Intramolecular Pd(0)-Catalyzed Reactions of (2-Iodoanilino)-aldehydes: A Joint Experimental–Computational Study

Pd(0)-Catalyzed Asymmetric Intramolecular Grignard-Type Reaction of Vinyl Iodide-Carbonyl

The insertion of carbonyl into C(sp2)-Pd(II) σ-bond (Grignard-type addition) was not established until the 1990s. While this elemental reaction has been well explored since then, its application in Pd(0) asymmetric catalysis remain elusive. Herein, we report the Pd(0)-catalyzed asymmetric intramolecular Grignard-type reaction of vinyl iodide-carbonyl in the presence of HCO2H additive, affording cyclic allylic alcohol with good to excellent enantioselectivity and diastereoselectivity. Mechanistic studies suggested that besides serving as an efficient reductant, HCO2H is also capable of facilitating protonation of the involved secondary alkoxyl-Pd(II), thus completely suppressing the β-H elimination. Moreover, no KIE was found in the competing reaction between vinyl iodide-aldehyde and 1-deuterated one, demonstrating the facile step of aldehyde insertion.

Palladium-catalyzed stereoselective domino arylation-acylation: an entry to chiral tetrahydrofluorenone scaffolds

A palladium-catalyzed domino arylation-cyclization of biocatalytically derived cyclic 1,3-dienes is demonstrated. The reaction introduces a high degree of structural complexity in a single step, giving access to tricyclic tetrahydrofluorenones with full regio- and stereoselectivity. The transformation proceeds through a novel acylation-terminated Heck-type sequence, and quantum chemical calculations indicate that C-H activation is involved in the terminating acylation step.

Concise catalytic asymmetric synthesis of (R)-4-amino Uhle's ketone

A practical and asymmetric synthesis of (R)-4-amino-5-oxo-1,3,4,5-tetrahydrobenz[cd]indole, an enantiopure framework shared by most ergot alkaloids, was accomplished. Our method involves a Rh(i)-catalyzed 6-exo-trig intramolecular cyclization of an appropriate 4-pinacolboronic ester d-tryptophan aldehyde followed by the oxidation of the resulting secondary benzylic alcohol with a Cu(i)-ABNO catalyst and final deprotection under acidic conditions. This new procedure offers significant advantages over previous synthetic approaches, including brevity, mild reaction conditions, preservation of chiral integrity, and high overall yield and avoids the use of stoichiometric amounts of strongly basic and pyrophoric organometallic reagents.

Palladium-Catalyzed Cross-Coupling of gem-Bis(boronates) with Aryl Halides: An Alternative To Access Quaternary α-Aryl Aldehydes

The compounds with a quaternary α-aryl aldehyde skeleton are important units in organic chemistry. Previously, the aryl group and carbonyl group are introduced in a stepwise manner. Herein, a novel route is developed to construct the quaternary α-aryl aldehydes with gem-bis(boronates) as precursors, in which the two groups are installed simultaneously. The gem-bis(boronates) are readily available from ketones; as a result, this methodology provides a more general strategy to produce the quaternary α-aryl aldehydes with broad scopes and synthetic convenience.

Axial stereocontrol in tropos dibenz[c,e]azepines: the individual and cooperative effects of alkyl substituents

6,7-Dihydro-5H-dibenz[c,e]azepines, a class of secondary amine incorporating a centre-axis chirality relay, can be prepared from N-(2-bromobenzyl)-N-(1-arylalkyl)methanesulfonamides via Pd-catalysed intramolecular direct arylation, and methylated at C(7) via the 5,7-trans diastereoselective addition of methylmagnesium bromide to the derived N-benzylazepinium tetraphenylborate. Using these methods, the 4,5-dimethylated and 4,5,7-trimethylated homologues 13 and 14 were obtained and shown by ¹H NMR spectroscopy to be axially biased in opposite senses, as defined by the respective pseudoaxial or pseudoequatorial orientation of the 5-substituent in the preferred conformers, while retaining their tropos nature (the Arrhenius activation energy, E_A, for the conformational exchange process in 14 was estimated to be 57 kJ mol^-1 using 2D-EXSY NMR spectroscopy at 233-248 K). These results serve to illustrate how substituent effects might be exploited in new designs of bridged biaryl ligand in which tropos dynamics operate in combination with a pre-existing axial stereochemical bias.

Rhodium(i)-catalyzed stereoselective [4+2] cycloaddition of oxetanols with alkynes through C(sp3)-C(sp3) bond cleavage

An efficient and convenient synthesis of highly functionalized dihydropyrans has been achieved through rhodium(i)-catalysed tandem C(sp³)–C(sp³) bond cleavage and annulation of oxetanols with alkynes.

An efficient and convenient synthesis of highly functionalized dihydropyrans has been achieved through rhodium(i)-catalysed tandem C(sp³)–C(sp³) bond cleavage and annulation of oxetanols with alkynes. An enantioselective version was enabled using a Binaphine ligand. Excellent site-selectivity and remarkable enantioretention are obtained for 2-substituted oxetanols.

Stereoselective synthesis of tricyclic compounds by intramolecular palladium-catalyzed addition of aryl iodides to carbonyl groups

Starting from γ-ketoesters with an o-iodobenzyl group we studied a palladium-catalyzed cyclization process that stereoselectively led to bi- and tricyclic compounds in moderate to excellent yields. Four X-ray crystal structure analyses unequivocally defined the structure of crucial cyclization products. The relative configuration of the precursor compounds is essentially transferred to that of the products and the formed hydroxy group in the newly generated cyclohexane ring is consistently in trans-arrangement with respect to the methoxycarbonyl group. A transition-state model is proposed to explain the observed stereochemical outcome. This palladium-catalyzed Barbier-type reaction requires a reduction of palladium(II) back to palladium(0) which is apparently achieved by the present triethylamine.

Synthesis of isatins by the palladium-catalyzed intramolecular acylation of unactivated aryl C(sp2)–H bonds

Synthesis of isatins from formyl-N-arylformamides is achieved via PdCl₂-catalyzed intramolecular acylation.

Direct construction of 2-alkylbenzo-1,3-azoles via C–H activation of alkanes for C–C and C–X (X = O, S) bond formation

2-Alkylated benzo(oxa)thiazoles were prepared directly from simple alkanes and aryl isocyanates/isothiocyanates in a one-pot procedure.

Controlling the ambiphilic nature of σ-arylpalladium intermediates in intramolecular cyclization reactions

The reactivity of main group organometallics, such as organolithium compounds (RLi) and Grignard reagents (RMgX), is quite straightforward. In these species the R group usually exhibits nucleophilic reactivity without any possibility of inducing electrophilic character. In contrast, in organopalladium complexes, researchers can switch the reactivity from electrophilic to nucleophilic relatively simply. Although σ-aryl and σ-vinylpalladium complexes are commonly used as electrophiles in C-C bond-forming reactions, recent research has demonstrated that they can also react with carbon-heteroatom multiple bonds in a nucleophilic manner. Nevertheless, researchers have completely ignored the issue of controlling the ambiphilic nature of such species. This Account describes our efforts toward selectively promoting the same starting materials toward either electrophilic α-arylation or nucleophilic addition reactions to different carbonyl groups. We could tune the properties of the σ-arylpalladium intermediates derived from amino-tethered aryl halides and carbonyl compounds to achieve chemoselective transformations. Therefore, chemists can control the ambiphilic nature of such intermediates and, consequently, the competition between the alternative reaction pathways by the adequate selection of the reaction conditions and additives (base, presence/absence of phenol, bidentate phosphines). The nature of the carbonyl group (aldehydes, ketones, esters, and amides) and the length of the tether connecting it to the aniline moiety also play an important role in the outcome of these processes. Our joint computational and experimental efforts to elucidate the reaction mechanism of these palladium-catalyzed transformations suggest that beyond the formation of the four-membered azapalladacycle, two major factors help to control the dual character of the palladium(II) intermediates derived from 2-haloanilines. First, their high nucleophilicity strongly modifies the interaction of the metal center with the carbonyl group. Second, the additive phenol exchanges the iodide ligand to give an arylpalladium(II) phenoxide complex, which has a beneficial effect on the arylation. The formation of this transient intermediate not only stabilizes the arylpalladium moiety, thus preventing the nucleophilic attack at the carbonyl group, but also assists the enolization reaction, which takes place in a more favorable intramolecular manner. The azapalladacycle intermediate is, in the words of J. R. R. Tolkien, "the one ring to bring them all and in the darkness to bind them." With this intermediate, we can easily achieve the synthesis of a variety of heterocyclic systems by selectively promoting electrophilic α-arylation or nucleophilic addition reactions from the same precursors.

Iron-catalyzed decarboxylative alkenylation of cycloalkanes with arylvinyl carboxylic acids via a radical process

A Fe(acac)₃-catalyzed decarboxylative coupling of 2-(aryl)vinyl carboxylic acids with cycloalkanes was developed by using DTBP as an oxidant through a radical process. This reaction tolerates a wide range of substrates, and products are obtained in good to excellent yields (71–95%). The reaction also shows excellent stereoselectivity, and only trans-isomers are obtained.