Rh-Catalyzed Borylative Cyclization of Acrylate-Containing 1,6-Enynes

A Rh-catalyzed regioselective, stereoselective carbocyclization/borylation of acrylate-containing 1,6-enynes was described, which offers a general and practical method for constructing versatile and densely functionalized pyrrolidines with the (Z) geometry or the uncommon (E) geometry at the double bond with different substituents of the alkyne, with sterically hindered and conjugative aryl groups favoring the latter, featuring a tetrasubstituted vinyl boronate, containing an all-carbon quaternary stereocenter.

Ruthenium-Catalyzed Geminal Hydroborative Cyclization of Enynes

Disclosed here is the first geminal (gem-) hydroborative cyclization of enynes. Different from known hydroborative cyclizations, this process adds hydrogen and boron to the same position, leading to a new reaction mode. With [Cp*RuCl]₄ as catalyst, a range of gem-hydroborated bicyclic products bearing a cyclopropane unit could be rapidly assembled from simple enyne substrates. Control experiments and density functional theory (DFT) calculations provided important insights into the reaction mechanism. Notably, two major competing pathways may operate with substrate-dependence. 1,6-Enynes favor initial oxidative cyclometalation to form a ruthenacyclopentene intermediate prior to engaging hydroborane, while other enynes (e.g., 1,7-enynes) that lack strong propensity toward cyclization prefer initial alkyne gem-(H,B)-addition to form an α-boryl ruthenium carbene followed by intramolecular olefin cyclopropanation. This process also represents the first ruthenium-catalyzed enyne hydroborative cyclization.

Copper-Catalyzed Diastereo- and Enantioselective Borylative Cyclization

Copper-catalyzed enantioselective borylative cyclization with various electrophiles via difunctionalization of unsaturated hydrocarbons is a powerful tool for the generation of interesting boron-containing carbocycles and heterocycles processes involving a chiral organocopper intermediate. Alkenes, allenes, and alkynes are versatile and easily accessible substrates that can be subjected to a wide range of reactions to produce densely functionalized, enantioenriched products. In this chapter, I discuss copper-catalyzed alkenes, allenes, and alkynes borofunctionalization and enantioselective cyclization via chiral organocopper intermediate. Copper-catalyzed enantioselective borylative cyclization and regiodivergent functionalization of alkenes, allenes, and alkynes, as well as the current mechanistic understanding of such processes, are given special attention in this review.

Borylative Cyclization of 1,6-Allenynes Driven by BCl3

A metal-free intramolecular borylative cyclization of 1,6-allenynes driven by BCl₃ was developed. This method provides a general and practical strategy to construct valuable pyrrolidines containing all-carbon quaternary centers or 3,5-dihydroazepine derivatives depending on the substituents of the allene, with conjugative and sterically hindered phenyl groups favoring the latter.

Stereoselective synthesis of pentasubstituted 1,3-dienes via Ni-catalyzed reductive coupling of unsymmetrical internal alkynes

The reductive coupling of alkynes represents a powerful strategy for the rapid synthesis of highly substituted 1,3-dienes. This method has the advantages of high atom and step economy, and readily available substrates. Unfortunately, the intermolecular coupling of unsymmetrical internal alkynes remains extremely challenging due to the difficulty in controlling self-dimerization and cross-coupling, as well as stereo- and regioselectivity. Previous reports are still limited to intramolecular processes or the use of stoichiometric amounts of metal catalyst. Herein, we report that nickel-catalyzed reductive coupling of two unsymmetrical internal alkynes can overcome the above-mentioned limitations by using a hemilabile directing group strategy to control the regioselectivity. A series of synthetically challenging penta-substituted 1,3-dienes are obtained in good yields with high regio- and enantioselectivity (mostly > 20/1 rr, >90% ee).

The reductive coupling of alkynes represents a powerful strategy for the rapid synthesis of highly substituted 1,3-dienes.

Silylboranes as Powerful Tools in Organic Synthesis: Stereo- and Regioselective Reactions with 1,n-Enynes

Bismetalated alkenes, accessible by element–element addition to alkynes, are valuable building blocks in organic synthesis, providing wide opportunities for divergent synthesis. Silaboration of alkynes with a pendant olefinic group, catalyzed by group 10 metal complexes, and subsequent transformation of the silicon and boron functional groups give access to densely functionalized 1,3-dienes and 1,3,5-trienes with defined stereo- and regiochemistry, 1,2-dienes, and carbocyclic and heterocyclic products.

1 Introduction

2 Background

3 Reactions with 1,3-Enynes

4 Cyclization 1,6-Enynes

5 Cyclization 1,7-Enynes

6 Cyclization of 1,n-Enynes (n > 7)

7 Cyclization of Dienynes and Enediynes

8 Cyclization of 1,6-Diynes

9 Conclusions

Nickel-catalyzed regio- and diastereoselective hydroarylative and hydroalkenylative cyclization of 1,6-dienes

By using methanol as the hydrogen source and commercially available nickel complex as the catalyst, the hydroarylative and hydroalkenylative cyclization of unsymmetrically substituted 1,6-dienes with organoboronic acid was developed to afford products with high regio- and diastereoselectivities.

Atom-Economical Ni-Catalyzed Diborylative Cyclization of Enynes: Preparation of Unsymmetrical Diboronates

We report a Ni-catalyzed diborylative cyclization of enynes that affords carbo- and heterocycles containing both alkyl- and alkenylboronates. The reaction is fully atom-economical, shows a broad scope, and employs a powerful and inexpensive catalytic Ni-based system. The reaction mechanism seems to involve activation of the enyne by Ni(0) through oxidative cyclometalation of the enyne prior to diboron reagent activation. An unprecedented dinuclear bis(organometallic) Ni(I) intermediate complex was isolated.

Atom-economical regioselective Ni-catalyzed hydroborylative cyclization of enynes: development and mechanism

We report a full study on the novel regioselective Ni-catalyzed hydroborylative cyclization of enynes using HBpin as the borylation agent.

Iron-Catalyzed Hydroborylative Cyclization of 1,6-Enynes

We report first Fe-catalyzed hydroborylative cyclization reaction. The process provides one C-C and one C-B bond in a single operation and shows a wide scope, allowing the formation of carbo- and heterocycles containing a homoallylic boryl unit that can be further functionalized. The reaction takes place in smooth conditions, with inexpensive catalytic system and full atom economy since HBpin is the borylation agent, in contrast to our previously reported Pd-catalyzed reaction. Both aryl and alkyl substituted alkynes are reactive, revealing a wide reaction scope. Mechanistic studies suggest the intermediacy of Fe^II -hydride active catalyst capable to react with the alkyne group prior to alkene insertion, and computational studies suggest the occurrence of barrierless σ-bond metathesis involving HBpin and Fe-C bonds along the catalytic cycle.

Oxidative bicyclization of N-tethered 1,7-enynes toward polycyclic 3,4-dihydroquinolin-2(1H)-ones via site-selective decarboxylative C(sp3)–H functionalization

A new Ag-catalyzed oxidative bicyclization of N-tethered 1,7-enynes with alkylcarboxylic acids for forming 41 examples of polycyclic 3,4-dihydroquinolin-2(1H)-ones has been established using readily accessible K₂S₂O₈ as an oxidant.

Nickel-catalyzed highly chemo- and stereoselective borylative cyclization of 1,6-enynes with bis(pinacolato)diboron

A highly chemo- and stereoselective nickel-catalyzed borylative cyclization of 1,6-enynes with bis(pinacolato)diboron.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.

Formation of C(sp(2))-boronate esters by borylative cyclization of alkynes using BCl3

BCl3 is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp(2) )boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2-carboboration of alkynes is also achieved using BCl3 to generate trisubstituted vinyl boronate esters.

Palladium(II)/Brønsted Acid-Catalyzed Enantioselective Oxidative Carbocyclization-Borylation of Enallenes

An enantioselective oxidative carbocyclization-borylation of enallenes that is catalyzed by palladium(II) and a Brønsted acid was developed. Biphenol-type chiral phosphoric acids were superior co-catalysts for inducing the enantioselective cyclization. A number of chiral borylated carbocycles were synthesized in high enantiomeric excess.

Domino Heck/borylation sequence towards indolinone-3-methyl boronic esters: trapping of the σ-alkylpalladium intermediate with boron

A suitable boron-complex has been used as a nucleophile to trap the σ-alkylpalladium intermediate for the synthesis of indolinone 3-methyl boronic esters, applying a domino Heck/borylation strategy.

Ligand-controlled divergent formation of alkenyl- or allylboronates catalyzed by Pd, and synthetic applications

Pd-catalysed reactions of allyl carbonates containing alkynes with B2(pin)2 afford either alkenylboronates by cascade borylative cyclisation, or allylboronates. The reaction outcome is controlled by a suitable choice of ligands. Boronates have been applied to a number of synthetic applications.