Achieving Chemo-, Regio-, and Stereoselectivity in Palladium-Catalyzed Reaction of γ-Borylated Allylic Acetates

Versatile Reactivity of Metalloid-Substituted π-Allylpalladium Species

π-Allylpalladium complexes can not only serve as electrophilic allylating agents for a broad range of nucleophiles, but also nucleophilic allylating agents for electrophiles depending on their electronic environments. In contrast to these typical reactivities of π-allylpalladium complexes, silylated and borylated π-allylpalladium species show unique reactivities that can allow versatile transformations in addition to simple allylation. Herein, four different types of transformations that are in principle achieved via the inherently reactive silylated and borylated π-allylpalladium species as common intermediates are described. An appropriate selection of ligands of the silylated and borylated π-allylpalladium species can allow control over the reaction pathways.

Copper-Mediated Synthesis of (E)-1-Azido and (Z)-1,2-Diazido Alkenes from 1-Alkene-1,2-diboronic Esters: An Approach to Mono- and 1,2-Di-(1,2,3-Triazolyl)-Alkenes and Fused Bis-(1,2,3-Triazolo)-Pyrazines

A stereoselective and convenient route has been demonstrated to access (Z)-1,2-diazido alkenes from the corresponding 1,2-diboronic esters via a copper-mediated reaction with sodium azide. Alternately, mono-functionalization was regioselectively carried out with trimethylsilyl azide as an azidation reactant. The in situ conversion of bis-azides to the corresponding bis-triazoles can be readily achieved in the presence of copper sulfate and sodium ascorbate, while the modification of the catalytic system opened a new convenient route to bis-triazolo-pyrazines, a new class of fused heterocycles.

Enantioselective α-Allylation of Acyclic Esters Using B(pin)-Substituted Electrophiles: Independent Regulation of Stereocontrol Elements through Cooperative Pd/Lewis Base Catalysis

Cooperation between a Lewis base and Pd catalyst enables the direct enantioselective α-functionalization of aryl and vinyl acetic acid esters using a bifunctional B(pin)-substituted electrophile. Critical to the success of this method was the recognition that both catalysts could control the necessary stereochemical aspects; the Lewis base catalyst controls the enantioselectivity of the reaction, whereas the Pd catalyst regulates alkenyl-B(pin) configuration. This is the first example of using cooperative catalysis to control both stereochemical features during Pd-catalyzed allylic alkylation.

Palladium-catalyzed enantioselective allylic alkylation of trifluoromethyl group substituted racemic and acyclic unsymmetrical 1,3-disubstituted allylic esters with malonate anions

We accomplished the enantioselective allylic alkylation of racemic and acyclic unsymmetrical 1,3-disubstituted allylic esters with a malonate anion by DYKAT.

Palladium-catalyzed chemoselective allylic substitution, Suzuki-Miyaura cross-coupling, and allene formation of bifunctional 2-B(pin)-substituted allylic acetate derivatives

A formidable challenge at the forefront of organic synthesis is the control of chemoselectivity to enable the selective formation of diverse structural motifs from a readily available substrate class. Presented herein is a detailed study of chemoselectivity with palladium-based phosphane catalysts and readily available 2-B(pin)-substituted allylic acetates, benzoates, and carbonates. Depending on the choice of reagents, catalysts, and reaction conditions, 2-B(pin)-substituted allylic acetates and derivatives can be steered into one of three reaction manifolds: allylic substitution, Suzuki-Miyaura cross-coupling, or elimination to form allenes, all with excellent chemoselectivity. Studies on the chemoselectivity of Pd catalysts in their reactivity with boron-bearing allylic acetate derivatives led to the development of diverse and practical reactions with potential utility in synthetic organic chemistry.

Breaking Conjugation: Unusual Regioselectivity with 2-Substituted Allylic Substrates in the Tsuji-Trost Reaction

η³-Allyl palladium complexes are key intermediates in Tsuji-Trost allylic substitution reactions. It is well known that (η³-1-aryl-3-alkyl substituted allyl)Pd intermediates result in nucleophilic attack at the alkyl substituted terminus. In contrast, the chemistry of (η³-1,2,3-trisubstituted allyl)Pd intermediates is relatively unexplored. Herein we probe the regioselectivity with 1,2,3-trisubstituted allylic substrates in Tsuji-Trost allylic substitution reactions. DFT investigation of cationic (η³-1-Ph-2-B(pin)-3-alkyl-allyl)Pd(PPh₃)₂ intermediates predict that nucleophilic attack should occur preferentially on anti-allyls rather than the syn-isomers to generate benzylic substitution products under Curtin-Hammett conditions. Experimentally, systematic studies with 1,2,3-trisubstituted allylic substrates revealed that a Linear Free Energy Relationship (LFER) is observed when Charton steric parameters of the C-2 substituents are plotted against the log of the ratio of regioisomers. Bulkier C-2 substituents in 1,2,3-trisubstituted η³-allyl palladium intermediates provide stronger preference for nucleophilic attack at anti-oriented benzylic termini. Additionally, the geometry of 1,4-elimination products supports the presence of anti-allyl palladium intermediates.

Synthesis of alkenyl boronates from allyl-substituted aromatics using an olefin cross-metathesis protocol

An efficient synthesis of 3-aryl-1-propenyl boronates from pinacol vinyl boronic ester and allyl-substituted aromatics by cross metathesis is reported. Although the allylbenzene derivatives are prone to isomerization reaction under metathesis conditions, we found that some ruthenium catalysts are effective for this methodology. This strategy thus provides an interesting alternative approach to alkyne hydroboration, leading to the preparation of unknown compounds. Moreover, the boron substituent can be replaced by various functional groups in good yields.

Aminoborylation/Suzuki-Miyaura tandem cross coupling of aryl iodides as efficient and selective synthesis of unsymmetrical biaryls

Sequential borylation of a first aryl iodide using a dialkylaminoborane followed by a Suzuki-Miyaura cross coupling of second aryl iodide ended up with an efficient, selective and practical synthesis of unsymmetrical biaryls. This tandem coupling shows a wide range of applicability.