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Frontier Chemistry-FZ C-C Bond Formations by Cross-Dehydrogenative Coupling of Csp3 –H Bonds Feng Zhang Frontiers of Chemistry Seminar 01-04-2014 01/04/2014 Feng Zhang @ Wipf Group 1 Feng Zhang @ Wipf Group Page 1 of 50 1/4/2014 Introduction § Carbon-carbon (C-C) bond forming reactions represent one of the most vital tools for the synthesis of complex natural products, bioactive molecules developed as drugs and agrochemicals. § The important of the synthesis of carbon-carbon bonds is reflected by the fact that Nobel Prizes in Chemistry have previously been given to this area: The Grignard reaction (1912), § the Diels-Alder reaction (1950), 2 Feng Zhang @ Wipf Group Page 2 of 50 1/4/2014 Introduction § the Wittig reaction (1979), § and olefin metathesis to Y. Chauvin, R. H. Grubbs, and R. R. Schrock (2005) 3 Feng Zhang @ Wipf Group Page 3 of 50 1/4/2014 Strategies for selective C-C bond formation § During the second half of the 20th century, transition metals, and in particular palladium, have been of importance for the development of reactions for the formation of carbon-carbon bonds. § Traditional coupling reaction for the construction of the C-C bond involves the coupling of an electrophilic species (C-X) with an unactivated substrate (C-H) or a reactive nucleophile, usually an organometallic reagent (C-M) Catalyst C X + C M C C + MX X = Cl, Br, I, OTf, OTs, OMs, etc.; M = H, MgX, Li, BX2, SnX3. Zn etc. 4 Feng Zhang @ Wipf Group Page 4 of 50 1/4/2014 Strategies for selective C-C bond formation § Mizoroki-Heck reaction R' H R' H Pd-catalyst RX + + HX H H H R § Suzuki reaction Pd-catalyst RBY2 + R'X R-R' + MX base § Negishi coupling reaction Pd-catalyst RZnY + R'X R-R' + MX R, R' = aryl, vinyl, alkyl X = halide, triflate, etc. § Sonogashira coupling reaction Pd cat., Cu cat. HC C R + R'X R' C C R base, rt 5 Feng Zhang @ Wipf Group Page 5 of 50 1/4/2014 Strategies for selective C-C bond formation Richard. F. Heck Ei-ichi Negishi Akira Suzuki § The year of 2010, the Nobel Prize in Chemistry is awarded to the formation of carbon-carbon single bonds through palladium-catalyzed cross-coupling reactions. Three researchers shared this Prize. § The great success of coupling reactions is the predictable regioselectivity of the C-C bond formation. The new C-C bond is selectively formed at positions which are determined by the leaving groups on both substrates. 6 Feng Zhang @ Wipf Group Page 6 of 50 1/4/2014 C-C bond formation via direct dehydrative coupling strategies § Recently, a number of alternative strategies for C–C bond formation utilizing abundantly available/unactivated precursors via C–H activation, decarboxylative coupling, use of arylhydrazones as coupling partners etc., have been developed. § In particular, the direct use of aryl alcohols/tautomerizable heterocycles/phenols (C–OH) as proelectrophiles is receiving huge attention because these building blocks are abundant in nature, have a relatively low toxicity and are an economical alternative for C–X species. Chem. Soc. Rev., 2013, 42, 1121-1146 7 Feng Zhang @ Wipf Group Page 7 of 50 1/4/2014 C-C bond formation via direct dehydrative coupling strategies § Direct dehydrative functionalization of the Csp3 –OH bond § Csp3 –Csp bond formation Chem. Commun., 2009, 6487–6489. § Csp3 –Csp2 bond formation J. Organomet. Chem., 2008, 693, 283–296. 8 Feng Zhang @ Wipf Group Page 8 of 50 1/4/2014 C-C bond formation via direct dehydrative coupling strategies § Csp3 –Csp3 bond formation Synlett, 1996, 557–559. § Direct dehydrative functionalization of the Csp2 –OH bond § Csp2 –Csp bond formation Org. Lett., 2010, 12, 2286–2289. 9 Feng Zhang @ Wipf Group Page 9 of 50 1/4/2014 C-C bond formation via direct dehydrative coupling strategies § Csp2 –Csp2 bond formation Org. Lett., 2012, 14, 2146–2149. 10 Feng Zhang @ Wipf Group Page 10 of 50 1/4/2014 C-C bond formation via direct dehydrogenative coupling strategies § The oxidative coupling of two different C-H bonds was termed cross- dehydrogenative coupling (CDC) by Dr. Li in 2004 and has become a growing field of interest. Chao-jun Li Acc. Chem. Res. 2009, 42, 335 – 344. 11 Feng Zhang @ Wipf Group Page 11 of 50 1/4/2014 Coupling of Csp3 –H with Csp –H Bonds § The group of Li succeeded in 2004 in developing the first catalytic alkynylation reaction of Csp3 –H bonds adjacent to a nitrogen atom as a proof-of-concept for the CDC. Aromatic alkynes provide the desired products in 60% to 82% yield whereas the use of aliphatic derivatives generally resulted in low to moderate yields (12–58%). The substituents on the aniline coupling partner influenced the reactivity, and the presence of either sterically demanding groups in the 2-position or electron-withdrawing groups on the 4- position resulted in a decrease of yield. J. Am. Chem. Soc. 2004, 126, 11810 – 11811. 12 Feng Zhang @ Wipf Group Page 12 of 50 1/4/2014 Coupling of Csp3 –H with Csp –H Bonds A silver-catalyzed oxidative coupling of terminal alkynes and benzylic ethers was introduced in 2010 by the group of Li Heterocycles 2010, 82, 555 – 562. Silver-catalyst alkynylation of benzylic ethers 13 Feng Zhang @ Wipf Group Page 13 of 50 1/4/2014 Coupling of Csp3 –H with Csp –H Bonds The group of Su showed that tetrahydroisoquinolines could be alkynylated in the presence of 1 equivalent of the oxidant DDQ under solvent-free, high-speed, ball-milling conditions. The reaction proceeded smoothly with both aliphatic and aromatic alkynes. J. Org. Chem. 2011, 76, 9144 – 9150. Functionalization of tetrahydroisoquinolines under ball-milling conditions 14 Feng Zhang @ Wipf Group Page 14 of 50 1/4/2014 Coupling of Csp3 –H with Csp –H Bonds More recently, Li and co-workers reported the first alkynylation of benzylic C-H bonds which are not adjacent to a heteroatom. Aromatic alkynes were smoothly converted, and the use of electron- rich derivatives resulted in slightly improved yields. Adv. Synth. Catal. 2010, 352, 1446 – 1450. CuOTf-catalyst CDC reaction of dephenylmethanes and aromatic alkynes 15 Feng Zhang @ Wipf Group Page 15 of 50 1/4/2014 Coupling of Csp3 –H with Csp –H Bonds The group of Nakamura presented an original approach for the synthesis of N-tethered 1,6-enynes by a zinc(II)-catalyzed redox CDC reaction. The use of 20 mol% of ZnBr allowed 2 the synthesis of various enynes from the coupling reaction between propargylic amines and terminal alkynes. The reaction does not require the addition of an external oxidant as the propargylic amine reacts as a hydrogen acceptor. The triple bond is reduced into the corresponding alkene. J. Am. Chem. Soc. 2012, 134, 2504 - 2507. Zinc-catalyst synthesis of N-tethered 1,6-enynes 16 Feng Zhang @ Wipf Group Page 16 of 50 1/4/2014 Coupling of Csp3 –H with Csp2 –H Bonds § Catalytic cross-dehydrogenative Friedel-Crafts-type arylations. The first catalytic coupling of Csp3 –H bonds with Csp2 –H bonds using a cross- dehydrogenative Friedel–Crafts approach was reported by the group of Li in 2005. In the following years, the scope of cross-dehydrogenative Friedel–Crafts-type arylation was significantly improved by the development of highly efficient catalyst systems Angew. Chem. Int. Ed. 2014, 53, 74 -100 17 Feng Zhang @ Wipf Group Page 17 of 50 1/4/2014 Coupling of Csp3 –H with Csp2 –H Bonds H R1 H N R1 N N 1.5 mol% CuSO4, 7.5 mol% CuI, 3.5 equiv pyridine N R 1 equiv TFAOH, O2 (1atm), DMF, 110 °C, 14h R 30 examples, 47-95% Angew. Chem. 2012, 124, 8443 – 8446; Angew. Chem. Int. Ed. 2012, 51, 8318 – 8321. Recently, an intramolecular version of the dehydrogenative Friedel–Crafts-type arylation has been developed. The reaction is believed to start with the copper-catalyzed aerobic oxidation of the hydrazone and the obtained carbonyl intermediate is activated by the copper-catalyst to Proposed mechanism of the cinnoline synthesis undergo a Friedel– Crafts cyclization. Demethylation with pyridine furnishes the desired products. 18 Feng Zhang @ Wipf Group Page 18 of 50 1/4/2014 Coupling of Csp3 –H with Csp2 –H Bonds § Friedl-Crafts-type cross-dehydrogenative benzylation The group of MeO C 2 MeO2C Shi introduced a Ph Ph FeCl2 (10 mol%), DDQ (2.5 equiv) FeCl2-catalyzed MeO H + H MeO benzylation DCE, 100 °C, 36h Ph Ph reaction of electron-rich arenes with diphenylmethan es Ph Ph Ph Ph Ph FeCl2 (10 mol%), DDQ (2.5 equiv) MeO + MeO + MeO Ph neat, 100 °C, 36h, 95% Ph Ph Ph MeO MeO Ph Ph FeCl2 (10 mol%), DDQ (2.5 equiv) MeO + MeO Ph neat, 100 °C, 36h, 95% MeO MeO Ph Ph Angew. Chem Int Ed, 2009, 48: 3817–3820 19 Feng Zhang @ Wipf Group Page 19 of 50 1/4/2014 Coupling of Csp3 –H with Csp2 –H Bonds § Metal-Free Procedures The group of Klussmann introduced a two-step procedure for the arylation of N protected tetrahydroisoquinolines in which the cationic intermediates are generated from the corresponding Peroxides in the absence of a transition- metal catalyst. Eur. J. Org. Chem. 2013, 666 – 671. Metal-free cross-dehydrogenative Friedel-Crafts arylation sequence 20 Feng Zhang @ Wipf Group Page 20 of 50 1/4/2014 Coupling of Csp3 –H with Csp2 –H Bonds § Cross-Dehydrogenative Coupling Involving Radical Intermediates § Intermolecular Radical Alkylations Various 2-aryl pyridines were successfully coupled with cyclohexane, cycloheptane, and cyclooctane. The reaction is sensitive toward the ring size. However, overalkylation was observed in many cases, and the products were obtained as isomeric mixtures.
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