The Critical Importance of Water in the Asymmetric Allylboration of N-Trimethylsilylbenzaldimines with B-Allyldiisopinocampheylborane

Palladium-Catalyzed Cascade Heck Coupling and Allylboration of Iododiboron Compounds via Diboryl Radicals

Geminal bis(boronates) are versatile synthetic building blocks in organic chemistry. The fact that they predominantly serve as nucleophiles in the previous reports, however, has restrained their synthetic potential. Herein we disclose the ambiphilic reactivity of α-halogenated geminal bis(boronates), of which the first catalytic utilization was accomplished by merging a formal Heck cross-coupling with a highly diastereoselective allylboration of aldehydes or imines, providing a new avenue for rapid assembly of polyfunctionalized boron-containing compounds. We demonstrated that this cascade reaction is highly efficient and compatible with various functional groups, and a wide range of heterocycles. In contrast to a classical Pd(0/II) scenario, mechanistic experiments and DFT calculations have provided strong evidence for a catalytic cycle involving Pd(I)/diboryl carbon radical intermediates.

Chemoselective reduction and oxidation of ketones in water through control of the electron transfer pathway

The selective synthesis of different products from the same starting materials in water, which is the most abundant solvent in nature, is a crucial issue as it maximizes the utilization of materials. Realizing such reactions for ketones is of considerable importance because numerous organic functionalities can be obtained via nucleophilic addition reactions. Herein, we report chemoselective reduction and oxidation reactions of 1,2-diketones in water, which initiates anionic electron transfer from the inorganic electride [Ca24Al28O64](4+)·4e(-), through controlling the pathway of the electrons to substrates. The generation of different radical species for transient intermediates was the key process required to control the reaction selectivity, which was achieved by reacting the anionic electrons with either diketones or O2, leading to the formation of ketyl dianion and superoxide radicals in the reduction and oxidation reactions, respectively. This methodology that utilizes electrides may provide an alternative to the pulse radiolysis of water in synthetic chemistry.

Silyl imine electrophiles in enantioselective catalysis: a Rosetta Stone for peptide homologation, enabling diverse N-protected aryl glycines from aldehydes in three steps

We report that N-(trimethylsilyl)imines serve in the Bis(AMidine)-catalyzed addition of bromonitromethane with a high degree of enantioselection. This allows for the production of a range of protected α-bromo nitroalkane donors (including Fmoc) for use in Umpolung Amide Synthesis (UmAS). Hence, peptide homologation with nonnatural aryl glycine amino acids is achieved in three steps from aromatic aldehydes, which are plentiful and inexpensive. Epimerization during the homologation step is circumvented by avoiding an α-amino acid intermediate.

Stereoselective formation of amines by nucleophilic addition to azomethine derivatives

This chapter describes state-of-the-art methods to prepare α-chiral amines by the addition of nonstabilized nucleophiles to imine derivatives. The first part of the chapter illustrates the most effective diastereoselective addition reaction (substrate controlled and chiral auxiliary based methods) whereas the second part focuses on catalytic asymmetric methods.

Asymmetric synthesis of 2 degrees- and 3 degrees-Carbinols via B-methallyl-10-(TMS and Ph)-9-borabicyclo[3.3.2]decanes

Simple Grignard procedures provide methallylboranes 1a and 1b in enantiomerically pure form from air-stable precursors in 98% and 95% yields, respectively. These reagents add smoothly to aldehydes and methyl ketones, respectively, providing branched 2 degrees- (6, 69-89%, 94-99% ee) and 3 degrees- (10, 71-87%, 74-96% ee) homoallylic alcohols.

Nonracemic 3 degrees-carbamines from the asymmetric allylboration of N-trimethylsilyl ketimines with B-allyl-10-phenyl-9-borabicyclo[3.3.2]decanes

The simple and efficient asymmetric synthesis of 3 degrees -carbamines 7 from N-TMS enamines (3) and either enantiomeric form of beta-allyl-10-(phenyl)-9-borabicyclo[3.3.2]decane (1) is reported. The high reactivity (<1 h, -78 degrees C) and enantioselectivity (60-98% ee) of these substrates can be attributed to the fact that the complexation of 3 with 1 facilitates its isomerization to the corresponding syn-N-TMS ketimine complex from which allylation can occur. In addition to providing the homoallylic amines 7 with predictable stereochemistry, the procedure also permits the efficient recovery of the chiral boron moiety (50-65%) as air-stable crystalline pseudoephedrine complexes 8, which are directly converted back to 1 with allylmagnesium bromide in ether (98%).

Convenient Synthesis of Stable Aldimine−Borane Complexes, Chiral δ-Amino Alcohols, and γ-Substituted GABA Analogues from Nitriles

A one-pot synthesis of stable aldimine-trialkylborane adducts, the first synthesis of C- and N-deuterated imine-borane complexes, and their application for a highly enantioselective (84-99% ee) synthesis of delta-amino alcohols and gamma-substituted gamma-aminobutyric acids, including deuterated amino acids from nitriles, are described.

[Development of efficient methods for synthesis of nitrogen-containing compounds using carbamates, acylhydrazines, and ammonia]

For the efficient synthesis of divergent nitrogen-containing compounds of pharmaceutical and agricultural importance, the development of efficient, complementary, and new synthetic methodologies is essential. One of the key subjects is how to introduce nitrogen atoms in to organic molecules. This review summaries our recent efforts on this issue, focusing on the use of carbamates, acylhydrazines, and ammonia as nitrogen sources.

Enantioselective Transfer Aminoallylation: Synthesis of Optically Active Homoallylic Primary Amines

A camphorquinone-derived chiral homoallylic amine was found to react with various aldehydes via imine formation and asymmetric 2-azonia-Cope rearrangement to give optically active homoallylic primary amines. A practical level of enantioselectivity with high functional group tolerance has been attained in this transformation.

Asymmetric Allylboration of Cyclic Imines and Applications to Alkaloid Synthesis

Treatment of cyclic imines with 3,3'-disubstituted binaphthol modified allylboronates provides the expected allylated products in good yields and with high stereoselectivities (91-99% ee). The products may be readily transformed into various alkaloids.

Sequential Pd-Catalyzed Asymmetric Allene Diboration/α-Aminoallylation

Pd-catalyzed enantioselective diboration of prochiral allenes provides adducts which participate in highly selective allylation reactions with primary imines. The allylation product is a vinyl boronate which may be oxidized to give nonracemic Mannich products (87-97% ee). Alternatively, enantiomerically enriched homoallylic amine derivatives may be obtained by protonation and Suzuki cross-coupling of the vinyl boronate.

Highly Diastereoselective and Enantioselective Preparation of Homoallylic Amines: Application for the Synthesis of β-Amino Acids and γ-Lactams

Reactions of N-silyl- and N-aluminoimines with B-allyldiisopinocampheylborane in the presence of methanol, followed by oxidative workup furnished homoallylic amines in good yields and high ee. A 11B NMR spectroscopy study revealed that the reactions do not proceed, even at room temperature, unless a molar equivalent of water or methanol is added. The first reagent-controlled asymmetric crotylboration and alkoxyallylboration of aldimines furnishing beta-methyl or beta-alkoxy homoallylic amines in very high diastereoselectivity and enantioselectivity are reported herein. Crotylboration and alkoxyallylboration of imines proceed only with the "allyl"-boron "ate" complexes, instead of the "allyl"-dialkylboron reagents used with aldehydes. The addition of methanol is necessary for these reactions as well. Application of this methodology for the conversion of representative nitriles to beta-amino acids in two steps has been described. Additionally, a procedure for the preparation of chiral delta-amino alcohols and gamma-lactams from nitriles is also reported.

α-Aminoallylation of Aldehydes with Ammonia: Stereoselective Synthesis of Homoallylic Primary Amines

Three-component reactions of aldehydes, ammonia, and allylboronates were found to provide homoallylic primary amines in high yields with high chemo- and stereoselectivities. A two-step, one-pot, stereoselective synthesis of an uncommon alpha-amino acid, alloisoleucine, was achieved utilizing this reaction.

Catalytic, enantioselective alkylation of alpha-imino esters: the synthesis of nonnatural alpha-amino acid derivatives

Methodology for the practical synthesis of nonnatural amino acids has been developed through the catalytic, asymmetric alkylation of alpha-imino esters and N,O-acetals by enol silanes, ketene acetals, alkenes, and allylsilanes using chiral transition metal-phosphine complexes as catalysts (1-5 mol %). The alkylation products, which are prepared with high enantioselectivity (up to 99% ee) and diastereoselectivity (up to 25:1/anti:syn), are protected nonnatural amino acids that represent potential precursors to natural products and pharmaceuticals. A kinetic analysis of the catalyzed reaction of alkenes with alpha-imino esters is presented to shed light on the mechanism of this reaction.

Water/MAO acceleration of the zirconocene-catalyzed asymmetric methylalumination of alpha-olefins

The zirconocene-catalyzed enantioselective methylalumination of terminal alkenes is greatly accelerated in the presence of water. Terminal olefins that are inert under the standard conditions can be readily methylated in good yields and with good to high enantioselectivities. Furthermore, methylaluminoxane is also shown to accelerate the reaction, albeit at a lesser rate.