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N-Heterocyclic Carbene-Mediated Transformations of Silicon Reagents ⇑ ⇑ ⇑ Lin He , Hao Guo, Ying Wang, Guang-Fen Du , Bin Dai

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N-Heterocyclic Carbene-Mediated Transformations of Silicon Reagents ⇑ ⇑ ⇑ Lin He , Hao Guo, Ying Wang, Guang-Fen Du , Bin Dai Tetrahedron Letters 56 (2015) 972–980 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet Digest Paper N-heterocyclic carbene-mediated transformations of silicon reagents ⇑ ⇑ ⇑ Lin He , Hao Guo, Ying Wang, Guang-Fen Du , Bin Dai The Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, School of Chemistry and Chemical Engineering, Shihezi University, Xinjiang Uygur Autonomous Region, 832000, China article info abstract Article history: This review summarizes developments in N-heterocyclic carbene-mediated transformation of silicon Received 29 July 2014 reagents over the past decade. Several reactions of silicon reagents, including the sila-Stetter reaction, Revised 7 December 2014 1,2-addition, carbon–carbon and carbon–oxygen double bond reduction, polymerization, and other mis- Accepted 2 January 2015 cellaneous reactions, are discussed. The mechanisms of these reactions are also presented. Available online 12 January 2015 Ó 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: N-heterocyclic carbenes Silicon reagents Stetter reaction 1,2-Addition Polymerization Reduction Contents Introduction. ......................................................................................................... 972 NHC-catalyzed sila-Stetter reaction of acylsilanes . ...................................................................... 973 1,2-Nucleophilic addition reactions . ...................................................................... 974 NHC-catalyzed cyanation and trifluoromethylation reactions . ............................................................ 974 NHC-catalyzed aldol reactions . ............................................................................ 974 NHC-catalyzed transformations of silanes. ...................................................................... 975 NHC-catalyzed polymerization involving silicon reagents ...................................................................... 976 Other reactions . ......................................................................................... 976 Ring-opening reactions . ............................................................................ 976 NHC-catalyzed MBH adducts . ............................................................................ 977 NHC-catalyzed annulation . ............................................................................ 978 NHC-catalyzed C–Si bond formation reaction ............................................................................ 978 Conclusion . ......................................................................................................... 979 Acknowledgments . ......................................................................................... 979 References and notes . ......................................................................................... 979 Introduction been used as versatile organocatalysts to catalyze a series of organic reactions.3 More than half a century ago, Breslow proposed The past decade has witnessed tremendous growth in the a catalytic model for the thiazolium salt-catalyzed benzoin con- chemistry of N-heterocyclic carbenes (NHCs). NHCs have served densation reaction, and in situ-generated thiazolylidene (the as essential building blocks in synthetic chemistry1 and, owing to NHC) was assumed to be the reactive species in this reaction.4 In their excellent electronic properties, have been utilized as strong 1991, the first stable NHC was isolated and characterized by Ardu- r-donor ligands for transition-metal catalysts.2 NHCs have also engo et al.5 These two ground-breaking works paved the way for the development of NHC catalysis. In 2004, the homoenolate reac- ⇑ Corresponding authors. Fax: +86 993 2057270. tion based on the NHC-catalyzed conjugate umpolung of enals was 6 E-mail address: [email protected] (L. He). independently developed by Glorious and Bode. In the same year, http://dx.doi.org/10.1016/j.tetlet.2015.01.034 0040-4039/Ó 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). L. He et al. / Tetrahedron Letters 56 (2015) 972–980 973 R1 O O O O sila-Stetter 30 mol% 3a, DBU n C 1 + 2 3 2 3 reaction y R SiMe R R S tio a 3 R R c n re n u io a a Br d t c t 1a 4 e c t io 2 a io n N R re n HO(H2C)2 Et R1 M u NHC 3a n r k e o X a i a t a y l l d i c y a o N Y 3 o 1 z 2 t a i R i R O R o l P m r n O e a m O + R2 R3 SNR R1R2R3SiX e 4 1 m n R SiMe3 o io O R u t i l p n if la e g r y n T in th g n R tio N OSiMe3 Form nyla An al tan on R1 nula S acti OH 1 S tion re S OH R N 7 5 R2 R3 R Scheme 1. NHC-mediated transformation of silicon reagent. R'OH OH O S R1 R2 R3 redox transformation of a-functional aldehydes was indepen- 2 N 7 R'OSiMe R dently reported by Bode and Rovis. In 2008, Ye and co-workers 3 6 demonstrated the first example of NHC-catalyzed cycloaddition 8 of ketenes. Chi and co-workers recently demonstrated that NHCs Scheme 2. NHC-catalyzed sila-Stetter reaction. can be used to activate esters,9 which may be assumed to be another novel activation model of NHCs. Over the past decade, a large number of reactions promoted by NHCs have been investigated, and many excellent reviews have 2 been published in this research field. Besides the diverse reactions O R HOAc already mentioned, NHCs also exhibit highly catalytic activities 1 R SiMe3 1 3 10 R O R toward organic transformations of silicon reagents. A number 1a 1 R O 8 of reactions, such as the sila-Stetter reaction, 1,2-addition, C@C 20 mol% 3a, DBU O @ + R2 and C O bond hydrosilylation, polymerization, and other miscella- O THF, i-PrOH R2 R3 neous reactions, have been studied. The current review mainly 4 4 R NH2 R2 R3 1 3 focuses on the development of NHC-mediated transformations that R N R involve silicon reagents (Scheme 1). 2 R4 9 NHC-catalyzed sila-Stetter reaction of acylsilanes Scheme 3. One-pot synthesis of furans and pyrroles using NHC-catalyzed sila- Stetter reaction. The NHC-catalyzed conjugate addition of acyl anions and Michael acceptors, also called the Stetter reaction,11 is a convenient approach for the synthesis of 1,4-dicarbonyl compounds. However, acyl anions that are generated in situ through NHC-catalyzed R3 umpolung of aldehydes can also react with a secondary aldehyde 2 NO2 molecule; thus, benzoin condensation is unavoidable in this pro- R 4 R O 11 R cess. This problem can be overcome by using acylsilanes instead R3 NO2 of aldehydes as acyl anion precursors. Scheidt and co-workers R2 4 demonstrated that NHCs generated in situ from thiazolium salts OSiX3 Z R F O and the corresponding base can promote the sila-Stetter reaction R S O 12 12 R of acylsilanes and a,b-unsaturated ketones efficiently. In this N 1 OH process, highly nucleophilic NHC attack of acylsilanes facilitates R 10 carbonyl anion R 1,2-Brook rearrangement to produce the silylated acyl anion equiv- 13a alent 5; with the assistance of alcohol, 5 converts to 6, which O undergoes the following conjugate addition to produce a 1,4-dicar- 14 bonyl compound (Scheme 2). Using an NHC-catalyzed umpolung strategy with acylsilanes, Scheme 4. Transformations of o-silyl thiazolium carbinols. Scheidt and co-workers subsequently developed the one-flask syn- 13 thesis of polysubstituted furans and pyrroles. In situ-generated Bis-spiroacetals are an important unit that exists in various bio- 1,4-dicarbonyl compounds can be transformed into furans by the active marine phycotoxins.15 Landais recently reported that this addition of HOAc. Under the mediation of p-toluenesulfonic acid valuable skeleton can be prepared through an NHC-promoted (TsOH), 1,4-diketones can undergo Paal–Knorr reactions with sila-Stetter-ketalization cascade reaction.16 Under the catalysis of amines to produce pyrroles in one pot (Scheme 3). NHCs, aliphatic acylsilanes couple with vinyl ketones to produce The same group prepared and isolated stable O-silyl thiazolium 1,4-diketones, followed by deprotection of silyl ether and spiroace- carbinols 10, which can be used as precursors of acyl anions. With talization under acidic conditions to produce bis-spiroacetals 15 in the assistance of a fluoride source, thiazolium carbinols can moderate to good yields (Scheme 5). undergo the conjugate addition with nitroalkenes or o-quinone Fluoroalkene dipeptide isosteres (FADIs)17 are important build- 14 methides efficiently (Scheme 4). ing blocks in peptidomimetic drug research. Okata and co-workers 974 L. He et al. / Tetrahedron Letters 56 (2015) 972–980 O 15 mol% 3a O TBSO + SiMe 2Ph OTBS TBSO DBU, i-PrOH OTBS O THF, 75 O 1b 2' ഒ 4' CSA 50mol% O O O CH3CN rt, 24h 15 Scheme 5. NHC-catalyzed synthesis of bis-spiroacetals. F X F F 30mol% NHC R OEt + TMSCN Cat. TMSX CN R TBS R1 R2 1 2 DBU, EtOH NH O 23 R R Boc X=O,22a NH O 24a Boc 19, FADIs X=NP,22b X=O, 16 X=NP,24b Scheme 8. NHC-catalyzed cyanosilylation reactions. EtOH NHC-catalyzed cyanation and trifluoromethylation reactions R R2 R 2 R1 1 N 1,2-Brook F N F F R Cyanohydrins and their silyl ethers are versatile synthons for R R3 rearrangement R 3 S S the synthesis of a variety of valuable building blocks, such as a- 20 NH O TBS NH OTBS hydroxy acids, a-amino alcohols, and 1,2-diols. The catalytic Boc Boc 17 18 addition of trimethylsilyl cyanide (TMSCN) to carbonyl compounds is one of the most common methods of preparing these types of Cl Cl Ph Ph molecules. tert-Amines, tert-phosphines, and other Lewis bases SN 21 NHC: SN can catalyze this nucleophilic addition efficiently. In 2006, Aoy- ama and co-workers successfully applied NHCs to catalyze cyano- 22 3b HO 3c silylation reactions of aldehydes, ketones, and imines; the last reaction is also known as the Strecker reaction. This investigation Scheme 6. NHC-mediated synthesis of FADIs using a,b-enoylsilanes. was shortly followed by a report by Song and co-workers,23 who utilized pre-isolated NHCs as catalysts and found that the NHC loading can be reduced to as low as 0.01 mol % without reducing yields.
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