N-Heterocyclic Carbene Catalyzed Photoenolization/Diels–Alder Reaction of Acid Fluorides

Article abstract of DOI:10.1002/anie.201914456

The combination of light activation and N-heterocyclic carbene (NHC) organocatalysis has enabled the use of acid fluorides as substrates in a UVA-light-mediated photochemical transformation previously observed only with aromatic aldehydes and ketones. Stoichiometric studies and TD-DFT calculations support a mechanism involving the photoactivation of an ortho-toluoyl azolium intermediate, which exhibits “ketone-like” photochemical reactivity under UVA irradiation. Using this photo-NHC catalysis approach, a novel photoenolization/Diels–Alder (PEDA) process was developed that leads to diverse isochroman-1-one derivatives.

Full text of DOI:10.1002/anie.201914456

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: N-Heterocyclic Carbene-Catalyzed Photoenolization Diels-Alder
Reaction of Acid Fluorides
Authors: Andreas Mavroskoufis, Keerthana Rajes, Paul Golz, Arush
Agrawal, Vincent Ruß, Jan Philipp Götze, and Matthew Neil
Hopkinson
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914456
Angew. Chem. 10.1002/ange.201914456
Link to VoR: http://dx.doi.org/10.1002/anie.201914456
http://dx.doi.org/10.1002/ange.201914456

10.1002/anie.201914456
Angewandte Chemie International Edition
COMMUNICATION
N-Heterocyclic Carbene-Catalyzed Photoenolization Diels-Alder
Reaction of Acid Fluorides
Andreas Mavroskoufis,[a] Keerthana Rajes,[a] Paul Golz,[a] Arush Agrawal,[a] Vincent Ruß,[a] Jan P.
Götze,[a] and Matthew N. Hopkinson*[a]
This work is dedicated to Prof. Dr. Hans-Ulrich Reißig on the occasion of his 70th birthday.
Abstract: The combination of light activation and N-heterocyclic
carbene (NHC) organocatalysis has enabled acid fluoride substrates
to engage in a UVA light-mediated photochemical transformation
previously observed only with aromatic aldehydes and ketones.
Stoichiometric studies and TD-DFT calculations support a mechanism
involving the photoactivation of an ortho-toluoyl azolium intermediate,
which exhibits “ketone-like” photochemical reactivity under UVA
irradiation. Using this photo-NHC catalysis approach, a novel
photoenolization Diels-Alder (PEDA) process was developed leading
to diverse isochroman-1-one derivatives.
comparable excited states to structurally related benzophenone
derivatives upon light irradiation. Furthermore, in providing a
second aromatic system for π-conjugation, the NHC can influence
the absorption characteristics of the carbonyl function, enabling
excitation at longer wavelengths than the parent carboxylic acid
derivative. Following a light mediated process of the type typically
observed with aromatic ketones and elimination of the NHC
organocatalyst, new classes of product for carbonyl
photochemistry would be provided.
Recent years have seen
a
resurgence of interest in
photochemical activation as a means of accessing new reactivity
modes in organic synthesis. In contrast to thermally activated
processes, photochemical reactions proceed via excited state
species and duly exhibit dramatically different reactivity and
selectivity. Reactions involving the activation of aromatic
aldehydes and ketones with UVA light are among the most widely
studied photochemical transformations. Upon excitation, the
biradical-like (n,π*) states of these compounds undergo a range
of synthetically important processes such as Norrish
fragmentations,
Yang
cyclizations
and
Paternò-Büchi
cycloadditions.[1] While such reactions are well established for
aldehydes and ketones, analogous transformations with
substrates at the carboxylic acid oxidation level are scarce. These
compounds typically absorb light at shorter wavelengths than the
corresponding ketones while many populate lowest energy (π,π*)
excited states with inherently different photochemical
reactivity.[2,3]
Inspired by the recent successes combining light activation
with other catalysis modes,[4,5] we wondered whether the scope of
carbonyl photochemistry could be expanded by merging light
activation with N-heterocyclic carbene (NHC) organocatalysis.[6]
An overview of our proposed “photo-NHC” catalysis concept is
shown in Scheme 1. As demonstrated in several elegant
processes, NHCs can react with activated aryl carboxylic acid
derivatives to afford benzoyl azolium intermediates. These
species are formally ketones and could be expected to populate
Scheme 1. Overview of photo-NHC catalysis. LG = leaving group.
The photo-NHC concept represents a new catalysis mode for
NHCs. These compounds most often facilitate umpolung
reactions of aldehydes while transformations involving acyl
azolium salts or enolates have been developed.[7] Most processes
are considered to proceed via polar mechanisms, however recent
studies have revealed new radical activation modes which exploit
the ability of NHCs to stabilize unpaired electron density.[8-10] In
photo-NHC catalysis, the role of the NHC is again different. In
temporarily changing the absorption profile and photochemical
reactivity of a carbonyl group during the catalytic cycle, the NHC
can be considered a “transient aryl group” that enables hitherto
unsuitable
substrates
to
engage
in
photochemical
transformations. Moreover, as the catalyst remains bound to the
substrates throughout the cycle, enantioselective variants could
be potentially realized using chiral NHCs.
We initially sought to validate the photo-NHC catalysis
concept by investigating an independently synthesized
[a]
M. Sc. A. Mavroskoufis, M. Sc. K. Rajes, M. Sc. P. Golz, M. Sc. A.
Agrawal, B.Sc. V. Ruß, Dr. J. P. Götze, Jun.-Prof. M. N. Hopkinson
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Takustrasse 3, 14195 Berlin (Germany)
E-mail: matthew.hopkinson@fu-berlin.de
stoichiometric acyl azolium salt. As
a
proof-of-concept
Website: https://www.bcp.fu-berlin.de/en/chemie/chemie/forschung/
OrgChem/hopkinson/index.html
transformation, a photoenolization process was selected starting
from ortho-toluoyl imidazolium salt 1.[11] Upon irradiation with UVA
light, excitation of the carbonyl followed by intramolecular 1,5-
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201914456
Angewandte Chemie International Edition
COMMUNICATION
hydrogen atom transfer (HAT) would lead to a hydroxyl o-
quinodimethane (o-QDM) species. Compound 1 was thus
prepared (see SI) and subjected to irradiation from UVA LEDs
(λmax = 365 nm) in a mixture of degassed CD3CN and CD3CO2D
(9:1). After 16 h, we were delighted to observe extensive
incorporation of deuterium (68%) at the o-methyl group consistent
with deuteration of an o-QDM intermediate while no reaction was
observed without light (Scheme 2a). This result demonstrates that
acyl azolium species are capable of both absorbing light at
wavelengths similar to other aromatic ketones and of exhibiting
analogous photochemical reactivity. In a further experiment, a
photoenolization Diels-Alder (PEDA) process leading to
isochroman-1-ones was investigated by reacting 1 with ketone 2a.
Analogous non-NHC-catalyzed reactions affording the
corresponding hemiacetals have been described for aromatic
aldehydes and ketones.[12,13] After 16 h of irradiation in degassed
MeCN, clean conversion to the desired product 3aa was observed
in 62% NMR yield while, as for the deuteration experiment, no
reaction occurred without light (Scheme 2b).
reactions without the NHC, base or light led only to trace amounts
of product (see SI).[20]
Scheme 2. Validation of the photo-NHC catalysis concept with acyl azolium salt
1. a) o-Benzylic deuteration via photoenolization. b) PEDA reaction with ketone
2a. Tf = triflyl.
Annulation reactions are among the most important classes of
NHC-catalyzed transformation.[14] As such, novel reactivity modes
for NHCs that open up new strategies for annulation reactions are
especially desirable. The proposed PEDA reaction has some
attractive features in comparison to alternative NHC-catalyzed
routes to isochroman-1-ones 3. Previously published processes
have employed either o-CH2Br-substituted benzaldehydes[15] or
o-CH2SiMe3-substituted esters,[16] which can be laborious to
prepare. The only example directly using simple o-toluic acid
derivatives was reported recently by Li, Yao and co-workers.[17,18]
This non-photochemical process, however, was successful only
for highly electron-deficient substrates whose o-benzylic protons
are sufficiently acidic to be deprotonated by an amine base.
We next turned our attention to the development of a NHC-
catalyzed PEDA process. After evaluating different leaving
groups, o-toluoyl fluorides 4[19] were identified as the most efficient
substrates with ketones 2. Optimization of the reaction conditions
resulted in an efficient system using the simple NHC precursor
1,3-dimethylimidazolium triflate (IMe·HOTf, 20 mol%) and
Cs2CO3 (2 equiv) in degassed MeCN. After 16 h under irradiation
from UVA LEDs, 3aa could be isolated in 84% yield while control
Scheme 3. Scope of the NHC-catalyzed PEDA reaction. Conditions: 4a-p (0.90
mmol), 2a-l (0.30 mmol), IMe∙HOTf (0.060 mmol), Cs2CO3 (0.60 mmol),
degassed MeCN (3 mL), 40 °C, 16 h, UVA LEDs. Isolated yields. [a]
Diastereomeric ratio (d.r.) measured by 1H NMR of the crude reaction mixture.
[b] Major diastereomer isolated.
The scope of the PEDA reaction was tested with a range of
substrates 4 and 2 (Scheme 3). As shown for the methyl-
substituted acid fluorides 4b-e, the yield of products 3 was found
to be dependent on the site of aromatic substitution. While the 5-,
6- and 7-methyl isochroman-1-ones were generated in good
yields (65-86%), the PEDA process with the sterically-demanding
2,6-dimethylbenzoyl fluoride 4e was comparatively inefficient
(17%). Halogen substituents amenable to subsequent cross-
coupling were well tolerated and the corresponding isochroman-
1-ones 3fa-3na were obtained in excellent yields up to 95%. The
successful application of the o-ethyl and o-benzyl-substituted acid
fluorides 4o (93%, d.r. 2.6:1) and 4p (48%, d.r. 10:1) is particularly
noteworthy as substrates with substituted o-benzyl positions were
not employed in previously reported NHC-catalyzed syntheses of
isochroman-1-ones.[15-17]
dienophiles 2 were tolerated including -CF3 (2b,c) and -OMe (2d)
A range of electronically diverse
This article is protected by copyright. All rights reserved.

10.1002/anie.201914456
Angewandte Chemie International Edition
COMMUNICATION
substituted derivatives. Unfortunately, other classes of non-
enolizable ketone such as isatins or 2-ketoesters could not be
employed, potentially due to competitive absorption of UVA light
by these substrates (see SI).
Time-dependent DFT calculations ((TD-)CAM-B3LYP/6-
311G**)[22] on o-toluoyl azolium intermediate A provide support
for the above mechanism. Irradiation of this species in the UVA
region results in a triplet excited state (T1(A)) with significant
unpaired electron density at the oxygen atom (Figure 1). 1,5-HAT
proceeds readily from this species, resulting in T1(B). Conversely,
excitation of acid fluoride 4a occurs only at shorter wavelengths
with the analogous HAT from the triplet state T1(4a) being highly
endergonic (see SI). The lack of photoenolization was
experimentally corroborated by subjecting 4a to the deuteration
conditions with CD3CN/CD3CO2D (see Scheme 2a). No
incorporation of deuterium was observed after 16 h, highlighting
the key role of the NHC in modifying the photochemistry of the
carbonyl function.
Scheme 4. Proposed reaction mechanism.
Figure 1. T1(A) equilibrium geometry showing the difference in DFT electronic
densities between the T1(A) and the electronic ground state at 0.002 contour
value (gains blue, losses red upon T1(A) formation).
A mechanistic proposal consistent with PEDA reactions of
ketones is shown in Scheme 4.[11-13] Upon deprotonation of the
NHC precursor with Cs2CO3, imidazolylidene IMe reacts with acid
fluoride 4 releasing F− and generating the o-toluoyl azolium
intermediate A.[19] This benzophenone-like species is excited
under UVA irradiation affording, after inter system crossing (isc),
a triplet excited state T1(A).[21] Fast 1,5-HAT from the o-benzylic
position to the radical-like carbonyl oxygen gives rise to the triplet
dienol biradical T1(B). Rotation of this species before relaxation
leads to the ground state o-QDM (E)-B, which can react with the
dienophile 2 in a cycloaddition process. This reaction could
feasibly occur in a concerted Diels-Alder fashion or, as shown in
Scheme 4, via a two-step sequence involving intermediate C.
Finally, elimination of the NHC from cycloadduct D in the
presence of the base releases the product 3 and completes the
catalytic cycle.
In conclusion, the merger of NHC organocatalysis with light
activation has enabled the annulation of o-toluoyl fluorides with
trifluoroacetophenones. Stoichiometric studies and TD-DFT
calculations support a mechanistic scenario where addition of the
NHC to the acid fluoride results in a temporary change in the
absorption characteristics and photochemical reactivity of the
carbonyl function during the catalytic cycle. This allows these
hitherto unsuitable substrates at the carboxylic acid oxidation
level to engage in a UVA light-mediated photochemical reaction
previously observed only with aromatic aldehydes and ketones.
We believe that photo-NHC catalysis could prove useful for
expanding the scope of carbonyl photochemistry and further
investigations are underway in our laboratory.
Acknowledgements
We thank Leonardo Kleebauer (FU Berlin) for initial experiments
and Prof. Frank Glorius (WWU Münster) for helpful discussions.
This work is funded by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation)
- Project Numbers
420535461; 393271229. Financial support through the DFG
Research Training Network “Fluorine as a Key Element” (GRK
This article is protected by copyright. All rights reserved.

10.1002/anie.201914456
Angewandte Chemie International Edition
COMMUNICATION
[11] First report: a) N. C. Yang, C. Rivas, J. Am. Chem. Soc. 1961, 83, 2213;
selected reviews: b) P. G. Sammes, Tetrahedron 1976, 32, 405; c)
“Photo-enolization and its applications” P. Klán, J. Wirz, A.
Gudmundsdottir in CRC Handbook of Organic Photochemistry and
Photobiology, 3rd ed. (Ed.: A. Griesbeck), CRC, Boca Raton, 2012, chap.
26, pp. 627-651; d) B. Yang, S. Gao, Chem. Soc. Rev. 2018, 47, 7926.
[12] K. Takaki, T. Fujii, H. Yonemitsu, M. Fujiwara, K. Komeyama, H. Yoshida,
Tetrahedron Lett. 2012, 53, 3974.
1582) and the FCI (Sachkostenzuschuss) is also gratefully
acknowledged. Spectroscopic measurements were conducted
with the assistance of the Core Facility BioSupraMol supported by
the DFG.
Keywords: N-Heterocyclic carbenes • photochemistry •
organocatalysis • acid fluorides • photoenolization
[13] An enantioselective PEDA reaction of benzophenones has been
reported using hydrogen-bonding organocatalysts: a) L. Dell'Amico, A.
Vega-Peñaloza, S. Cuadros, P. Melchiorre, Angew. Chem. Int. Ed. 2016,
55, 3313; selected reports involving photoenolization of benzophenones:
b) Y. Masuda, N. Ishida, M. Murakami, J. Am. Chem. Soc. 2015, 137,
14063; b) X. Yuan, S. Dong, Z. Liu, G. Wu, C. Zou, J. Ye, Org. Lett. 2017,
19, 2322; c) T. Ide, S. Masuda, Y. Kawato, H. Egami, Y. Hamashima,
Org. Lett. 2017, 19, 4452; d) L. Dell'Amico, V. M. Fernández-Alvarez, F.
Maseras, P. Melchiorre, Angew. Chem. Int. Ed. 2017, 56, 3304; e) S.
Cuadros, L. Dell'Amico, P. Melchiorre, Angew. Chem. Int. Ed. 2017, 56,
11875; review: f) S. Cuadros, P. Melchiorre, Eur. J. Org. Chem. 2018,
2884.
[1]
Selected reviews: a) T. Bach, J. P. Hehn, Angew. Chem. Int. Ed. 2011,
50, 1000; b) “Chemical Photocatalysis” (Ed.: B. König), De Gruyter,
Berlin, Boston, 2013; c) R. Brimioulle, D. Lenhart, M. M. Maturi, T. Bach,
Angew. Chem. Int. Ed. 2015, 54, 3872; d) D. Aarti, K. Radhika, K. Dinesh,
J. Pooja, C. Ashu, C. K. Ramesh, Curr. Org. Chem. 2015, 19, 2156; e)
M. D. Kärkäs, J. A. Porco, C. R. J. Stephenson, Chem. Rev. 2016, 116,
9683; f) C. Chen, Org. Biomol. Chem. 2016, 14, 8641; g) D. Ravelli, S.
Protti, M. Fagnoni, Chem. Rev. 2016, 116, 9850; h) M. Oelgemöller, N.
Hoffmann, Org. Biomol. Chem. 2016, 14, 7392; i) J. A. Dantas, J. T. M.
Correia, M. W. Paixão, A. G. Corrêa, ChemPhotoChem 2019, 3, 506; j)
M. D'Auria, Photochem. Photobiol. Sci. 2019, 18, 2297.
[14] Selected reviews on NHC organocatalysis: a) D. Enders, O. Niemeier, A.
Henseler, Chem. Rev. 2007, 107, 5606; b) J. Izquierdo, G. E. Hutson, D.
T. Cohen, K. A. Scheidt, Angew. Chem. Int. Ed. 2012, 51, 11686; c) M.
N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 2014, 510,
485; d) D. M. Flanigan, F. Romanov-Michailidis, N. A. White, T. Rovis,
Chem. Rev. 2015, 115, 9307.
[2]
[3]
J. D. Coyle, Chem. Rev. 1978, 78, 97.
Norrish type II eliminations of aromatic esters have been developed that
proceed through photoinduced electron transfer: a) D. P. DeCosta, A. K.
Bennett, J. A. Pincock, J. Am. Chem. Soc. 1999, 121, 3785; reviews on
Norrish-type reactivity with phthalimide derivatives: b) Y. Kanaoka, Acc.
Chem. Res. 1978, 11, 407; c) U. C. Yoon, P. S. Mariano, Acc. Chem.
Res. 2001, 34, 523; d) M. Oelgemöller, A. G. Griesbeck, J. Photochem.
Photobiol. C 2002, 3, 109; e) G. McDermott, D. J. Yoo, M. Oelgemöller,
Heterocycles 2005, 65, 2221.
[15] a) D. Janssen-Müller, S. Singha, T. Olyschläger, C. G. Daniliuc, F.
Glorius, Org. Lett. 2016, 18, 4444; b) D.-F. Chen, T. Rovis, Synthesis
2017, 49, 293.
[16] H. Wang, X. Chen, Y. Li, J. Wang, S. Wu, W. Xue, S. Yang, Y. R. Chi,
Org. Lett. 2018, 20, 333.
[4]
Selected reviews: a) M. N. Hopkinson, B. Sahoo, J.-L. Li, F. Glorius,
Chem. Eur. J. 2014, 20, 3874; b) K. L. Skubi, T. R. Blum, T. P. Yoon,
Chem. Rev. 2016, 116, 10035; c) J. Twilton, C. Le, P. Zhang, M. H. Shaw,
R. W. Evans, D. W. C. MacMillan, Nature Rev. Chem. 2017, 1, 0052.
Photochemical activation has been combined with other organocatalysis
modes: review: M. Silvi, P. Melchiorre, Nature 2018, 554, 41.
[17] Y. Hu, D. Pan, L. Cong, Y. Yao, C. Yu, T. Li, C. Yao, ChemistrySelect
2018, 3, 1708.
[18] Related reports: with indoles: a) X. Chen, S. Yang, B.-A. Song, Y. R. Chi,
Angew. Chem. Int. Ed. 2013, 52, 11134; with salicylic acids: b) X. Chen,
H. Wang, K. Doitomi, C. Y. Ooi, P. Zheng, W. Liu, H. Guo, S. Yang, B.-
A. Song, H. Hirao, Y. R. Chi, Nat. Commun. 2017, 8, 15598; with
anthranilic acids: c) A. Lee, J. L. Zhu, T. Feoktistova, A. C. Brueckner, P.
H.-Y. Cheong, K. A. Scheidt, Angew. Chem. Int. Ed. 2019, 58, 5941.
[19] Selected examples: a) S. J. Ryan, L. Candish, D. W. Lupton, J. Am.
Chem. Soc. 2009, 131, 14176; b) S. J. Ryan, S. D. Schimler, D. C. Bland,
M. S. Sanford, Org. Lett. 2015, 17, 1866; c) R. M. Gillard, J. E. M.
Fernando, D. W. Lupton, Angew. Chem. Int. Ed. 2018, 57, 4712.
[20] The NMR yield of 3aa obtained under catalytic conditions (88%) is
significantly higher than that observed in the stoichiometric reaction from
1 (62%). The origins of this discrepancy are not clear but could reflect
the very different reactant stoichiometries.
[5]
[6]
[7]
Seminal report combining NHC organocatalysis and photoredox
catalysis: D. A. DiRocco, T. Rovis, J. Am. Chem. Soc. 2012, 134, 8094.
Selected reviews: a) H. U. Vora, P. Wheeler, T. Rovis, Adv. Synth. Catal.
2012, 354, 1617; b) S. J. Ryan, L. Candish, D. W. Lupton, Chem. Soc.
Rev. 2013, 42, 4906; c) X.-Y. Chen, Q. Liu, P. Chauhan, D. Enders,
Angew. Chem. Int. Ed. 2018, 57, 3862.
[8]
Highlight: a) R. Song, Y. R. Chi, Angew. Chem. Int. Ed. 2019, 58, 8628;
selected examples: b) Y. Du, Y. Wang, X. Li, Y. Shao, G. Li, R. D.
Webster, Y. R. Chi, Org. Lett. 2014, 16, 5678; c) N. A. White, T. Rovis,
J. Am. Chem. Soc. 2014, 136, 14674; d) N. A. White, T. Rovis, J. Am.
Chem. Soc. 2015, 137, 10112; e) Y. Zhang, Y. Du, Z. Huang, J. Xu, X.
Wu, Y. Wang, M. Wang, S. Yang, R. D. Webster, Y. R. Chi, J. Am. Chem.
Soc. 2015, 137, 2416; f) W. Yang, W. Hu, X. Dong, X. Li, J. Sun, Angew.
Chem. Int. Ed. 2016, 55, 15783; g) X. Wu, Y. Zhang, Y. Wang, J. Ke, M.
Jeret, R. N. Reddi, S. Yang, B.-A. Song, Y. R. Chi, Angew. Chem. Int.
Ed. 2017, 56, 2942; h) W. Yang, D. Ma, Y. Zhou, X. Dong, Z. Lin, J. Sun,
Angew. Chem. Int. Ed. 2018, 57, 12097; i) T. Ishii, Y. Kakeno, K. Nagao,
H. Ohmiya, J. Am. Chem. Soc. 2019, 141, 3854; j) T. Ishii, K. Ota, K.
Nagao, H. Ohmiya, J. Am. Chem. Soc. 2019, 141, 14073.
[21] UV-Vis measurements show that azolium salt 1 exhibits significantly
higher absorbance at λ = 365 nm than acid fluoride 4a or ketone 2a with
a
cut-off at ca. 400 nm. Nevertheless, given the much higher
concentrations of 4 and 2, sensitization by either compound cannot be
entirely ruled out under catalytic conditions (see SI).
[22] a) Gaussian 16, Revision C.01, M. J. Frisch et al., Gaussian, Inc.,
Wallingford CT, 2016; b) T. Yanai, D. P. Tew, N. C. Handy, Chem. Phys.
Lett. 2004, 393, 51; c) P. C. Hariharan, J. A. Pople, Theor. Chim. Acta
1973, 28, 213.
[9]
Radical species have been proposed as intermediates in NHC-catalyzed
reactions of aldehydes: J. Rehbein, S.-M. Ruser, J. Phan, Chem. Sci.
2015, 6, 6013.
[10] NHCs have been employed to stabilize organic and inorganic radicals:
review: a) C. D. Martin, M. Soleilhavoup, G. Bertrand, Chem. Sci. 2013,
4, 3020; see also: b) J. K. Mahoney, D. Martin, C. E. Moore, A. L.
Rheingold, G. Bertrand, J. Am. Chem. Soc. 2013, 135, 18766.
This article is protected by copyright. All rights reserved.

10.1002/anie.201914456
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Andreas Mavroskoufis, Keerthana
Rajes, Paul Golz, Arush Agrawal,
Vincent Ruß, Jan P. Götze, Matthew N.
Hopkinson*
Page No. – Page No.
N-Heterocyclic Carbene-Catalyzed
Photoenolization Diels-Alder
Reaction of Acid Fluorides
Photo-NHC Catalysis has been introduced as a strategy for engaging hitherto
unsuitable carbonyl substrates at the carboxylic acid oxidation level in
photochemical transformations of aldehydes and ketones. This approach enables
an NHC-catalyzed photoenolization Diels-Alder (PEDA) reaction of acid fluorides
leading to isochroman-1-ones. A mechanism where the NHC acts as a “transient
aryl group” that temporarily changes the photochemical properties of the carbonyl
substrate during the catalytic cycle is proposed.
This article is protected by copyright. All rights reserved.