Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
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-Universität Marburg, Hans-Meerwein-Straße 4, D-35032 Marburg, Germany
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S Supporting Information *
ABSTRACT: 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.
U
Scheme 2. Context of This Work I
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 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 azaanalogues,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 photochemistry 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 corresponding 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, © XXXX American Chemical Society
Received: May 6, 2019
A
DOI: 10.1021/acs.orglett.9b01585 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 4. Synthetic Access to α-Naphthyl Acrylates
Scheme 3. Context of This Work II
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 orthoxylylene 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 [%]
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
42a 72 83 89 95 98 b
93c
a
1 equiv of t-BuNH2 added. 56% conv. bExclusion of light. c1 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 electronwithdrawing group (18g) and the synthetically versatile pinacolate (18c) also remain intact during the photocyclization. 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
Letter
Organic Letters phenyl analogue 33. This is most likely due to the triplet energy of 33 being inaccessible to photocatalyst 28. Noteworthy 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
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 (αnaphthyl) 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. Stereoinformation 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
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 2,6disubstituted aromatic ketones 24, trapping experiments with dienophiles such as N-phenyl maleimide during the diastereoselective 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 conformaC
DOI: 10.1021/acs.orglett.9b01585 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 8. Synthetic Transformations of 17a and 17n
Scheme 9. Synthetic Routes to Naphthocyclobutenes
described. Methyl (α-naphthyl) acrylates bearing an orthosubstituent with a hydrogen atom produce naphthocyclobutenes upon Ir(Fppy)3-mediated photosensitization. Mechanistic 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.
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tion of disubstituted ortho-xylylene 41 favors the intramolecular sigmatropic H-shift over the intermolecular cycloaddition. 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. The photochemical synthesis of naphthocyclobutenes (15 → 44, R = COOCH3) described here complements previous accesses (Scheme 9). In an early report, the Wolff rearrangement 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 In conclusion, a novel carbon analogue of the Yang cyclization furnishing substituted naphthocyclobutenes is
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01585. 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
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 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
The authors declare no competing financial interest. D
DOI: 10.1021/acs.orglett.9b01585 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
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DOI: 10.1021/acs.orglett.9b01585 Org. Lett. XXXX, XXX, XXX−XXX
