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.

Domino Sequences Involving Stereoselective Hydrazone-Type Heck Reaction and Denitrogenative [1,5]-Sigmatropic Rearrangement

Although the Heck reactions of alkene partners with various electrophiles have achieved great success, the variant focused on carbon═heteroatom counterparts still remains elusive. Herein, we report a Pd(0)-catalyzed asymmetric intramolecular hydrazone-type Heck reaction of N-[(Z)-3-iodoallyl]-aminoacetaldehyde and hydrazine hydrate (NH₂NH₂-H₂O), wherein the required hydrazone is in situ generated via an acid-promoted condensation. A key strategic advantage of this Heck paradigm is that the resultant Heck product allylic diazene rapidly undergoes stereospecific denitrogenative [1,5]-sigmatropic rearrangement, eventually furnishing a domino sequence toward 3-substituted tetrahydropyridine (THP) with high enantioselectivity. The substrate-induced diastereoselective version has also been realized, exclusively giving cis-2,5-disubstituted THPs. The utility of this sequence is demonstrated by the formal synthesis of multiple valuable bioactive targets, including 3-ethylindoloquinolizine, preclamol, and niraparib.

Pd(0)-Catalyzed Intramolecular Reductive Heck Reaction of Vinyl Iodide and Oxime Ether: Enantioselective Synthesis of Cyclic Allylic N-Alkoxy Amine

Whereas the intramolecular reductive Heck reaction of aryl/vinyl halide and alkene has been well documented, the oxime analogue remains extremely elusive. Herein we report the Pd(0)-catalyzed intramolecular reductive Heck reaction of vinyl iodide and oxime ether with the use of formic acid as the reductant. It is found that the TsOH additive plays a crucial role in the reaction efficiency, and the (S)-SEGPhos ligand enables cyclic allylic N-alkoxy amine products with high enantioselectivity.

Evidence and exploitation of dicationic ammonium-nitrilium superelectrophiles: direct synthesis of unsaturated piperidinones

Exploiting superacid activation, the reactivity of aminonitriles was enhanced through the transient formation of highly reactive ammonium-nitrilium superelectrophiles. Demonstrated by using in situ low-temperature NMR experiments and confirmed by X-ray diffraction analysis, these dications can be intramolecularly trapped by non-activated alkenes to generate unsaturated piperidinones, including enantioenriched ones, in a straightforward way.

Palladium-catalyzed heck-type cascade cyclization of (Z)-1-iodo-1,6-dienes with N-tosyl hydrazones

A new way to synthesize tetrahydropyridine derivatives bearing a terminal alkene due to highly regioselective β-H elimination of alkylpalladium species.

Pd(0)-Catalysed asymmetric reductive Heck-type cyclization of (Z)-1-iodo-1,6-dienes and enantioselective synthesis of quaternary tetrahydropyridines

The asymmetric reductive Heck reaction of (Z)-1-iodo-1,6-dienes has been developed, which affords quaternary tetrahydropyridines with good to excellent enantioselectivities.

Oxime ethers as versatile precursors in organic synthesis: a review

This review is a survey of the literature describing synthetic applications of oxime ethers. The cyclization and metal-catalyzed cross-coupling reactions of oxime ethers in recent years are also highlighted.

Access to functionalized bicyclo[4,3,0]nonenes via palladium-catalyzed oxidative cyclization of 2-allylcyclohexyl oximes

A new palladium-catalyzed oxidative cyclization process leading to the functionalized bicyclo[4,3,0]nonenes is serendipitously discovered during attempts to form aza-heterocycle by the amino-Heck reaction of trans-2-vinylclohexyl phosphinyloxime. Under the influence of Pd(dba)2/Et3N/1:1 N2-O2 (1:1, v/v) (Method A) or Pd(OAc)2/Et3N/O2 (Method B), the reactions afford the substituted cis-1-hydroxyl-8-formyl-bicyclo[4,3,0]non-8(9)-enes or bicycle[4,3,0]non-1(9)-en-8-ones in varying yields with the incorporation of molecular oxygen into the structures. The 5,6-bicyclic scaffold of these products is presumably derived from tandem double intramolecular cyclization followed by the ring-opening of an aza-palladium(II) tricyclic intermediate.

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.