[2+2] photochemical cycloaddition with in situ generation of
siloxyketene. This mild and operationally simple reaction
proceeds in an atom-economic fashion with broad substrate
scope, including aldimines, ketimines, hydrazones, and fused
nitrogen heterocycles, affording a variety of important b-
lactams with satisfactory diastereoselectivities in most cases.
This reaction also features good functional-group tolerance,
facile scalability and product diversification. Experimental and
computational studies suggest that a-ketoacylsilanes can serve
Scheme 1. Photochemistry of acylsilanes.
as photochemical precursors by engaging in a 1,3 silicon shift
to the distal carbonyl group.
insertion (X = C, N, O, Si, B, etc);[5] (ii) undergo addition to
electrophilic compounds, for instance cross-coupling with
organoboronic esters,[6] benzoin reaction with aldehydes,[7]
addition to alkynes[8] or electron-poor alkenes,[9] and partic-
ipation in electrocyclization processes.[10] In addition to
siloxycarbenes, other reactive intermediates generated from
acylsilanes and their applications in organic synthesis remain
to be addressed.[11]
The b-lactam (2-azetidinone) is a unique and important
scaffold in terms of its bioactivity and utility as synthetic
intermediate, and thus straightforward methods for its
formation are highly valued in organic synthesis
(Figure 1).[12] The formal [2+2] cycloaddition of ketenes
with imines, the so-called Staudinger cycloaddition, undoubt-
edly offers the most direct access to b-lactams.[12,13] The most
common methods for generation of ketenes are: 1) de-
(hydro)halogenation of acid chlorides by trialkylamines or
zinc; 2) ring opening of cyclobutenones; 3) Wolff rearrange-
ment of a-diazoketones; 4) pyrolysis of anhydrides or esters;
5) cracking of diketenes. Despite the above advancements,
the identification of new ketene precursors and their reac-
tivities towards cycloadditions with imines are still of
significant importance. With our continuing interest in acyl
silane chemistry,[5h] we turned our attention to a-ketoacylsi-
lanes, which are a rarely explored class of compounds.[14]
Besides [1,2]-silyl migration of acylsilanes to form siloxycar-
benes, we envisaged that a-ketoacylsilanes could theoretically
O
rganosilanes are highly important compounds and widely
used in organic synthesis and materials science.[1] In partic-
ular, acylsilanes, with the silicon atom directly attached to the
carbonyl group, are a fascinating class of compounds which
exhibit unique physical and chemical properties. The use of
acylsilanes in a variety of synthetic routes and improved
methodologies for their preparation have turned acylsilanes
into important reagents for organic chemistry.[2] In recent
years, much effort has been devoted towards the development
of both their preparation and novel transformations.
Interestingly, acylsilanes display unusual spectroscopic
properties owing to the inductive release of electrons from the
silicon atom to the carbonyl moiety. The relatively low energy
of the n-p* electron transition leads to an abnormally long
absorption wavelength with a relatively large extinction
coefficient. Silyl ketones can therefore undergo interesting
photochemical transformations, other than a-cleavage. Pio-
neered by Brook and co-workers, siloxycarbenes can be
generated photochemically via the 1,2-Brook rearrangement
of acyl silanes (Scheme 1a).[3] Generally speaking, the reac-
tivity patterns of siloxycarbenes correspond with nucleophilic
carbene chemistry,[3,4] which have been attributed to reso-
nance stabilization of the singlet state by donor substituents.
Such intermediates possess the ability to (i) undergo X-H
[*] Dr. J.-H. Ye, P. Bellotti, T. O. Paulisch, Dr. C. G. Daniliuc,
Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstraße 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202102451.
Figure 1. Bioactive b-lactam-containing compounds.
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undergo [1,3]-silyl migration to form ketene intermediates.
Herein, we report a mild, visible-light-induced method to
generate ketenes from a-ketoacylsilanes without additives
such as stochiometric base or metal, and their cycloadditions
with imines (Scheme 1b).
We commenced our investigations by studying the reac-
tion of 1-(tert-butyldimethylsilyl)-2-phenylethane-1,2-dione
(1a) and N-benzylideneaniline (2a) under light irradiation.
After extensive optimization, we were pleased to obtain the
desired b-lactam 3a in 95% yield with excellent diastereose-
lectivity (> 19:1) using acetonitrile as solvent under irradi-
ation with blue LEDs (5 W, 455 nm) (Table 1, entry 1). Other
Scheme 2. Mechanistic rationale for the stereochemical outcome of
the Staudinger reaction.
Table 1: Optimization of the reaction conditions.[a]
initial exo-attack from the nitrogen to the more accessible
ketene face generates a zwitterionic adduct I, which can
=
undergo either conrotatory ring closure (cis product) or C N
bond isomerization to II, followed by cyclization (trans
product). In the presence of electron-withdrawing groups,
the enhanced rate of isomerization[18] likely causes erosion of
the cis-diastereoselectivity. In addition to the phenyl group,
N-substitution can include benzyl (Bn) and tert-butylcarba-
mate (Boc), giving the corresponding b-lactam 3r and 3s in
excellent yields and moderate dr values. Substrates bearing
naphthalene and heteroaromatic rings, such as pyridine,
thiophene, benzothiophene, and furan were all suitable,
resulting in their corresponding products with high yields
and moderate to good diastereoselectivities (3t–3x). Inter-
estingly, a glyoxylate-derived imine was successfully subjected
to the reaction conditions, albeit in moderate yield and low dr
value. It is worth noting that reactive functional groups such
as halogens (3b–3e), Bpin (3 f), amide (3j), nitrile (3l), nitro
(3m), Boc (3s), and ester (3y), were all well tolerated in this
reaction, thus allowing for downstream chemistry.
Entry
Solvent
Yield [%][b]
1
2
3
4
5
6
CH3CN
THF
1,4-dioxane
n-Hexane
CH2Cl2
97 (95)[c]
70
74
83
87
85
N.D.
N.D.
87
PhCF3
7[d]
8[d,e]
9[f]
CH3CN
CH3CN
CH3CN
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.1 mmol), solvent (1 mL),
5 W blue LEDs (lmax =455 nm), rt, 1.5 h. [b] Determined by crude GC-
FID with dodecane as internal standard. [c] Isolated yield. The diaste-
reoisomeric ratio of product 3a is >19:1, determined by 1H-NMR. [d] In
the dark. [e] 1008C. [f] 1a (0.1 mmol), 2a (0.2 mmol). N.D.=not
detected.
We then investigated different ketimines and hydrazones.
N-(diphenylmethylene)aniline underwent smooth cycloaddi-
tion to give 4a in 98% yield. An N-phenylketimine derived
from acetone also gave the product 4b with good yield.
Moreover, N-alkyl imines with different functional groups,
including trifluoromethyl (4d), nitrile (4e), ester (4 f), and
thioether (4g), all afforded the corresponding b-lactams in
excellent yields. A heterocyclic substrate bearing pyrimidine
reacted smoothly and gave the product 4i in 71% yield.
Interestingly, an N-alkyl imine containing alkene moiety (4h)
proved to be an amenable substrate, without detection of
cyclobutanone, indicating that the [2+2] ketene-imine cyclo-
addition is favored over the [2+2] ketene-alkene cycloaddi-
tion. Besides these, N,N-disubstituted hydrazone also gave
the desired product 4j in good yield, while N-monosubsti-
tuted hydrazone only gave the desired product 4k in
moderate yield.
We next turned our attention to heterocycles as imine
coupling partners. Phenanthridine displayed remarkable
reactivity, giving product 5a in both excellent yield and
diastereoselectivity. Since dibenzazepines constitute an essen-
tial component of second generation or atypical antipsychot-
ics, these tricyclic moieties were subjected to the photochemi-
cally [2+2] cycloaddition conditions. Dibenzoxazepine,
dibenzothiazepine, and 11H-dibenzo[b,e]azepine all under-
solvents gave suboptimal yields and unsatisfactory diastereo-
selectivity (entries 2–6). Without irradiation, most of the
starting material 1a was not converted, even upon increasing
the temperature to 1008C (entries 7–8). Interestingly, com-
parable results could be obtained with 1a as the limiting
reagent. The results of the sensitivity assessment[15] revealed
that the most important reaction parameter is oxygen content,
while temperature, concentration, water, and light intensity
showed little impact on the reaction outcome.
With the optimal reaction conditions (Table 1, entry 1) in
hand, we first investigated the reaction scope with respect to
the aldimines (Table 2). N-phenyl imines derived from
aromatic aldehydes were first examined in this reaction.
Both excellent yields and diastereoselectivities were obtained
using substrates bearing electron-rich or electron-neutral
substituents on the aromatic rings (3b–3j). ortho-Substituted
aldimine 2o underwent the reaction to smoothly afford b-
lactam 3o with satisfactory results. Electron withdrawing
group substituted imines afforded good to excellent yields,
but with decreased diastereoselectivities (3k–3n). Notably,
the electronic influence on diastereoselectivity was also
observed when aniline-derived aldimines (3p–3q) were
employed. These findings are in accordance with the mech-
anistic scenarios proposed by Cossꢀo[16] and Xu[17] (Scheme 2):
2
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Table 2: Scope of visible-light-induced cycloaddition of a-ketoacylsilanes with imines.[a]
1
[a] The standard reaction condition (Table 1, entry 1), isolated yields. Diastereomeric ratio (dr) was determined by H-NMR. The explicit relative
stereochemistry for examples with dr <10:1 was not shown. [b] Modified conditions. See the Supporting Information for full experimental details.
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went cycloaddition efficiently, giving access to the products
with very high yields and diastereoselectivities (5b–5d). As
their analogue, dibenzazepin-11-one could give the product
5e with a comparable yield and good diastereoselectivity. 4-
Azacoumarin afforded the product 5 f in synthetically useful
yield with separable diastereomers. Both N-methyl quinox-
alinone and quinoxalinone showed excellent reactivity and
selectivity (5g and 5h[19]). To our delight, phenylthiazoline
also reacted well, giving the product 5i containing the penam
core structure of penicillins.
With phenylthiazoline as cycloaddition partner, different
a-ketoacylsilanes were examined. We were pleased to find
that electronic effects did not significantly influence the
reactivity and selectivity of the process. a-Ketoacylsilanes
with electron-neutral, electron-donating, and electron-with-
drawing substituents on the aromatic ring were all well-
tolerated. Moreover, the substituents on the phenyl ring could
be located at the para (6a–6m), meta (6n–6p), and ortho (6q–
6r) position. The obtained crystal structure of compound 6r
confirmed the anti-relationship between the two aromatic
rings, which is in agreement with the above-mentioned
mechanistic rationale.[19] As shown, the manifold provided
the desired products in the presence of different halogens
(6a–6d, 6r), sulfone (6 f), ether (6g–6h), amine (6i), thio-
ether (6j), TMS (6k), and phosphine (6l) decoration on the
phenyl moiety. Remarkably, both alkyne (6m) and alkene (6n
and 6t) were well-accommodated in this reaction. Naphtha-
lene and chromene containing substrates were also tolerated
and converted into the desired product 6s and 6t. Besides
TBS, the translocating silyl group could also be TIPS (6u).
Finally, alkyl a-ketoacylsilanes also showed good reactivity
and excellent diastereoselectivity (6v and 6w).
The scalability of the reaction is demonstrated in Fig-
ure 2a, in which product 3a was prepared in good yield with
slightly diminished diastereoselectivity. Moreover, compara-
ble results could be obtained under higher concentration and
lower loading of the a-ketoacylsilane, giving product 3 f by
simple recrystallization, bypassing the need for chromato-
graphic purification. The products were further derivatized to
illustrate potential synthetic applications (Figure 2b): desily-
lation (7), oxidative deborylation (8), and Suzuki–Miyaura
coupling (9) reactions all proceeded in high yields. Impor-
tantly, crystal structures of compounds 7 and 8 could be
obtained, thus confirming the anti-relationship between the
two aromatic rings.[19]
Figure 2. (A) Gram-scale reaction. (B) Synthetic transformations of b-
lactam products. (C) Trapping experiments.
with additional trapping agents. These observations indicate
that the ketene is likely generated from the single-step [1,3]-
silyl shift.
Computational investigations into the reaction mecha-
nism (Figure 3) show that direct excitation of a-ketoacylsilane
1a in the visible light region is feasible and gives rise to the
1
vertically excited n!p* state *[A], which undergoes geo-
metrical relaxation to 1*A (DG1*[A]/1*A = ꢀ7.8 kcalmolꢀ1). The
3
n!p* triplet state A was shown to be the lowest excited
triplet state, whose population from 1*A was computed to be
thermodynamically favorable (DG1*A/3A = ꢀ9.7 kcalmolꢀ1).
From triplet state 3A, two competitive pathways towards
ketene intermediate 3B could be envisioned, either via direct
[1,3]-silyl migration, or via [1,2]-silyl migration and subse-
quent [1,2]-phenyl shift: The former [1,3]-silyl migration is
To gain more insight into this reaction, mechanistic studies
were conducted. Attempts to prove the formation of the
ketene intermediate using additional trapping agents (Fig-
ure 2c) were fruitful. The ketene intermediate can be trapped
by methanol, piperidine, and 4-toluenethiol to give the
corresponding carboxylic acid derivatives in high yields. As
for the generation of ketenes by photolysis, two possibilities
of the photorearrangement were considered. One process
involves a single-step [1,3]-silyl shift[20] to the distal carbonyl
group. Alternatively, a two-step mechanism, that a [1,2]-shift
of silicon to the adjacent carbonyl group with generation of
a siloxycarbene, followed by a Wolff rearrangement, could be
operative. It is noteworthy that we did not detect the product
generated from the hypothetic reaction of the siloxycarbene
Figure 3. Calculated reaction profiles for the excitation of a-ketoacylsi-
lane 1a, and two potential pathways for the formation of the
siloxyketene (B3LYP/def2-TZVPP).
4
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accompanied by a small activation barrier of 6.0 kcalmolꢀ1
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908 – 950.
1
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favored (DG3B/1B = ꢀ20.9 kcalmolꢀ1), rendering the overall
reaction pathway both kinetically and thermodynamically
feasible. In comparison, a [1,2]-silyl migration of a-ketoacyl-
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3
3C. Since the following reaction pathway of C to form the
singlet ketene intermediate 1B is significantly higher in energy
than the pathway of [1,3]-silyl migration, we propose the
latter to be the operative mechanism. (For more details, see
the Supporting Information.)
In summary, we have developed a protocol for the visible-
light-induced generation of siloxyketenes and their cyclo-
addition with imines to directly access valuable b-lactams,
complementing other well-established methodologies. A
variety of imines, including aldimines, ketimines, hydrazones,
and different fused nitrogen heterocycles, were found to be
compatible. This protocol shows noteworthy functional-group
compatibility, satisfactory diastereoselectivity, straightfor-
ward scalability, and facile product diversification. Mecha-
nistic studies indicate that the reaction likely proceeds via the
formation of a singlet siloxyketene from the T1 state of the a-
ketoacylsilane precursor. Ultimately, we believe that this
methodology will greatly influence method development in
the area of the photochemistry of acylsilanes.
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Acknowledgements
Generous financial support by the Alexander von Humboldt
Foundation and the Deutsche Forschungsgemeinschaft (Leib-
niz Award; SFB 858; ChemBIon-GRK2515) is gratefully
acknowledged. We thank Toryn Dalton and Elena Sophia
Horst (all WWU) for helpful discussions and experimental
support.
Conflict of interest
The authors declare no conflict of interest.
Keywords: acylsilane · cycloaddition · siloxyketene · visible light ·
b-lactam
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Manuscript received: February 17, 2021
Revised manuscript received: March 9, 2021
Accepted manuscript online: March 17, 2021
Version of record online: && &&, &&&&
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Photochemistry
J.-H. Ye, P. Bellotti, T. O. Paulisch,
C. G. Daniliuc,
F. Glorius*
&&&& — &&&&
Visible-Light-Induced Cycloaddition of a-
Ketoacylsilanes with Imines: Facile
Access to b-Lactams
The synthesis of b-lactams from a-ketoa-
cylsilanes and imines is reported, which
proceeds via a [2+2] photochemical
cycloaddition with in situ generation of
siloxyketene. This reaction proceeds in an
atom-economic fashion with broad sub-
strate scope, including aldimines, keti-
mines, hydrazones, and fused nitrogen
heterocycles, and good functional-group
tolerance, affording a variety of b-lactams
with satisfactory diastereoselectivities.
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
7
These are not the final page numbers!