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8 Ir(I)-Catalyzed C–H Borylation Arylboronic acids and esters are invaluable tools for the chemical community. These powerful reagents are used for a variety of transformations, most notably the Suzuki-Miyaura cross-coupling reaction. This reaction is used to selectively construct C–C bonds through the combination of an organo-boron nucleophile with a suitable aryl, alkenyl, or alkyl halide or trifl ate. While the Suzuki-Miyaura reaction has become commonplace within the synthetic community, one limitation of this method is the limited ability to access the requisite organo-boron species. Historically, methods for the synthesis for aryl C–B bonds have relied upon the use of harshly basic reaction conditions or substrates containing prefunctionalized carbon centers. These shortcomings require that additional steps must be taken to either protect sensitive functionality or install the necessary functional handle prior to C–B bond formation (Scheme 1). R X X = Cl, Br, I R DG H Aldrich.com R'MX R'Li R X Pd(0), Base HB(OR'')2 or (OR'') 2B B(OR'')2 R X = Br, I, OTf R R MX B(OR'')3 R B(OR'')2 DG B(OR'')3 R DG Li B(OR'')2 B(OR'') 2 Scheme 1: Classical methods for C–B bond formation. Harshly basic reaction conditions Multiple steps and manipulations Requires prefunctionalized starting materials The direct formation of aryl C–B bonds from aryl C–H bonds thus represents a powerful strategy for streamlining the synthesis of these useful reagents (Scheme 2). 1 R H [M] HB(OR'')2 or (OR'')2B B(OR'')2 R B(OR'')2 Scheme 2: Metal-catalyzed direct C–H borylation. Catalysis Josephine Nakhla, Ph.D. Market Segment Manager Organometallics and Catalysis josephine.nakhla@sial.com No harsh reagents or reaction conditions No need for prefunctionalization Atom economical TO ORDER: Contact your local Sigma-Aldrich offi ce (see back cover), or visit Aldrich.com/chemicalsynthesis. Building upon their previous work within the area, 2 Professor John F. Hartwig has disclosed a method for the direct conversion of aryl C–H bonds to aryl C–B bonds through the use of an Ir(I) catalyst and B2pin2 (Table 1). 3 This powerful system displays excellent regioselectivity that can be easily predicted by sterics and leads to the rapid synthesis of highly useful arylboronic esters. H 3C H3CO R arene product CH3 H H H H3C CH3 H H3CO OCH3 OCH3 Cl Cl Cl Bpin Bpin Bpin 1/2[IrCl(COD)]2/bpy B2pin2, 80 oC, 16 h. 95 % 83 % 86 % H Bpin H3CO H 83 % Table 1: Ir(I)-Catalyzed aryl C–H borylation. Cl R Bpin yield arene product yield H 3C H3C H3CO CH3 CH3 Br CH3 H 3C H3C H3CO H3CO This method provides a simple and direct r<strong>out</strong>e to arylboronic esters that fully avoids the use of harshly basic reaction conditions and does not require multiple reaction steps and manipulations. Importantly, this reaction employs catalysts and reagents that are all readily accessible, and now available from Aldrich. Reference: (1) Cho, J. Y.; Tse, M. K.; Holmes, D.; Maleczka Jr., R. E.; Smith III, M. R. Science, 2002, 295, 305 (2) (a) Chen, H.; Hartwig, J. F. Angew. Chem. Int. Ed. Engl. 1999, 38, 3391. (b) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995. (3) Hartwig, J. F. et al. J. Am. Chem. Soc. 2002, 124, 390. (4) Hartwig, J. F. et al. Chem. Rev. 2010, 110, 890. H H H CH3 CH3 Br Bpin CH3 Bpin Bpin Bpin 58 % 86 % 72 % 73 %
Iridium(I) Catalysts, Bipyridine Ligands, and Borylation Reagents from Aldrich: Ir(I) Catalysts Bipyridine Ligands Borylation Reagents For a complete list of C–H borylation reagents available from Aldrich Chemistry, please visit Aldrich.com/borylation Carboranes as Superweak Anions The chemistry of weakly coordinating anions, or superweak anions, continues to be actively investigated within many laboratories for a variety of purposes. These useful molecules often allow for the isolation of extremely reactive salts of cations, making them applicable to the ever growing list of chemical tasks that require highly reactive cations. These uses include the catalytic polymerization of olefi ns, the catalytic formation of C–C bonds, the manufacture of high-current-density lithium batteries, and the activation of C–H bonds. Discover how carboranes from Aldrich can advance your research. Carboranes from Aldrich: F B F B B F Cl Ir Ir Cl 683094 CH3 O Ir Ir O B CH3 685062 [Ir(COE) 2 Cl] 2 377155 Ir 685011 F B B F F F F B B F F B F B Cs B Cs B F N N 36759 473294 H3C CH3 N N 569593 H3C CH3 N N 513040 H3CO OCH3 N N 536040 t-Bu t-Bu N N 515477 F F B B F B B F B B F B B F F B B F F F 723509 720887 H3C H3C H3C H3C H3C H3C H H B B O O B B O O O B O CH3 CH3 CH3 CH3 655856 O O B B O O 473286 O B O 188913 O B O 518808 F F Ready to scale up? For competitive quotes on larger quantities or custom synthesis, contact your local Sigma-Aldrich offi ce, or visit safcglobal.com. K K CH3 CH3 CH3 CH3 O CH3 B O CH3 Frustrated Lewis Pairs as Hydrogenation Catalysts Catalysis The hydrogenation of organic substrates with molecular hydrogen (H2) has been used for purposes ranging from the large-scale upgrading of crude-oil, to the synthesis of fi ne chemicals used in food, agriculture, and the pharmaceutical industry. While the majority of methods rely on the use of costly precious metal catalysts, recent work from the lab of Professor Douglas W. Stephan has illustrated the use of frustrated Lewis pairs for the same purpose. 1,2 These powerful non-metallic catalysts contain both Lewis acidic (borane) and Lewis basic (phosphine) moieties that cannot be quenched internally due to steric constraints. Because of this unquenched reactivity, these organic catalysts are used to activate a variety of small molecules, including the heterolytic cleavage of H2, leading to a powerful catalyst system for the hydrogenation of imines and aziridines (Scheme 3). Scheme 3: Frustrated Lewis pairs for the hydrogenation of imines and aziridines. Reference: (1) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124. (2) (a) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050. (b) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 9136. Frustrated Lewis Pairs from Aldrich: F F t-Bu N 703087 5 mol % t-Bu NH H Ph 5 atm H 2 H Ph H 98 % F F F F H B F F F F F Ph F Ph N Ph F t-Bu P t-Bu F H + 703095 5 atm H 2 F F For a complete list of Frustrated Lewis pairs available from Aldrich Chemistry, please visit Aldrich.com/fl p F F F F 10 mol % F BH F F F F F F H3C Ph NH H + F H3C 703087 703095 Ph Ph 98 % P CH 3 CH3 CH3 CH3 9
Page 1 and 2: Aldrich Cyclopropylboronic acid MID
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Page 19 and 20: Organotrifl uoroborates as Coupling
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Page 23 and 24: Thiazoles Thiazoles have been frequ
Page 25 and 26: Olefi n Metathesis Employing the se
Page 27 and 28: Unlock a smarter, greener laborator
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