Allylic substitution versus Suzuki cross-coupling: capitalizing on chemoselectivity with bifunctional substrates

LiOtBu-Promoted trans-Stereoselective and β-Regioselective Hydroboration of Propargyl Alcohols

A convenient and efficient trans-stereoselective and β-regioselective hydroboration of propargyl alcohols was achieved simply with LiOtBu as the base and (Bpin)2 as the boron reagent in dimethyl sulfoxide at room temperature. Both terminal and internal propargyl alcohols with diverse structures and functional groups underwent the transformation smoothly to produce β-Bpin-substituted (E)-allylic alcohols, of which the synthetic potentials were demonstrated by the downstream conversions of boronate, alkenyl, and hydroxyl groups.

Synthesis and reactivity of alkynyl boron compounds

Over the last century, there have been considerable developments in organoboron chemistry due to the stability, non-toxicity, and easy commercial availability of various boronic esters. Several organoboron reagents have emerged and play an increasingly important role in everyday organic synthesis. Among them, alkynyl boron compounds have attracted significant attention due to their easy synthesis and diverse reactivity. In this review, we summarize the advancement of research on alkynyl boron compounds, highlighting their importance in the synthesis of valuable compounds.

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.

Traversing Steric Limitations by Cooperative Lewis Base/Palladium Catalysis: An Enantioselective Synthesis of α-Branched Esters Using 2-Substituted Allyl Electrophiles

Cooperative catalysis enables the direct enantioselective α-allylation of linear prochiral esters with 2-substituted allyl electrophiles. Critical to the successful development of the method was the recognition that metal-centered reactivity and the source of enantiocontrol are independent. This feature is unique to simultaneous catalysis events and permits logical tuning of the supporting ligands without compromising enantioselectivity.

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.

Gold-catalyzed allylation of aryl boronic acids: accessing cross-coupling reactivity with gold

A sp(3)-sp(2) C-C cross-coupling reaction catalyzed by gold in the absence of a sacrificial oxidant is described. Vital to the success of this method is the implementation of a bimetallic catalyst bearing a bis(phosphino)amine ligand. A mechanistic hypothesis is presented, and observable transmetalation, C-Br oxidative addition, and C-C reductive elimination in a model gold complex are shown. We expect that this method will serve as a platform for the development of novel transformations involving redox-active gold catalysts.

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.

ortho-Selective nucleophilic addition of amines to 3-borylbenzynes: synthesis of multisubstituted anilines by the triple role of the boryl group

Nucleophilic addition of amines to 3-[(dan)boryl]benzynes (dan = 1,8-diaminonaphthalene) generated by a fluoride ion proceeded with high ortho-selectivity to give 2-borylaniline derivatives, under conditions that are tolerant to various functional groups. The (dan)boryl group of the adduct was hydrolyzed into a boronic acid under acidic conditions, which could further serve for various C-C, C-O, C-N, and C-H bond-formation reactions. The overall process provides a promising entry for preparing multisubstituted aniline derivatives.

Formal regiocontrolled hydroboration of unbiased internal alkynes via borylation/allylic alkylation of terminal alkynes

In accessing trisubstituted vinyl boronates from terminal alkynes, a propargyl directing (2-pyridyl)sulfonyl group allows terminal alkynes to undergo Cu-catalyzed B2(pin)2-borylation and subsequent Cu-catalyzed allylic alkylation with Grignard reagents without affecting the pinacolboronate moiety, thereby formally enabling a highly stereo- and regiocontrolled access to hydroboration products of unbiased dialkyl internal alkynes.

Stereoselective vinylation of aryl N-(2-pyridylsulfonyl) aldimines with 1-alkenyl-1,1-heterobimetallic reagents

Vinylation of aryl N-(2-pyridylsulfonyl) aldimines with versatile 1-alkenyl-1,1-borozinc heterobimetallic reagents is disclosed. In situ hydroboration of air-stable B(pin)-alkynes followed by chemoselective transmetalation with dimethylzinc and addition to aldimines provides B(pin)-substituted allylic amines in 53-93% yield in a one-pot procedure. The addition step can be followed by either B-C bond oxidation to provide α-amino ketones (71-98% yield) or Suzuki cross-coupling to furnish trisubstituted 2-arylated (E)-allylic amines (51-73% yield).

Iridium-Catalyzed Allylic Amination Route to α-Aminoboronates: Illustration of the Decisive Role of Boron Substituents

The development of a new route to α-aminoboronates using an iridium-catalyzed allylic amination on boronated substrates is described. Unlike the boronate group, the trifluoroborato substituent was found to govern the regioselectivity exclusively in favor of branched products. The transformation of an allylic substitution product into an α-aminoboronic ester in an efficient way validated the implementation of this approach.

Palladium-catalyzed allylic substitution with (η6-arene-CH2Z)Cr(CO)(3)-based nucleophiles

Although the palladium-catalyzed Tsuji-Trost allylic substitution reaction has been intensively studied, there is a lack of general methods to employ simple benzylic nucleophiles. Such a method would facilitate access to "α-2-propenyl benzyl" motifs, which are common structural motifs in bioactive compounds and natural products. We report herein the palladium-catalyzed allylation reaction of toluene-derived pronucleophiles activated by tricarbonylchromium. A variety of cyclic and acyclic allylic electrophiles can be employed with in situ generated (η(6)-C(6)H(5)CHLiR)Cr(CO)(3) nucleophiles. Catalyst identification was performed by high throughput experimentation (HTE) and led to the Xantphos/palladium hit, which proved to be a general catalyst for this class of reactions. In addition to η(6)-toluene complexes, benzyl amine and ether derivatives (η(6)-C(6)H(5)CH(2)Z)Cr(CO)(3) (Z = NR(2), OR) are also viable pronucleophiles, allowing C-C bond-formation α to heteroatoms with excellent yields. Finally, a tandem allylic substitution/demetalation procedure is described that affords the corresponding metal-free allylic substitution products. This method will be a valuable complement to the existing arsenal of nucleophiles with applications in allylic substitution reactions.

Diastereoselective aziridination of 2-B(pin)-substituted allylic alcohols: an efficient approach to novel organoboron compounds

We report that 2-B(pin)-substituted allylic alcohols are good substrates for diastereoselective aziridinations in the presence of PhI(OAc)(2) and N-aminophthalimide. Under the aziridination conditions, the valuable B-C bond remains intact, affording a variety of novel boron-substituted aziridines in good yields and excellent diastereoselectivities. Oxidation of the aziridine B-C bond enables generation of syn-1,3-aminohydroxy-2-ketones with high diastereoselectivity.