Secondary Alkyl Halides in Transition-Metal-Catalyzed Cross-Coupling Reactions

On the stereochemical course of palladium-catalyzed cross-coupling of allylic silanolate salts with aromatic bromides

The stereochemical course of palladium-catalyzed cross-coupling reactions of an enantioenriched, alpha-substituted, allylic silanolate salt with aromatic bromides has been investigated. The allylic silanolate salt was prepared in high geometrical (Z/E, 94:6) and high enantiomeric (94:6 er) purity by a copper-catalyzed S(N)2' reaction of a resolved allylic carbamate. Eight different aromatic bromides underwent cross-coupling with excellent constitutional site-selectivity and excellent stereospecificity. Stereochemical correlation established that the transmetalation event proceeds through a syn S(E)' mechanism which is interpreted in terms of an intramolecular delivery of the arylpalladium electrophile through a key intermediate that contains a discrete Si-O-Pd linkage.

Stereospecific Suzuki cross-coupling of alkyl alpha-cyanohydrin triflates

Scalemic alpha-cyanohydrin triflates undergo Pd-catalyzed cross-coupling with aryl, heteroaryl, and vinyl boronic acids under mild conditions. Coupling proceeds with complete inversion of configuration at the stereogenic carbon. The resultant nitrile can be easily converted into a variety of alternative functional groups of value in organic synthesis and thus achieves a higher level of molecular complexity than the products of traditional Suzuki reactions.

Enantioselective alkenylation via nickel-catalyzed cross-coupling with organozirconium reagents

A new family of organometallic compounds, organozirconium reagents, are shown to serve as suitable partners in cross-coupling reactions of (activated) secondary alkyl electrophiles. Thus, the first catalytic method for coupling secondary alpha-bromoketones with alkenylmetal reagents has been developed, specifically, a mild, versatile, and stereoconvergent carbon-carbon bond-forming process that generates potentially labile beta,gamma-unsaturated ketones with good enantioselectivity.

Nickel/bis(oxazoline)-catalyzed asymmetric Kumada reactions of alkyl electrophiles: cross-couplings of racemic alpha-bromoketones

The first asymmetric Kumada reactions of alkyl electrophiles are described, specifically, cross-couplings of racemic alpha-bromoketones with aryl Grignard reagents. Several features of this investigation are of interest. First, the couplings proceed at remarkably low temperature (-40 or -60 degrees C), which enables the asymmetric synthesis of racemization-prone alpha-arylketones. Second, dialkyl ketones undergo enantioselective coupling in good ee and yield. Third, readily available bis(oxazolines) are shown for the first time to be effective ligands for cross-couplings of alkyl electrophiles, thereby opening the door to new opportunities in asymmetric catalysis.

Palladium-catalyzed cross-coupling reaction of secondary benzylic bromides with grignard reagents

A mild palladium-catalyzed Kumada-Corriu reaction of secondary benzylic bromides with aryl and alkenyl Grignard reagents has been developed. In the presence of the Xantphos ligand, the undesired beta-elimination pathway is minimized, affording the corresponding cross-coupling products in acceptable to good yields. The reaction proceeds with inversion of the configuration.

Palladium-catalyzed hydroalkylation of styrenes with organozinc reagents to form carbon-carbon sp3-sp3 bonds under oxidative conditions

An unconventional route for the formation of sp(3)-sp(3) C-C bonds from various styrenes and an organozinc reagent in a formal alkene hydroalkylation process is detailed. Mechanistically, this process is proposed to proceed by initial transmetalation followed by formation of a Pd-H species, which is subsequently trapped by the styrene.

Ni-catalyzed Sonogashira coupling of nonactivated alkyl halides: orthogonal functionalization of alkyl iodides, bromides, and chlorides

Ni-catalyzed Sonogashira coupling of nonactivated, beta-H-containing alkyl halides, including chlorides, is reported. The coupling is tolerant to a wide range of functional groups, including ether, ester, amide, nitrile, keto, heterocycle, acetal, and aryl halide, in both coupling partners. The coupling can be selective for a specific C-X bond (X = I, Br, Cl) and allows for orthogonal functionalization of alkyl halides with multiple reactive sites.

Functional group tolerant Kumada-Corriu-Tamao coupling of nonactivated alkyl halides with aryl and heteroaryl nucleophiles: catalysis by a nickel pincer complex permits the coupling of functionalized Grignard reagents

A nickel(II) pincer complex [((Me)NN(2))NiCl] (1) catalyzes Kumada-Corriu-Tamao cross coupling of nonactivated alkyl halides with aryl and heteroaryl Grignard reagents. The coupling of octyl bromide with phenylmagnesium chloride was used as a test reaction. Using 3 mol % of 1 as the precatalyst and THF as the solvent, and in the presence of a catalytic amount of TMEDA, the coupling product was obtained in a high yield. The reaction conditions could be applied to cross coupling of other primary and secondary alkyl bromides and iodides. The coupling is tolerant to a wide range of functional groups. Therefore, alkyl halides containing ester, amide, ether, thioether, alcohol, pyrrole, indole, furan, nitrile, conjugated enone, and aryl halide moieties were coupled to give high isolated yields of products in which these units stay intact. For the coupling of ester-containing substrates, O-TMEDA is a better additive than TMEDA. The reaction protocol proves to be efficient for the coupling of Knochel-type functionalized Grignard reagents. Thus aryl Grignard reagents containing electron-deficient and/or sensitive ester, nitrile, amide, and CF(3) substituents could be successfully coupled to nonactivated and functionalized alkyl iodides. The catalysis is also efficient for the coupling of alkyl iodides with functionalized heteroaryl Grignard reagents, giving rise to pyridine-, thiophene-, pyrazole-, furan-containing molecules with additional functionalities. Concerning the mechanism of the catalysis, [((Me)NN(2))Ni-(hetero)Ar] was identified as an intermediate, and the activation of alkyl halides was found to take place through a radical-rebound process.