Efficient Generation and Synthetic Applications of Alkyl-Substituted Siloxycarbenes: Suppression of Norrish-Type Fragmentations of Alkanoylsilanes by Triplet Energy Transfer

Article abstract of DOI:10.1002/chem.201904635

Acylsilanes have been known to undergo isomerization to siloxycarbenes under photoirradiation and the thus generated carbenes can be utilized for various synthetic reactions. But this carbene formation is not necessarily efficient with some alkanoylsilanes because Norrish-type fragmentations compete, which limit the synthetic utility of alkanoylsilanes as carbene precursors. In this study, generation of siloxycarbenes from alkanoylsilanes by visible-light-induced energy transfer was examined by using an Ir complex, [Ir{dF(CF₃)ppy}₂(dtbpy)]PF₆, and was successfully applied to the C?C coupling reactions with boronic esters or aldehydes. This methodology efficiently suppressed undesired Norrish-type reactions and broadened synthetic utility of alkanoylsilanes.

Full text of DOI:10.1002/chem.201904635

A Journal of
Accepted Article
Title: Efficient Generation and Synthetic Applications of Alkyl-
Substituted Siloxycarbenes: Suppression of Norrish-type
Fragmentations of Alkanoylsilanes by Triplet Energy Transfer
Authors: Kento Ishida, Hokuto Yamazaki, Chihiro Hagiwara, Manabu
Abe, and Hiroyuki Kusama
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To be cited as: Chem. Eur. J. 10.1002/chem.201904635
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Efficient Generation and Synthetic Applications of Alkyl-
Substituted Siloxycarbenes: Suppression of Norrish-type
Fragmentations of Alkanoylsilanes by Triplet Energy Transfer
Kento Ishida[a], Hokuto Yamazaki[a], Chihiro Hagiwara[a], Manabu Abe[b], Hiroyuki Kusama*[a]
Abstract: Acylsilanes have been known to undergo isomerization to
siloxycarbenes under photoirradiation and thus-generated carbenes
can be utilized for various synthetic reactions. But this carbene
formation is not necessarily efficient with some kinds of
alkanoylsilanes because Norrish-type fragmentations compete, which
limit synthetic utility of alkanoylsilanes as carbene precursors. In this
study, generation of siloxycarbenes from alkanoylsilanes by visible-
light-induced energy transfer was examined by using an Ir complex,
[Ir{dF(CF3)ppy}2(dtbpy)]PF6, and was successfully applied to the C-C
coupling reactions with boronic esters or aldehydes. This
methodology efficiently suppressed undesired Norrish-type reactions
and broadened synthetic utility of alkanoylsilanes.
aroylsilanes (1, R1 = aryl), the use of alkanoylsilanes (1, R1 = alkyl)
has been quite limited.[8]
Scheme 1. (a) Photochemical generation of siloxycarbenes (b) Photo-induced
coupling reactions between acylsilanes and electrophiles (our previous works)
Photocatalytic organic transformations have attracted
significant attentions in current synthetic chemistry. Photoredox
process involving radical intermediate is one of the hot topics in
this field.[1] Another interesting aspect of photocatalytic reactions
is triplet energy transfer processes which enable generation of
triplet-excited states of organic substrates through energy transfer
from photo-excited catalysts.[1b,1c,1e,2] From a synthetic point of
view, triplet energy transfer is considered to have some benefits[3]
as compared to direct photoexcitation of organic substrates: (i)
highly energetic singlet-excited states of substrates, which
sometimes induce undesired reactions, can be skipped, and (ii) it
enables excitation of substrates with longer-wavelength light than
the inherent absorption band of substrates.
We have already reported intermolecular coupling reactions
of photochemically-generated siloxycarbenes 2 with boronic
esters or aldehydes under neutral or Lewis acidic conditions
(Scheme 1b).[7] During the course of these studies, we noticed
that serious undesired photochemical processes competed with
the siloxycarbene formation in the reactions with some kinds of
alkanoylsilanes. For example, UV-irradiation (365 nm) to a
mixture of 4-phenylbutanoylsilane 1a[9] and boronic ester 5 gave
styrene along with the desired coupling product 3a in low yield
(Scheme 2). The formation of styrene clearly indicated that
Norrish type II fragmentation[10][11] competed with the desired
photochemical isomerization to siloxycarbene 2 from excited
states of alkanoylsilane 1a.
It has been known that triplet-excited states of acylsilanes 1
undergo
a 1,2-silyl migration to generate transient and
nucleophilic siloxycarbene intermediates 2 (Scheme 1a).[4] Since
Brook
and
co-workers
originally
discovered
this
photoisomerization
through reactions
of siloxycarbene
intermediates 2 with polar compounds such as alcohols over fifty
years ago,[5] various synthetic reactions utilizing photochemically-
generated siloxycarbenes 2 have been developed by several
groups[6] including us.[7] However, in contrast to the reactions of
Scheme 2. Photochemical reaction with 1a and 5
[a]
[b]
Dr. K. Ishida, H. Yamazaki, C. Hagiwara, Prof. Dr. H. Kusama
Department of Chemistry, Faculty of Science
Gakushuin University
1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588 (Japan)
E-mail: hiroyuki.kusama@gakushuin.ac.jp
Prof. Dr. M. Abe
Similarly, photo-irradiation to a mixture of a-branched
alkanoylsilane 1b and boronic ester 5 afforded a decarbonylated
product 6 as a major product (Scheme 3). In this case, a Norrish
type I fragmentation[12] apparently competed with siloxycarbene
formation.
Department of Chemistry, Graduate School of Science
Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526
(Japan)
Supporting information for this article is given via a link at the end of
the document.
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With this in mind, we tried to find an appropriate
photosensitizer by using an alkanoylsilane 1c as a model
substrate. To estimate efficiency of siloxycarbene formation,
ethanol was employed as a carbene trapping reagent because
the reaction of photochemically-generated siloxycarbenes with
alcohols have been known to be facile (Table 1).[5] At first, photo-
irradiation was conducted with a 500W super-high-pressure Hg
lamp through a 365 nm band pass filter in the absence of
photosensitizers (direct photo-excitation condition). An acetal 7
was obtained in good yield, which confirmed that the
siloxycarbene actually reacted with ethanol smoothly (entry 1). On
the other hand, no reaction occurred with photoirradiation through
a 436 nm band pass filter, indicating that the formation of
siloxycarbene from 1c did not proceed at all under these
conditions (entry 2). Then, we examined reactions with various
photosensitizers under 436 nm photo-irradiation conditions and
found that the use of 2 mol% of an iridium catalyst 8 afforded the
desired acetal 7 in quantitative yield (entry 3).[14] Since the triplet
energies of the Ir complex 8 and the alkanoylsilane 1c are almost
same (8: 60.8 kcal/mol[15], 1c: 60.6 kcal/mol[16]), this result
suggested that expected triplet energy transfer proceeded
efficiently.
Scheme 3. Photochemical reaction with 1b and 5
These results clearly indicate that Norrish type
fragmentation reactions are serious problems to expand synthetic
applicability of photochemically-generated siloxycarbenes.
Generally, Norrish type fragmentations are known to take place
both from singlet and triplet excited states of carbonyls, but, in the
reactions of alkanoylsilanes, clear evidence about which excited
state is involved in Norrish type reactions is not available in
literatures. If the above Norrish reactions took place from the
singlet-excited-state of the alkanoylsilanes, triplet energy transfer
methodology would be a reasonable solution to realize efficient
generation of siloxycarbenes. Thus, we decided to develop the
generation of siloxycarbenes 2 from alkanoylsilanes 1 through
triplet energy transfer[13] and tried to suppress undesired Norrish
type fragmentations to expand synthetic utility of alkanoylsilanes.
Table 1. Examination of photosensitizers by using ethanol as
a
Strategy for the generation of siloxycarbenes 2 via triplet
sensitization is shown in Scheme 4. A photosensitizer (PS) having
its absorption band in visible region would be a reasonable
candidate to prevent the direct photo-excitation of co-existing
alkanoylsilanes 1, because the n-p* absorption of 1 lies around
360-370 nm. Triplet energy of PS should be comparable or larger
than that of an alkanoylsilane 1 to realize energy transfer from a
triplet-excited state of PS (3PS*) to the ground state of the
alkanoylsilane 1. With these requirements, the triplet-excited state
of the alkanoylsilane 1 (31*) can be generated without the
formation of its singlet-excited-state, and generation of the
corresponding siloxycarbene 2 will be realized efficiently. By
using this strategy, undesired reactions from singlet-excited state
of alkanoylsilanes 1 can be eliminated.
siloxycarbene-trapping reagent
Yield of 7
entry
Conditions
Time
[%][a]
1
2
hn (365 nm)
hn (436 nm)
1 h
5 h
65
-
3
hn (436 nm) + 8 (2 mol%)
1 h
quant
[a] Determined by 1H-NMR analysis.
To confirm the energy transfer between a triplet-excited
state of 8 (38*) and a ground state of 1c, quenching experiments
were conducted by measuring luminescence spectra of 8 under
various concentration of the alkanoylsilane 1c. As shown in
Scheme 5a, the emission intensity decreased with increasing the
concentration of alkanoylsilane 1c. These data indicated that the
triplet-excited state of 8 (38*) was quenched by 1c through triplet
energy transfer.[17]
Scheme 4. Generation of siloxycarbenes from alkanoylsilanes by visible-light-
induced energy transfer (this work)
A plausible mechanism of this reaction is described in
Scheme 5b. The Ir complex 8 is photo-excited under visible-light
irradiation conditions and the resulting singlet-excited state of 8
(18*) is smoothly converted to its triplet-excited state (38*) by
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intersystem crossing. Then, energy transfer from 38* to the ground
state of 1c occurs to give a triplet-excited state of 1c (31c*). 31c*
undergoes a 1,2-silyl migration to generate siloxycarbene 2,[18]
which reacts with ethanol to afford the acetal 7.
Table 2. Application to the coupling reaction with boronic esters[a]
Scheme 5. (a) Quenching experiment of the Ir complex 8 under various
concentration of 1c (b) Plausible reaction mechanism
[a] Isolated yields. Yields in the parentheses are those obtained under
direct photo-excitation conditions. (see ref. 7a) [b] 2 equiv. of boronic
ester were employed. [c] The crude material of photochemical reaction
was treated with 1M HCl aq. under air.
With the energy transfer conditions in hands, we then turned
our attention into applicability of this methodology to the carbon-
carbon bond-forming reactions using boronic esters or aldehydes.
We first examined the reactions of alkanoylsilane 1d without a
reactive g-hydrogen with boronic esters (Table 2). Gratifyingly,
visible-light-irradiation to mixtures of alkanoylsilane 1d and
various boronic esters in the presence of 1 mol% of the Ir complex
8 afforded the desired coupling products, which could be
converted to the corresponding ketones 9a-d. Yields of the
products 9a-d were comparable to those obtained under direct
photo-excitation conditions with UV light.[7a]
Table 3. Application to the coupling reaction with aldehydes[a]
This methodology was also applicable to the reactions with
aldehydes (Table 3). Visible-light-irradiation to mixtures of
alkanoylsilanes 1 and aldehydes in the presence of 1 mol% of the
Ir complex 8 and 5 mol% of zinc iodide afforded the desired a-
siloxyketones 4a-d in good yields. Again, yields of the products
4a-d were comparable to those obtained under direct photo-
excitation conditions.[7b]
[a] Isolated yields. Yields in the parentheses are those obtained under
direct photo-excitation conditions. (see ref. 7b) [b]
butyraldehyde were employed.
5 equiv. of
Thus, the energy transfer methodology could be
successfully applied to the reactions of the alkanoylsilanes 1c-e
with boronic esters or aldehydes. Then, we tried to suppress the
serious side reaction in the reaction of 4-phenylbutanoylsilane 1a.
As described in the beginning, the reaction of 1a with the boronic
ester 5 under UV-irradiation conditions afforded styrene as a
byproduct through Norrish type II fragmentation (Scheme 2). But,
under the triplet sensitization conditions, formation of styrene was
significantly suppressed and the yield of the desired coupling
product 3a was improved to 83% yield (Table 4, entry1).
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Table 4. Coupling reactions with alkanoylsilanes bearing weak C-H
bonds at the g position
Scheme 6. Coupling reaction between a-branched acylsilane 1b and boronic
ester 5 under triplet sensitization conditions
Condi
tions[a]
Time
[h]
Yield of 3
Yield of 10
entry
R
[%][b]
[%][b]
This methodology could be also applied to the reaction of
a-tertiary alkanoylsilane 1h, which was highly susceptible to
Norrish I fragmentation. The desired coupling reaction between
1
2
Ph (1a)
B
A
4.5
4
83 (3a)
33 (3c)
7
1h and
a boronic ester 11 proceeded well under triplet
OBn (1f)
19
sensitization conditions, while the same reaction under direct
photo-excitation conditions gave a poor result (Scheme 7).[19]
Thus, triplet sensitization methodology efficiently suppressed
Norrish type fragmentation reactions of alkanoylsilanes, thereby
enhancing the synthetic utility of alkanoylsilanes as siloxycarbene
precursors.
3
4
5
OBn (1f)
vinyl (1g)
vinyl (1g)
B
A
B
4
2
5
87 (3c)
11 (3d)
73 (3d)
N.D.
23
trace
[a] A: hn (365 nm), B: hn (436 nm) + 1 mol% 8 [b] Yields were
determined by 1H-NMR analysis.
This kind of improvement was also observed with other
alkanoylsilanes bearing a reactive g-hydrogen. UV-irradiation to
alkanoylsilanes bearing a benzyloxy or a vinyl group at g-position
(1f or 1g) in the presence of boronic ester 5 resulted in complex
mixtures including fragmentation products 10, and the yields of
the desired coupling products 3c and 3d were quite low (entries 2
and 4). In contrast, under triplet sensitization conditions, formation
of the fragmentation products 10 was almost completely
suppressed in both cases, and the yields of the coupling products
3c and 3d were dramatically improved to 87 and 73%,
respectively (entries 3 and 5). These results indicated that Norrish
type II fragmentation of alkanoylsilanes mainly proceeded from
the singlet-excited states and, from the triplet-excited states,
isomerization to siloxycarbenes was much faster than the
fragmentation reaction.
Then, we turned our attention to the reactions of the a-
branched alkanoylsilane 1b. As already mentioned in Scheme 3,
direct photoexcitation of 1b with UV-light afforded the
decarbonylated compound 6 as a major product through Norrish
type I fragmentation. With this substrate, the energy transfer
conditions again worked nicely, and the desired coupling product
3b was obtained in 76% yield without accompanying the
decarbonylated product 6 (Scheme 6). Thus, Norrish type I
fragmentation of 1b took place from its singlet-excited state, and
the triplet sensitization methodology completely eliminated this
undesired process. To our knowledge, this is the first successful
Scheme 7. Coupling reaction between tert-alkyl silyl ketone 1h and boronic
ester 11
From a synthetic point of view, visible light-induced energy
transfer process has another merit that can be utilized for the
reaction with UV-labile compounds. For example, UV-irradiation
(365 nm) to
a
mixture of alkanoylsilane 1d and 2-
allylbenzaldehyde in the presence of 5 mol% of zinc iodide
resulted in complex mixture (Scheme 8a). In this case, formation
of 2-(1-propenyl)benzaldehydes was observed. This means
photo-enolization[20] of 2-allylbenzaldehyde by g-hydrogen
abstraction took place because this aldehyde also absorbed UV-
light. But the triplet sensitization conditions promoted the desired
coupling reaction, giving an a-hydroxyketone 4e in good yield
without accompanying isomerization of 2-allylbenzaldehyde
(Scheme 8b). Considering the triplet energies of substrates
(alkanoylsilanes : ca. 60 kcal/mol, aromatic aldehydes : ca. 70
kcal/mol[21]), this result is attributed to chemoselective energy
transfer from the triplet-excited state of 8 to the ground state of
1d.
C-C
coupling
reaction
of
photochemically-generated
siloxycarbenes derived from a sec-alkyl silyl ketone.
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[3]
[4]
For examples of advantages of triplet sensitization compared to direct
photo-excitation, see ref 2a, 2b, 2d, and 2g.
Dalton and co-workers reported the possibility of the generation of
siloxycarbenes via a triplet-excited state of alkanoylsilanes. See: R. A.
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699.
Scheme 8. Photochemical reactions with 1d and 2-allylbenzaldehyde
[5]
[6]
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In summary, we have realized the generation of
siloxycarbenes from various kinds of alkanoylsilanes by visible-
light-induced energy transfer[13], and this methodology was
successfully applied to the C-C coupling reactions with boronic
esters or aldehydes. Noteworthy is that Norrish type
fragmentations of alkanoylsilanes were clarified to proceed mainly
from singlet-excited states and these undesired reactions were
significantly suppressed by triplet sensitization. We believe that
the present results clearly show the synthetic utility of
alkanoylsilanes as siloxycarbene precursors.
For recent examples of reactions utilizing photochemically-generated
siloxycarbenes, see : a) H.-J. Zhang, P. Becker, H. Huang, R. Pirwerdjan,
F.-F. Pan, C. Bolm, Adv. Synth. Catal. 2012, 354, 2157-2161; b) P.
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Acknowledgements
For examples of reactions of photochemically-generated siloxycarbenes
from alkanoylsilanes, see a) A. G. Brook, H. W. Kucera, R. Pearce, Can.
J. Chem. 1971, 49, 1618-1621; b) A. G. Brook, R. Pearce, J. B. Pierce,
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Jr., Org. Biomol. Chem. 2004, 2, 3155-3161; and ref 5a, 5b, 6a, 7a, 7b
In general, n-p* absorption bands of alkanoylsilanes are observed
around 360-370 nm.
We would like to thank Prof. Yoshiyuki Inaguma (Gakushuin
University) for his support on the CV measurement. This research
was supported by JSPS KAKENHI Grant Number JP18H04658
(Hybrid Catalysis) (HK).
[9]
Keywords: acylsilanes • energy transfer • photochemistry •
[10] a) P. J. Wagner, Acc. Chem. Res. 1971, 4, 168-177; b) C. P. Casey, R.
A. Boggs, J. Am. Chem. Soc. 1972, 94, 6457-6463.
siloxycarbenes • triplet sensitizer
[11] Frei et al. have reported competitions between siloxycarbene formation
and g-H abstraction from photo-excited states of alkanoylsilanes, see ref
8c and 8d.
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[13] Quite recently, triplet sensitization to alkanoylsilanes was reported by
Glorius et al. See: J.-H. Ye, L. Quach, T. Paulisch, F. Glorius, J. Am.
Chem. Soc. 2019, 141, 16227-16231. But no information on Norrish type
reactions was available in this report. We independently studied the
triplet energy transfer methodology and its synthetic applications. We
have reported the present work in 14th International Kyoto Conference
on Organic Chemistry (IKCOC14), November 13th, 2018.
[2]
For recent examples of organic transformations utilizing triplet
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[14] For details of the examination, see SI.
[15] M. S. Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, Jr., G.
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[16] The triplet energy of alkanoylsilane 1c was determined by measuring a
phosphorescence spectrum (See SI).
[17] Single electron transfer (SET) reactions between a triplet-excited state of
Ir complex
8 a ground state of alkanoylsilane 1c are
(38*) and
thermodynamically disfavored according to their redox potentials (1c:
ERed = -1.13 V, EOx = + 1.66 V (see SI); 38*: EIr(III)*/Ir(IV) = -0.89 V, EIr(III)*/Ir(II)
= +1.21 V (see ref. 1c) (vs SCE)). Therefore, the decrease of emission
intensity of Ir complex 8 is attributed to energy transfer. For redox
chemistry of acylsilanes, see: L. Capaldo, R. Riccardi, D. Ravelli, M.
Fagnoni, ACS Catal. 2018, 8, 304-309.
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[18] Initially generated siloxycarbene is triplet, but this must be readily
converted to its singlet state and then reacts with ethanol because
ground state of siloxycarbene is singlet.
[20] a) K. C. Nicolaou, D. L. F. Gray, J. Tae, J. Am. Chem. Soc. 2004, 126,
613-627; b) B. Grosch, C. N. Orlebar, E. Herdtweck, M. Kaneda, T. Wada,
Y. Inoue, T. Bach, Chem. Eur. J. 2004, 10, 2179-2189.
[19] Reactions of 1h with other boronic esters such as 5 were unsuccessful
even under triplet sensitization conditions probably due to the bulky tert-
alkyl substituent. The use of less sterically hindered catecholate was
important.
[21] M. Montalti, A. Credi, L. Prodi, M. T. Gandolfi, HANDBOOK OF
PHOTOCHEMISTRY THIRD EDITION, CRC Press, Boca Raton, FL,
2006.
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K. Ishida, H. Yamazaki, C. Hagiwara, M.
Abe, H. Kusama*
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Efficient Generation and Synthetic
Applications of Alkyl-Substituted
Siloxycarbenes: Suppression of
Norrish-type Fragmentations of
Alkanoylsilanes by Triplet Energy
Transfer
Enhancement of the utility of alkanoylsilanes: Generation of siloxycarbenes
from alkanoylsilanes by triplet energy transfer was examined by using an Ir complex
and was successfully applied to C-C coupling reactions with boronates or
aldehydes. This methodology efficiently suppressed undesired Norrish-type
fragmentation reactions and broaden synthetic utility of alkanoylsilanes.
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