Selective palladium-catalyzed arylation(s) of benzaldehyde derivatives by N-heterocarbene ligands (original) (raw)
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Studies on Palladium-Catalyzed Benzylic Arylation
2009
Contents General Introduction Chapter 1 Palladium-Catalyzed 2-Pyridylmethyl Transfer from 2-(2-Pyridyl)ethanol Derivatives to Organic Halides by Chelation-Assisted Cleavage of Unstrained sp 3 C-sp 3 C Bonds Chapter 2 Palladium-Catalyzed Direct Arylation of Aryl(azaaryl)methanes with Aryl Halides Providing Triarylmethanes Chapter 3 Palladium-Catalyzed Benzylic Arylation of N-Benzylxanthone Imine Chapter 4 Palladium-Catalyzed Benzylic Direct Arylation of Benzyl Sulfone Publication List Acknowledgment tol tolyl Xantphos 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene Halogen-magnesium exchange Cross-homo scrambling Scheme 6. Br Br m 1 mol% Pd(PPh 3) 4 + Br m = MgCl, Et 2 O, reflux, 20 h m = ZnBr, THF, 50 ˚C, 16 h 20% 68% Scheme 7. X Mg metal MgX + Undesirable dimerization Scheme 5. General Introduction derivative in 22% yield. Because organoboron compounds are generally synthesized via hydroboration, benzylboron compounds are less readily available and are uncommon coupling partners in Suzuki-Miyaura cross-coupling reaction. Flaherty et al. reported the palladium-catalyzed benzylation of aryl halides with commercially available benzylboron compounds (Scheme 9). 11 As described above, benzylzinc derivatives appear to be the most favorable benzylmetals in all respects for palladium-catalyzed benzylation. Although palladium-catalyzed cross-coupling reactions of benzylzincs display high reactivity and chemoselectivity, the need still exists for preparation of benzylzinc reagents by treatment of benzyl halides with a stoichiometric amount of Zn powder. Furthermore, synthesis of benzyl halides often requires several steps. An alternative attractive route for the palladium-catalyzed benzylation of aryl halides is the use of stable organic compounds bearing no metallic atom as nucleophilic partners (Scheme 10).
Intramolecular Palladium-Catalyzed Alkane C− H Arylation from Aryl Chlorides
Journal of the American Chemical Society, 2010
The first examples of efficient and general palladium-catalyzed intramolecular C(sp3)−H arylation of (hetero)aryl chlorides, giving rise to a variety of valuable cyclobutarenes, indanes, indolines, dihydrobenzofurans, and indanones, are described. The use of aryl and heteroaryl chlorides significantly improves the scope of C(sp3)−H arylation by facilitating the preparation of reaction substrates. Careful optimization studies have shown that the palladium ligand and the base/solvent combination are crucial to obtaining the desired class of product in high yields. Overall, three sets of reaction conditions employing PtBu3, PCyp3, or PCy3 as the palladium ligand and K2CO3/DMF or Cs2CO3/pivalic acid/mesitylene as the base/solvent combination allowed five different classes of products to be accessed using this methodology. In total, more than 40 examples of C−H arylation have been performed successfully. When several types of C(sp3)−H bond were present in the substrate, the arylation was found to occur regioselectively at primary C−H bonds vs secondary or tertiary positions. In addition, in the presence of several primary C−H bonds, selectivity trends correlate with the size of the palladacyclic intermediate, with five-membered rings being favored over six- and seven-membered rings. Regio- and diastereoselectivity issues were studied computationally in the prototypal case of indane formation. DFT(B3PW91) calculations demonstrated that C−H activation is the rate-determining step and that the creation of a C−H agostic interaction, increasing the acidity of a geminal C−H bond, is a critical factor for the regiochemistry control.
Efficient palladium-catalyzed direct arylation of azines and diazines using ligand-free conditions
Tetrahedron, 2009
The use of the palladium-catalyzed direct arylation was successfully tested on different electron-deficient heterocycles. The results demonstrate the effectiveness of the method based on the intramolecular coupling reaction providing polyazacyclic systems. This new application was obtained by using ligandfree conditions with the mixture of Pd(OAc) 2 and TBAC as catalytic system. With suitable substrates different products arising from regioselective coupling were observed.
Palladium-Catalyzed Regioselective Direct Arylation of Benzofurazans at the C-4 Position
Advanced Synthesis & Catalysis, 2017
The palladium-catalyzed direct arylation of benzofurazans with aryl bromides to access 4arylbenzofurazans proceeds in moderate-to-high yields using phosphine-free palladium acetate as the catalyst and potassium acetate as an inexpensive base. A wide variety of (hetero)aryl bromides, including bromopyridine and bromothiophene derivatives has been successfully employed. Palladium-catalyzed one-pot C4,C7-diarylation of benzofurazane was also described using a larger amount of aryl bromide. Moreover, the derivatization of 4arylbenzofurazans into 4-arylquinaxolines is also reported.
Direct Acylation of Aryl Chlorides with Aldehydes by Palladium−Pyrrolidine Co-catalysis
Organic Letters, 2010
A palladium catalyst system has been developed that allows for the direct acylation of aryl chlorides with aldehydes. The choice of ligand, as well as the presence of pyrrolidine and molecular sieves is shown to be critical to the catalysis, which appears to proceed via an enamine intermediate. The reaction was successful for a wide range of aryl chlorides and tolerant of functionality on the aldehyde component, giving easy access to alkyl aryl ketones in modest to good yields.
2011
The cationic Ir(III) acetone complex (POCOP)Ir(H) 2 (acetone) + (POCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) was shown to catalyze the reduction of a variety of tertiary amides to amines using diethylsilane as reductant. Mechanistic studies established that a minor species generated in the reaction, the neutral silyl trihydride Ir(V) complex (POCOP)IrH 3 (SiEt 2 H), was the catalytically active species. High concentrations of this species could be conveniently generated by treatment of readily available (POCOP)IrHCl with tert-butoxide in the presence of Et 2 SiH 2 under H 2 . Thus, using this mixture in the presence of a trialkylammonium salt, a wide array of tertiary amides, including extremely bulky substrates, are rapidly and quantitatively reduced to tertiary amines under mild conditions with low catalyst loading. A detailed mechanistic study has been carried out and intermediates identified. In brief, (POCOP)IrH 3 (SiEt 2 H) reduces the amide to the hemiaminal silyl ether that, in the presence of a trialkylammonium salt, is ionized to the iminium ion, which is then reduced to the tertiary amine by Et 2 SiH 2 . Good functional group compatibility is demonstrated, and a high catalyst stability has provided turnover numbers as high as 10 000.