Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rhodium-Catalyzed Carbonylative Synthesis of Benzosilinones Bo Chen†,‡ and Xiao-Feng Wu*,†,‡ †
Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou 310018, People’s Republic of China Leibniz-Institut für Katalyse e. V., Universität Rostock, Albert-Einstein-Straβe 29a, 18059 Rostock, Germany
‡
Org. Lett. Downloaded from pubs.acs.org by QUEEN MARY UNIV OF LONDON on 04/01/19. For personal use only.
S Supporting Information *
ABSTRACT: In this work, a novel and practical procedure for the synthesis of benzosilinones by carbonylative cyclization has been developed. Various benzosilinones were isolated in moderate to good yields, using rhodium as the catalyst with good functional group tolerance. Not only symmetric alkynes but also nonsymmetric alkynes are applicable with excellent regioselectivity and good yields. Remarkably, this is the first procedure for benzosilinone synthesis which is general and practical.
O
attempt at testing was carried out with diphenylacetylene in a Schlenk tube using Mo(CO)6 as the CO source. To our delight, the target 1,1-dimethyl-2,3-diphenylbenzo[b]silin4(1H)-one can be successfully obtained with [RhCl(cod)]2 as the catalyst in toluene at 80 °C for 12 h. Then immediately systematic studies of the reaction parameters were carried out (Table 1), we finally succeeded to get the desired product in 76% yield and 74% isolated yield. Conversely, no desired product was generated if we used Pd(OAc)2 as the catalyst (entry 2). RhCl3 can give the desired benzosilinone as well, but with lower efficiency (entry 3). Changing bipyridine to other bidentate nitrogen ligands or PPh3 (similar results were obtained with DPPE or Xantphos as the ligand) all gave decreased yields (entries 4−7), and 55% yield can be obtained without any ligand in the reaction (entry 8). Examination of various solvents indicated that toluene is the best reaction media for this transformation (entries 9−11). Bases screening disclosed that TMEDA is the optimal base here (entries 12− 15). When the amount of Mo(CO)6 was varied, we found that 1 equiv of Mo(CO)6 was needed to give the best yield (entries 16 and 17). For the substrate analogues, (2-iodophenyl)dimethylsilane gave even better reactivity in this reaction (entry 18). However, no desired product could be achieved from (2-chlorophenyl)dimethylsilane, which was mainly due to the high energy of the C−Cl bond (entry 19). Moreover, X-ray single-crystal structure analysis of 3aa was performed and confirmed the product structure (Figure 1). Notably, the product of silylformylation can be detected in our optimization stage.5 Encouraged by these findings and taking advantage of the use of aryl bromides, we then tested the generality of (2bromoaryl)dimethylsilane in this carbonylative cyclization reaction. As shown in Scheme 1, the 2-bromoaryldimethylsilyl moiety with electron-donating or -withdrawing substituents can be well tolerated, delivering the desired products in moderate to good yields (3aa−ga). Additionally, 71% isolated
rganosilicon compounds have received increasing interest in recent years due to their wide applications in organic chemistry, material science, and optoelectronic and electronic devices.1 Among them, silicon-containing benzofused heterocycle is even more attractive. However, most of the efforts have been focused on the preparation of four- and five-membered benzo-fused silicon-containing heterocycles.2,7−9 The method for preparing benzosilinones is still very rarely reported.3 Kende’s group developed a procedure based on the oxidation of 1-silyl-1,4-dihydronaphthalene.3a Recently, Song and co-workers described one example using BF3·OEt2-promoted cyclization/oxidation of (Z)-2-(dimethyl(1-phenyl-2-(trimethylsilyl)vinyl)silyl)benzaldehyde in their study on bis-silylation of alkynes.3b More recently, Anslyn’s group found that Na3PO4 is able to catalyze the direct oxidation of siliconrhodamine pyronine to the corresponding silicon rhodamine xanthone in good yield.3c However, the substrates’ applicability is still very limited in those cases. On the other hand, transition-metal-catalyzed carbonylative transformations have been accepted as a powerful toolbox for the preparation of carbonyl-containing compounds. However, in the attempts to combine silane with carbonylation, usually two types of transformations occur: (1) reductive carbonylation of aryl halides4a and (2) silylformylation of alkenes and alkynes,5 where silane acts as the reductant in both cases. No carbonylative synthesis of benzosilinone could be realized, which might due to the interaction between carbon monoxide and silane.6 We have been interested in overcoming this challenge during the past years, but without real success.4b Recently, inspired by the excellent contributions from Xi’s group7 and Chatani’s group,8 and also the DFT understanding from Lin’s group,9 we have finally been able to reach this goal. Based on the achievements of Xi and Chatani, we initially selected 2-(trimethylsilyl)phenyl bromide and diphenylacetylene as the model substrates under CO pressure, but no desired benzosilinone could be detected; then (trimethylsilyl)phenylboronic acid was prepared and tested, but again no desired product could be obtained. After introspection, we believe (2-bromophenyl)dimethylsilane can be a choice. An © XXXX American Chemical Society
Received: March 15, 2019
A
DOI: 10.1021/acs.orglett.9b00930 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Selected Results of the Optimization of the Reaction Conditionsa
entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
variations from the standard conditions Pd(OAc)2 (4 mol %) instead of [RhCl(cod)]2 (2 mol %) RhCl3·3H2O (4 mol %) instead of [RhCl(cod)]2 (2 mol %) PPh3 instead of bipridine L1 instead of bipyridine L2 instead of bipyridine L3 instead of bipyridine no ligand THF instead of toluene m-Xylene instead of toluene DMF instead of toluene K2CO3 instead of TMEDA KOAc instead of TMEDA Et3N instead of TMEDA DBU instead of TMEDA using 0.8 equiv of Mo(CO)6 using 0.3 equiv of Mo(CO)6 (2-iodophenyl)dimethylsilane instead of 1a (2-chlorophenyl)dimethylsilane instead of 1a
Scheme 1. Synthesis of Benzosilinones: Variation of Silanesa
yield[%]b 76 (74)c 0 57 49 49 53 40 55 34 60 0 18 10 65 6 60 43 79 0
a
Reaction scale: 0.20 mmol, isolated yields.
(3an, 3ao). Several symmetric and unsymmetrical aliphatic alkynes were also incorporated, delivering the corresponding products in moderate to good yields (3as−av). Notably, excellent regioselectively can be achieved when alkylphenylacetylenes are used as examples of unsymmetrical alkynes and yielded the corresponding heterocycles bearing an alkyl group at the 2-position in good yields (3aw−aaa). However, it is also important to mention that the reaction failed when phenylacetylene or nitro-substituted internal alkyne was applied (see the Supporting Information). Concerning the reaction pathway, on the basis of our results and the literature,5,10,11 a possible reaction mechanism is proposed and shown in Scheme 3. At the beginning, rhodium inserts into the Si−H bond to form a Si−Rh−H complex A. Then the formed compound A adds to the internal alkyne to obtain complex B. Meanwhile, carbon monoxide will be released from Mo(CO)6 based on the coordination of TMEDA with a Mo metal center. The free carbon monoxide then coordinates and inserts into the Rh−C bond to give the silylformylation product C,5 which will be subsequently transformed into the final product with the assistance of rhodium catalyst.11 Finally, the active rhodium catalyst will be regenerated in the presence of base for the next catalytic cycle. Finally, in order to make this process more practical, we studied the possibility of performing this novel transformation under carbon monoxide pressure (for optimization details, see the Supporting Information). To our delight, under the same reaction conditions, by simply replacing Mo(CO)6 with 5 bar of carbon monoxide gas, the reaction proceeded well and gave the corresponding products in moderate to good yields (Scheme 4). When substrates were varied, different functional groups on both substrates were all well tolerated, even the bromide group and heterocycles. In conclusion, we have developed a novel and practical procedure for the synthesis of benzosilinones from (2-
a
Reaction scale: 0.20 mmol. bGC yield. cIsolated yield. THF = tetrahydrofuran. DMF = N,N-dimethylformide. TMEDA = N,N,N′,N′- tetramethylethan-1,2-diamin. DUB= 1,8diazabicyclo[5.4.0]undec-7-ene.
Figure 1. Crystal structure of 3aa (CCDC 1876192); thermal ellipsoids are drawn at the 30% probability level.
yield of 1,1-diphenyl-2,3-diphenylbenzo[b]silin-4(1H)-one (3ha) can be produced from its parent molecule as well. Then the substrate scope of different types of internal alkynes was investigated (Scheme 2). Moderate to good yields can be achieved in using symmetric aromatic alkynes including heterocyclic or polycyclic alkynes (3ab−am). Interestingly, when ortho-position was occupied, a mixture of atropisomers was obtained because of the hindrance of the ortho-position (3ap−ar). Subsequently, unsymmetrical aromatic alkynes were tested, and we obtained a mixture of isomers with a 1:1 ratio B
DOI: 10.1021/acs.orglett.9b00930 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Scheme 4. Synthesis of Benzosilinones with CO Gasa
Scheme 2. Synthesis of Benzosilinones: Variation of Alkynes*
a
Reaction scale: 0.20 mmol, isolated yields.
bromophenyl)dimethylsilane and internal alkynes. With rhodium as the catalyst and Mo(CO)6 as the CO source, the desired silane-containing six-membered benzo-fused heterocycles were isolated in moderate to good yields with good functional group tolerance. When unsymmetrical alkylphenylacetylenes were used as the substrates, excellent regioselectively was achieved with good yields as well. To the best of our knowledge, this is the first procedure for benzosilinone synthesis which is general and practical.
* Reaction scale: 0.20 mmol, isolated yields. aThe form of isomer is like 3an. bThe form of atropisomer is like 3ap.
Scheme 3. Proposed Reaction Mechanism
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00930. Experimental procedures and analysis data for all new compounds. X-ray crystallographic data for compound 3aa (PDF) Accession Codes
CCDC 1876192 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. C
DOI: 10.1021/acs.orglett.9b00930 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiao-Feng Wu: 0000-0001-6622-3328 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.B. thanks the Chinese Scholarship Council (CSC) for financial support. The analytical support of Dr. Anke Spannenberg, Dr. W. Baumann, Dr. C. Fisher, S. Buchholz, and S. Schareina (all in LIKAT) is gratefully acknowledged. We also appreciate the generous support from Professor Armin Börner and Professor Matthias Beller in LIKAT.
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REFERENCES
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DOI: 10.1021/acs.orglett.9b00930 Org. Lett. XXXX, XXX, XXX−XXX
