Synthesis of Naphthocyclobutenes from α-Naphthyl Acrylates by Visible-Light Energy-Transfer Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b01585

Methyl (α-naphthyl) acrylates bearing an ortho-substituent with a hydrogen atom produce naphthocyclobutenes upon Ir(Fppy)₃-mediated photosensitization. This reaction can be described as a carbon analogue of the Norrish-Yang reaction: upon triplet excitation of the acrylate a 1,5-HAT results in a 1,4-diradical which forms the cyclobutene. Diastereoselectivities up to >20:1 were observed for the ring-closure reaction.

Full text of DOI:10.1021/acs.orglett.9b01585

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Naphthocyclobutenes from α‑Naphthyl Acrylates by
Visible-Light Energy-Transfer Catalysis
Theodor Peez, Veronika Schmalz, Klaus Harms, and Ulrich Koert*
̈
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße 4, D-35032 Marburg, Germany
S
* Supporting Information
ABSTRACT: Methyl (α-naphthyl) acrylates bearing an
ortho-substituent with a hydrogen atom produce naphthocy-
clobutenes upon Ir(Fppy)3-mediated photosensitization. This
reaction can be described as a carbon analogue of the
Norrish−Yang reaction: upon triplet excitation of the acrylate
a 1,5-HAT results in a 1,4-diradical which forms the
cyclobutene. Diastereoselectivities up to >20:1 were observed
for the ring-closure reaction.
pon photoexcitation and intersystem crossing (ISC),
ketones bearing a γ-hydrogen atom (1) form the
corresponding triplet 2. After 1,5-hydrogen atom transfer
(HAT, Scheme 1) the resulting 1,4-diradical intermediate 3
Scheme 2. Context of This Work I
U
Scheme 1. Norrish Type II Photochemistry
substrates 15 bearing an ortho-substituent with a hydrogen
produce naphthocyclobutenes 18. We will show that this
reaction can be described as a carbon analogue of the
Norrish−Yang II photoreaction: upon sensitization of 15 to
the triplet 16 a subsequent 1,5-HAT gives the 1,4-diradical 17.
By ISC, cyclobutene 18 is formed. This process is either direct
or proceeds via a ground state 19 which could be described as
an ortho-xylylene.
For ortho-alkyl styrenes 20 the photoinduced HAT to 21 is
known (Scheme 3).7 Yet, no Yang-type cyclization to 23 was
described. Instead, the formation of ortho-xylylene 22 was
detected by trapping with dienophiles. This cycloaddition has
extensively been applied to dienes 26 (R = H)8 and their aza-
analogues,9 resulting from the photoenolization of aromatic
either forms cyclobutanol 4 (Yang cyclization) or fragments to
an enol 5 and an alkene 6.1 This Norrish−Yang photo-
chemistry is well established and used in complex molecule
synthesis.2 Carbon to oxygen HAT (2 → 3) is a
thermodynamically favored irreversible process and is well
established, as in photoenolization (7 → 8).3 The correspond-
ing carbon to carbon HAT (10 → 11) is reversible and less
developed.4 Here, we show carbon to carbon HAT to be a
useful tool for the construction of naphthocyclobutenes.
In previous work, we have employed energy transfer (EnT)
catalysis5 to sensitize methyl (α-naphthyl) acrylates 12 to
triplet 13. In the presence of a primary amine as a proton
shuttle, substituted acenaphthenes 14 are formed (Scheme 2).6
Substituents such as an ester or a chlorine in ortho-position to
the acrylate inhibit acenaphthene formation. Remarkably,
Received: May 6, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01585
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Letter
Scheme 3. Context of This Work II
Scheme 4. Synthetic Access to α-Naphthyl Acrylates
to steric encumberment of the halogen. This could be
circumvented by employing naphthyl iodides (30) in
conjunction with the less active Ph(PPh3)4 as catalyst. In
cases where the corresponding naphthyl acetic acids 31 were
readily accessible, condensation of formaldehyde under phase
transfer conditions was employed.
The formation of the cyclobutene ring was found to be
compatible with a variety of substitution and annelation
patterns (Scheme 5). Substitutions by an additional phenyl
ring (18b) or an alkyl residue (18d) are tolerated at different
positions of the naphthyl ring.
ketones 24 (R = H). 2,6-Disubstitution of these ketones leads
to cyclobutenols 25.10 For this case, the presence of an ortho-
xylylene intermediate could not be confirmed by trapping with
a dienophile.3a,10a,b
The starting point for our studies was the finding that
acrylate 15a formed naphthocyclobutene 18a as the main
product under the conditions employed for the synthesis of
acenaphthenes6 (Table 1, entry 1). In order to optimize the
Table 1. Course of Optimization
Scheme 5. Scope of the Cyclization on the Naphthyl Ring
#
Ir-cat.
Ir-cat. [mol %]
T [°C]
t [h]
yield 18a [%]
a
1
2
3
4
5
6
7
8
27
27
27
28
28
28
28
28
3
3
3
3
1
1
1
1
40
50
60
60
60
100
100
60
36
120
48
48
48
18
18
48
42
72
83
89
95
98
b
c
93
a
b
c
1 equiv of t-BuNH2 added. 56% conv. Exclusion of light. 1 mmol
scale.
yield, the amine base was omitted and reaction time prolonged,
which resulted in complete conversion (entry 2). Raising the
temperature as well as employing energy transfer catalyst 28
with a slightly smaller van der Waals radius,11 higher solubility
in toluene12 and slightly lower triplet state energy13 than 27
improved the yield and allowed lowering of catalyst loading to
1 mol % (entries 3, 4, and 5). The yield could be maintained at
a higher temperature which allowed shortening of reaction
time (entry 6). A control excluding light was negative (entry
7). Changes in solvent or energy transfer catalyst proved to be
detrimental (see Supporting Information for details and effect
of reaction conditions).
With the reaction conditions optimized, the scope of the
photocyclization with respect to variations on the naphthyl
ring was investigated. To this end, a convenient synthetic
access to α-naphthyl acrylates 15 was needed (Scheme 4). We
found our previously employed Stille coupling of stannane 29
with bromides using a PdCl2/P(tBu)3 system6 to furnish
inseparable byproducts of cine-substitution,14 most likely due
Heteroaromatics such as quinolines and isoquinolines can be
used instead of the naphthalene (18e,f,g). An electron-
withdrawing group (18g) and the synthetically versatile
pinacolate (18c) also remain intact during the photocycliza-
tion. The annelation also proceeded smoothly on a larger
aromatic system (18h). Switching positions of the methyl
group and acrylate furnished isomeric naphthocyclobutene 18i.
The reaction could not be performed on indole 32 or the
B
DOI: 10.1021/acs.orglett.9b01585
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
phenyl analogue 33. This is most likely due to the triplet
energy of 33 being inaccessible to photocatalyst 28. Note-
worthy is the fact that substitution on the naphthyl ring
necessitated increased reaction times.
Substitution at the ortho-methyl group could lead to
naphthocyclobutenes with additional substituents on the
four-membered ring (Scheme 6). One additional methyl
Stern−Volmer quenching experiment (Figure S5) providing
evidence for a reaction via the triplet state. CV measurements
show the redox potential of acrylate 15a to lie outside of that
of Ir(Fppy)3 (Figure S1).13 Thus, a triplet energy transfer
instead of a redox-based mechanism seems likely. In order to
examine the proposed 1,5-HAT, isotope-labeled analogue 15t
was subjected to photocyclization (Scheme 7).
Scheme 6. Scope of the Cyclization on the ortho-Substituent
Scheme 7. Mechanistic Considerations
The formation of deuterated naphthocyclobutene 18t
provides evidence for a 1,5-HAT. The kinetic isotope effect
(KIE) of 4.23 is in the range of a primary KIE17 and shows C−
H/D bond dissociation to be the rate-determining step of the
cyclization. These experiments provide evidence for the HAT
mechanism in Scheme 2 (16 → 17). For the stereochemical
outcome of the photocyclizations, the ISC of the 1,4-diradical
has to be considered.18 In the case of an alkyl substituent,
steric hindrance of the two sp3 substituents favors an ISC
pathway from the triplet diradical 34 T1 to the cis-cyclobutene
18k. In contrast, the substitution pattern on the triplet
diradical 35 favors an increasing ionic character within the
ISC19 which can be schematically simplified as 36 and results
in the trans-cyclobutene 18l. As for the aforementioned 2,6-
disubstituted aromatic ketones 24, trapping experiments with
dienophiles such as N-phenyl maleimide during the diaster-
eoselective photocyclizations did not indicate the presence of
ortho-xylylenes (Supporting Information, Section 3.6). As the
steric strain in these tri- or tetrasubstituted intermediates
should be rather high, we consider the involvement of
electrocyclic ring-closing reactions for the diastereoselective
cyclobutene formations as less plausible.
Benzocyclobutenes have proven to be useful intermediates
in organic synthesis.20,8h With different naphthocyclobutenes
in hand, we began examining their synthetic utility (Scheme
8). Reduction of the ester 18a with LiAlH4 gave the alcohol 37.
Heating of acid 38 most likely led to the ortho-xylylene 39 by
electrocyclic ring opening which then gave lactone 40 by
electrocyclic ring closure.21 Heating of ester 18a to 180 °C
gave the acrylate 15a, which can be rationalized by 1,5
sigmatropic H-shift of the disubstituted ortho-xylylene 41. This
reaction did not occur at 100 °C. No trapping of 41 by
cycloaddition with a dienophile (42) was possible. As shown
by the energy-optimized geometry, the nonplanar conforma-
group resulted in the diastereoselective formation of the
corresponding napthocyclobutene 18k. Its X-ray crystal
structure secured the formation of the four-membered ring
and the relative configuration at the two stereocenters. NOE
data were used to assign the relative configurations for the
other cases. With one alkyl substituent the diastereomer with a
cis configuration of the alkyl group and the ester was observed
(18k, 18q). With an ether substituent the diastereomer with a
trans configuration of the ether group and the ester was formed
(18l,m,n). A limitation to diastereoselectivity is presented by
18o. Noteworthy is the formation of naphthocyclobutenes
with two adjacent quarternary centers such as 18m,o,p,r. No
Yang-type cyclization was observed for the difluoromethyl (α-
naphthyl) acrylate 15s. As acrylates 15k−s are present as
atropisomers on the NMR time scale (∼22 ms15), a
conservation of stereochemical information was envisioned.
15m was thus synthesized in enantiopure form. Stereo-
information was found to be erased, and the racemic
naphthocyclobutene 18m was formed, as also observed in
related photoenolization processes.16
In addition to the substrate scope studies, mechanistic
aspects of the napthocyclobutene formation were addressed.
Plotting of conversion against reaction time showed the
reaction to be pseudo-zeroth-order (Figure S7). Energy
transfer from Ir(Fppy)3 to acrylate 15a was confirmed by a
C
DOI: 10.1021/acs.orglett.9b01585
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Scheme 8. Synthetic Transformations of 17a and 17n
Scheme 9. Synthetic Routes to Naphthocyclobutenes
described. Methyl (α-naphthyl) acrylates bearing an ortho-
substituent with a hydrogen atom produce naphthocyclobu-
tenes upon Ir(Fppy)3-mediated photosensitization. Mechanis-
tic experiments show a carbon−carbon 1,5-HAT followed by
cyclization to be the operational method. The functional group
tolerance on the methyl group as well as the naphthyl ring was
investigated, allowing donor−acceptor structures to be
synthesized, among others, in high diastereoselectivity. This
work shows carbon to carbon HAT to be a valuable tool for
the construction of C−C bonds by employing energy transfer
catalysis.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01585.
tion of disubstituted ortho-xylylene 41 favors the intra-
molecular sigmatropic H-shift over the intermolecular cyclo-
addition. ortho-Xylylenes resulting from benzocyclobutenes can
be planar which favors their known cycloaddition reactions. As
substrates 18k, l, and m represent donor−acceptor-substituted
naphthocyclobutenes, we were intrigued by the possibility to
access 1,4-dipolar structures by heterolytic bond cleavage.
Indeed, subjecting naphthocyclobutene 18n to typical
conditions for the opening of donor−acceptor cyclopropanes
gave the aldehyde 43.
■
S
Experimental details, spectroscopic and analytical data of
all new compounds, and mechanistic studies (PDF)
Accession Codes
CCDC 1913562 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The photochemical synthesis of naphthocyclobutenes (15
→ 44, R = COOCH3) described here complements previous
accesses (Scheme 9). In an early report, the Wolff rearrange-
ment of 49 was used by Horner to obtain naphthocyclobutene
carboxylic acid.22 Later, [2 + 2] photocycloadditions of
electron-deficient alkenes (46) to electron-rich naphthalenes
(45) lead to derivatives which found application in synthesis.23
Flash-vacuum pyrolysis,24 the Finkelstein reaction,25 or [4 + 2]
cycloadditions to arynes26 have also been employed. In the
recent past, transition-metal-enabled reactions have been
developed, such as an annulation via Gold vinylidene
intermediate 4727 or aryne−zirconocene complex 48.28
Palladium-catalyzed C−H activation has been employed to
construct heteroaromatic and benzocyclobutene analogues of
18.29 Intramolecular cycloadditions of allenes to alkynes have
also been shown to furnish naphthocyclobutenes 44 with
certain substitution patterns.30
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: koert@chemie.uni-marburg.de.
ORCID
Ulrich Koert: 0000-0002-4776-8549
Author Contributions
Experimental work was accomplished by T.P. and V.S., and the
X-ray crystal structure was solved by K.H.
Notes
In conclusion, a novel carbon analogue of the Yang
cyclization furnishing substituted naphthocyclobutenes is
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.9b01585
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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ACKNOWLEDGMENTS
■
Financial support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
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