Synthesis and Reactions ofgem-Borazirconocenes

Boryl-Dictated Site-Selective Intermolecular Allylic and Propargylic C-H Amination

For internal alkenes possessing two or more sets of electronically and sterically similar allylic protons, the site-selectivity for allylic C-H functionalization is fundamentally challenging. Previously, the negative inductive effect from an electronegative atom has been demonstrated to be effective for several inspiring regioselective C-H functionalization reactions. Yet, the use of an electropositive atom for a similar purpose remains to be developed. α-Aminoboronic acids and their derivatives have found widespread applications. Their current syntheses rely heavily on functional group manipulations. Herein we report a boryl-directed intermolecular C-H amination of allyl N-methyliminodiacetyl boronates (B(MIDA)s) and propargylic B(MIDA)s to give α-amino boronates with an exceptionally high level of site-selectivities (up to 300:1). A wide variety of highly functionalized secondary and tertiary α-amino boronates are formed in generally good to excellent yields, thanks to the mildness of the reaction conditions. The unsaturated double and triple bonds within the product leave room for further decorations. Mechanistic studies reveal that the key stabilization effect of the B(MIDA) moiety on its adjacent developing positive charge is responsible for the high site-selectivity and that a closed transition state might be involved, as the reaction is fully stereoretentive. An activation effect of B(MIDA) is also found.

α-Aminoboronates: recent advances in their preparation and synthetic applications

α-Aminoboronic acids and their derivatives are useful as bioactive agents. Thus far, three compounds containing an α-aminoboronate motif have been approved by the Food and Drug Administration (FDA) as protease inhibitors, and more are currently undergoing clinical trials. In addition, α-aminoboronic acids and their derivatives have found applications in organic synthesis, e.g. as α-aminomethylation reagents for the synthesis of chiral nitrogen-containing molecules, as nucleophiles for preparing valuable vicinal amino alcohols, and as bis-nucleophiles in the construction of valuable small molecule scaffolds. This review summarizes new methodology for the preparation of α-aminoboronates, including highlights of asymmetric synthetic methods and mechanistic explanations of reactivity. Applications of α-aminoboronates as versatile synthetic building blocks are also discussed.

α-Boryl Organometallic Reagents in Catalytic Asymmetric Synthesis

Recent years have witnessed an increase in the popularity of α-boryl organometallic reagents as versatile nucleophiles in asymmetric synthesis. These compounds have been adopted in chemo- and stereoselective coupling reactions with a number of different electrophiles. The resulting enantioenriched boronic esters can be applied in stereospecific carbon-carbon or carbon-heteroatom bond construction reactions, enabling a two-step strategy for the construction of complex structures with high efficiency and functional group compatibility. Due to these reasons, tremendous effort has been devoted to the preparation of enantiomerically enriched α-boryl organometallic reagents and to the development of stereoselective reactions of related racemic or prochiral materials. In this review, we describe the enantio- or diastereoselective reactions that involve α-boryl organometallic reagents as starting materials or products and we showcase their synthetic utility.

Are Organozirconium Reagents Applicable in Current Organic Synthesis?

The search for mild, user-friendly, easily accessible, and robust organometallic reagents is an important feature of organometallic chemistry. Ideally, new methodologies employing organometallics should be developed with respect to practical applications in syntheses of target compounds. In this short review, we investigate if organozirconium reagents can fulfill these criteria. Organozirconium compounds are typically generated via in situ hydrozirconation of alkenes or alkynes with the Schwartz reagent. Alkyl and alkenylzirconium reagents have proven to be convenient in conjugate additions, allylic substitutions, cross-coupling reactions, and additions to carbonyls or imines. Furthermore, the Schwartz reagent itself is a useful reducing agent for polar functional groups.

1 Introduction

2 Synthesis and Generation of the Schwartz Reagent

3 Structure and Properties of Cp2Zr(H)Cl

4 Reactivity of Organozirconium Reagents

4.1 Asymmetric Conjugate Addition

4.2 Asymmetric Allylic Alkylations

4.3 Desymmetrization Reactions

4.4 Cross-Coupling Reactions

4.5 1,2-Additions

5 Conclusions

Chemoselective Cross-Coupling of gem-Borazirconocene Alkanes with Aryl Halides

The direct and chemoselective conversion of the carbon-metal bond of gem-dimetallic reagents enables rapid and sequential formation of multiple carbon-carbon and carbon-heteroatom bonds, thus representing a powerful method for efficiently increasing structural complexity. Herein, we report a visible-light-induced, nickel-catalyzed, chemoselective cross-coupling reaction between gem-borazirconocene alkanes and diverse aryl halides, affording a wide range of alkyl Bpin derivatives in high yields with excellent regioselectivity. This practical method features attractively simple reaction conditions and a broad substrate scope. Additionally, we systematically investigated a Bpin-directed chain walking process underlying the regioselectivity of alkylzirconocenes, thus uncovering the mechanism of the remote functionalization of internal olefins achieved with our method. Finally, DFT calculations indicate that the high regioselectivity of this reaction originates from the directing effect of the Bpin group.

Synthesis of α-aminoboronic acids

This review describes available methods for the preparation of α-aminoboronic acids in their racemic or in their enantiopure form. Both, highly stereoselective syntheses and asymmetric procedures leading to the stereocontrolled generation of α-aminoboronic acid derivatives are included. The preparation of acyclic, carbocyclic and azacyclic α-aminoboronic acid derivatives is covered. Within each section, the different synthetic approaches have been classified according to the key bond which is formed to complete the α-aminoboronic acid skeleton.

Rh-catalyzed borylation of N-adjacent C(sp3)-H bonds with a silica-supported triarylphosphine ligand

Direct C(sp(3))-H borylation of amides, ureas, and 2-aminopyridine derivatives at the position α to the N atom, which gives the corresponding α-aminoalkylboronates, has been achieved with a heterogeneous catalyst system consisting of [Rh(OMe)(cod)]2 and a silica-supported triarylphosphine ligand (Silica-TRIP) that features an immobilized triptycene-type cage structure with a bridgehead P atom. The reaction occurs not only at terminal C-H bonds but also at internal secondary C-H bonds under mild reaction conditions (25-100 °C, 0.1-0.5 mol % Rh).

Boroalkyl group migration provides a versatile entry into α-aminoboronic acid derivatives

A reaction exemplifying migration of boron-substituted carbon is described. We show that α-boroalkyl groups of transient boroalkyl acyl azide intermediates readily migrate from carbon to nitrogen. This process allows access to a new class of stable molecules, α-boryl isocyanates, from α-borylcarboxylic acid precursors. The methodology facilitates synthesis of a wide range of α-aminoboronic acid derivatives, including α,α-disubstituted analogues.

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.

Novel Vinyl Phosphonates and Vinyl Boronates by Halogenation, Allylation, and Propargylation of α-Boryl- and α-Phosphonozirconacyclopentenes

[structure: see text] Alpha-phosphonozirconacyclopentenes or alpha-borylzirconacyclopentenes react by bromination, iodination, allylation, and propargylation to generate unique vinyl boronates and vinyl phosphonates not obtainable by other methods. The reaction proceeds in two steps, with both high regio- and stereoselectivity. With the vinyl boronates, the Zr-Csp2 bond is initially cleaved by 1 equiv of electrophile. With the phosphonates, either the Zr-Csp2 bond (allyl bromide, Br(2)) or the Zr-Csp3 bond (I(2), propargyl bromide) may be initially cleaved. The addition of a second equivalent of an electrophile results in disubstitution.