Transition Metal-Free Catalytic C-H Zincation and Alumination

C-H metalation is the most efficient method to prepare aryl-zinc and -aluminium complexes that are ubiquitous nucleophiles. Virtually all C-H metalation routes to form Al/Zn organometallics require stoichiometric, strong Brønsted bases with no base-catalyzed reactions reported. Herein we present a catalytic in amine/ammonium salt (Et3N/[(Et3N)H]+) C-H metalation process to form aryl-zinc and aryl-aluminium complexes. Key to this approach is coupling an endergonic C-H metalation step with a sufficiently exergonic dehydrocoupling step between the ammonium salt by-product of C-H metalation ([(Et3N)H]+) and a Zn-H or Al-Me containing complex. This step, forming H2/MeH, makes the overall cycle exergonic while generating more of the reactive metal electrophile. Mechanistic studies supported by DFT calculations revealed metal-specific dehydrocoupling pathways, with the divergent reactivity due to the different metal valency (which impacts the accessibility of amine-free cationic metal complexes) and steric environment. Notably, dehydrocoupling in the zinc system proceeds through a ligand-mediated pathway involving protonation of the β-diketiminate Cγ position. Given this process is applicable to two disparate metals (Zn and Al), other main group metals and ligand sets are expected to be amenable to this transition metal-free, catalytic C-H metalation.

Silyl Radical Cascade Cyclization of 2-Isocyanothioanisole toward 2-Silylbenzothiazoles through Radical Initiator-Inhibitor Symbiosis

A demethylative silyl radical cascade cyclization of 2-isocyanothioanisoles toward 2-silylated benzothiazole building blocks has been developed. The development of a "radical initiator-inhibitor symbiosis" system solves the challenge of otherwise dominant methyl radical-triggered side reactions brought about by kinetically unfavored generation of reactive silyl radical species. The products accessed in this protocol are amendable to various downstream functionalization reactions, including the quick construction of a topoisomerase II inhibitor via a Hiyama cross-coupling reaction and of an antiviral agent via a fluoride-/hydroxide-free nucleophilic substitution to acyl chloride.

Metallation of sensitive fluoroarenes using a potassium TMP-zincate supported by a silyl(bis)amido ligand

Combining a bulky bis(amide) and a reactive one-coordinate TMP (2,2,6,6-tetramethylpiperidide) ligand, a new mixed K/Zn heteroleptic base has been developed for regioselective zincation of fluoroarenes. This special ligand set allows for trapping and structural authentication of the first intermediates of direct Zn–H exchange of fluoroarenes obtained via deprotonative metallation, providing mechanistic insights of the processes involved.

Providing unique structural insights, regioselective zincation of a range of fluoroarenes has been accomplished using a novel heterobimetallic K/Zn amide base.

A meta-Selective C-H Alumination of Mono-Substituted Benzene by Using An Alkyl-Substituted Al Anion through Hydride-Eliminating SN Ar Reaction

Reaction of an Al-centered anion with toluene proceeded to form C-H cleaved product with a perfect meta-selectivity and a relatively small kinetic isotope effect (KIE, k_H /k_D =1.51). DFT calculations suggested a two-step reaction mechanism and electronically controlled meta-selectivity arising from the electron-donating methyl group. The reaction with other mono-substituted arenes was also investigated.

Rare-Earth Catalyzed C-H Bond Alumination of Terminal Alkynes

Organoaluminum reagents' application in catalytic C-H bond functionalization is limited by competitive side reactions, such as carboalumination and hydroalumination. Herein, rare-earth tetramethylaluminate complexes are shown to catalyze the exclusive C-H bond metalation of terminal alkynes with the commodity reagents trimethyl-, triethyl-, and triisobutylaluminum. Kinetic experiments probing alkyl-group exchange between rare-earth aluminates and trialkylaluminum, C-H bond metalation of alkynes, and catalytic conversions reveal distinct pathways of catalytic aluminations with triethylaluminum versus trimethylaluminum. Most significantly, kinetic data point to reversible formation of a unique [Ln](AlR₄ )₂ ⋅AlR₃ adduct, followed by turnover-limiting alkyne metalation. That is, C-H bond activation occurs from a more associated organometallic species, rather than the expected coordinatively unsaturated species. These mechanistic conclusions allude to a new general strategy for catalytic C-H bond alumination that make use of highly electrophilic metal catalysts.

Room temperature catalytic carbon-hydrogen bond alumination of unactivated arenes: mechanism and selectivity

We report the first catalytic methods for the transformation of C-H bonds of unactivated arenes into C-Al bonds. The catalytic reactions occur at 25 °C (benzene, toluene and xylenes) with palladium loadings as low as 0.1 mol%. Remarkably, the C-H activation of toluene and xylenes proceeds with ortho- and meta-selectivity. This selectivity is highly unusual and complementary to both Friedel-Crafts and the majority of C-H borylation methods. Through a detailed mechanistic analysis (Eyring analysis, KIE, DFT, QTAIM) we show that unusual Pd-Al intermetallic complexes are on the catalytic cycle and that the selectivity is determined by weak attractive dispersion forces in the transition state for C-H bond breaking.

Synthesis of Polyflourinated Biphenyls; Pushing the Boundaries of Suzuki-Miyaura Cross Coupling with Electron-Poor Substrates

Polyfluorinated biphenyls are interesting and promising substrates for many different applications. Unfortunately, all current methods for the syntheses of these compounds only work for a hand full of molecules or only in very special cases. Thus, many of these compounds are still inaccessible to date. Here we report a general strategy for the synthesis of a wide range of highly fluorinated biphenyls. In our studies we investigated crucial parameters, such as different phosphine ligands and the influence of various nucleophiles and electrophiles with different degrees of fluorination. These results extend the scope of the already very versatile Suzuki–Miyaura reaction toward the synthesis of very electron-poor products, making these more readily accessible. The presented methodology is scalable and versatile without the need for elaborate phosphine ligands or Pd-precatalysts.

C-H Functionalization of Azines

Azines, which are six-membered aromatic compounds containing one or more nitrogen atoms, serve as ubiquitous structural cores of aromatic species with important applications in biological and materials sciences. Among a variety of synthetic approaches toward azines, C-H functionalization represents the most rapid and atom-economical transformation, and it is advantageous for the late-stage functionalization of azine-containing functional molecules. Since azines have several C-H bonds with different reactivities, the development of new reactions that allow for the functionalization of azines in a regioselective fashion has comprised a central issue. This review describes recent advances in the C-H functionalization of azines categorized as follows: (1) S_NAr reactions, (2) radical reactions, (3) deprotonation/functionalization, and (4) metal-catalyzed reactions.

Ester-directed Ru-catalyzed C–O activation/C–C coupling reaction of ortho-methoxy naphthoates with organoboroneopentylates

A new, catalytic and general synthetic methodology for the construction of biaryls and heterobiaryls by the cross-coupling of ortho-methoxy naphthoates with organoboroneopentylates is disclosed.

TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation–alkylaluminium-trapping reagents?

Surprisingly lithium TMP-aluminate reagents are not capable of directly aluminating anisole as previously thought but operateviasequential lithiation–alkylaluminium trapping.

A convenient alumination of functionalized aromatics by using the frustrated Lewis pair Et3 Al and TMPMgCl⋅LiCl

A straightforward and efficient alumination of functionalized arenes by using the frustrated Lewis pair Et3 Al and TMPMgCl⋅LiCl (TMP=2,2,6,6-tetramethylpiperidyl) has been developed. In particular, halogenated electron-rich aromatics can be smoothly functionalized by using the frustrated Lewis pair Et3 Al and TMPMgCl⋅LiCl. Compared with previously described alumination methods, this procedure avoids extensive cooling and the need for an excess of base. This in situ procedure has proven to be most practical and allows for regio- and chemoselective metalation of a wide range of aromatics with sensitive functional groups (CONEt2 , CO2 Me, CN, OCONMe2 ) or halogens (F, Cl, Br, I). The resulting aromatic aluminates, which were characterized by using NMR spectroscopy, were subjected to allylations, acylations, and palladium-catalyzed cross-coupling reactions after transmetalation to zinc. It was shown that the nature of the Zn salt used for transmetalation is crucial. Thus, compared with ZnCl2 (2 equiv), the use of Zn(OPiv)2 (2 equiv; OPiv=pivalate) allows the subsequent quenching reactions to be performed with only a slight excess of electrophile (1.2 equiv) and provides interesting functionalized aromatics in good yields.

Direct synthesis of polysubstituted aluminoisoxazoles and pyrazoles by a metalative cyclization

Alumino-heteroles are obtained from simple precursors in a fully chemo- and regioselective manner by a metalative cyclization. The carbon-aluminum bond is still able to react further with several electrophiles, without the need of transmetalation. This synthetic route provides a novel entry to heterocyclic organoaluminum reagents as well as a straightforward access to 3,4,5-trisubstituted isoxazoles and 1,3,4,5-tetrasubstituted pyrazoles.