1,4-Pd Migration-Enabled Synthesis of Fused 4-Membered Rings.

1,4-Palladium migration has been widely used for the functionalization of remote C-H bonds. However, this mechanism has been limited to aryl halide precursors. This work reports an unprecedented Pd0-catalyzed cyclobutanation protocol producing valuable fused cyclobutanes starting from cycloalkenyl (pseudo)halides. This reaction takes place via alkenyl-to-alkyl 1,4-Pd migration, followed by intramolecular Heck coupling. The method performs best with cyclohexenyl precursors, giving access to a variety of substituted bicyclo[4,2,0]octenes. Reactants containing an N-methyl or methoxy group give rise to fused azetidines or oxetanes, respectively, via the same mechanism. Kinetic and deuterium-labeling studies point to a rate-limiting C(sp3)-H activation step.

Direct Synthesis of Cyclopropanes from gem-Dialkyl Groups through Double C-H Activation.

Cyclopropanes are important structural motifs found in numerous bioactive molecules, and a number of methods are available for their synthesis. However, one of the simplest cyclopropanation reactions involving the intramolecular coupling of two C-H bonds on gem-dialkyl groups has remained an elusive transformation. We demonstrate herein that this reaction is accessible using aryl bromide or triflate precursors and the 1,4-Pd shift mechanism. The use of pivalate as the base was found to be crucial to divert the mechanistic pathway toward the cyclopropane instead of the previously obtained benzocyclobutene product. Stoichiometric mechanistic studies allowed the identification of aryl- and alkylpalladium pivalates, which are in equilibrium via a five-membered palladacycle. With pivalate, a second C(sp3)-H activation leading to the four-membered palladacycle intermediate and the cyclopropane product is favored. A catalytic reaction was developed and showed a broad scope for the generation of diverse arylcyclopropanes, including valuable bicyclo[3.1.0] systems. This method was applied to a concise synthesis of lemborexant, a recently approved anti-insomnia drug.

Synthesis of Amides and Esters by Palladium(0)-Catalyzed Carbonylative C(sp3 )-H Activation.

The 1,4-palladium shift strategy allows the functionalization of remote C-H bonds that are difficult to reach directly. Reported here is a domino reaction proceeding by C(sp3 )-H activation, 1,4-palladium shift, and amino- or alkoxycarbonylation, which generates a variety of amides and esters bearing a quaternary ß-carbon atom. Mechanistic studies showed that the aminocarbonylation of the σ-alkylpalladium intermediate arising from the palladium shift is fast using PPh3 as the ligand, and leads to the amide rather than the previously reported indanone product.

Cooperative Pd/Cu Catalysis: Multicomponent Synthesis of Tetrasubstituted Imidazolones from Methyl α-Isocyanoacetates, Primary Amines, and Aryl(vinyl) Iodides.

Three-component reaction of methyl α,α-disubstituted α-isocyanoacetates, primary amines, and aryl(vinyl) halides in the presence of Pd(OAc)2 (0.05 equiv) and Cu2O (1.0 equiv) provided 2,3,5,5-tetrasubstituted imidazolones via the formation of three chemical bonds. A copper-mediated migratory insertion of the isocyano group into the N-H bond of the amine followed by lactamization and Pd-catalyzed cross-coupling of the in situ generated amidinyl copper species with aryl(vinyl) halides accounted for the reaction outcome.

Silver Nitrate-Catalyzed Isocyanide Insertion/Lactamization Sequence to Imidazolones and Quinazolin-4-ones: Development and Application in Natural Product Synthesis.

Silver nitrate-catalyzed reaction of methyl α,α-disubstituted α-isocyanoacetates with primary amines afforded 3,5,5-trisubstituted imidazolones in good to excellent yields. A silver salt-catalyzed insertion of the isocyano group into the N-H bond of the amine followed by in situ lactamization accounted for the reaction outcome. The same transformation between methyl 2-isocyanobenzoate and amines afforded quinazolin-4-ones in excellent yields. The utility of this chemistry was illustrated by the development of concise syntheses of (±)-evodiamine and rutaecarpine.

Enantioselective Synthesis of Quaternary α-Amino Acids Enabled by the Versatility of the Phenylselenonyl Group.

A novel Cinchona alkaloid-catalyzed enantioselective conjugate addition of α-alkyl substituted α-nitroacetates to phenyl vinyl selenone was developed. The resulting enantio-enriched α,α-dialkyl substituted α-nitroacetates were subsequently converted to various cyclic and acyclic quaternary α-amino acids, taking advantage of the rich functionalities of the resulting Michael adducts. Novel protocols allowing chemoselective reduction of phenyl selenone to phenyl selenide and reduction of alkyl phenyl selenones to alkanes are also reported.

Functionalized chromans and isochromans via a diastereoselective Pd(0)-catalyzed carboiodination.

A diastereoselective approach to isochromans and chromans via Pd(0)-catalyzed carboiodination is reported. The transformations using this methodology display excellent yields and diastereoselectivities as well as broad functional group compatibility. The selectivity observed in these cyclizations, forming isochroman or chroman targets, is postulated to originate from the minimization of A(1,2) strain and axial-axial interactions, respectively. This method has also been used to highlight the concept of reversible oxidative addition to carbon-iodine bonds in polyiodinated substrates.