Stereodivergent Hydroboration of Allenes

Borane-catalysed C2-selective indole reductive functionalisation

Indolines are common motifs within pharamceuticals and natural products. Boron catalysis enables the chemoselective allylation of indoles to give allylic indolines in excellent diastereoselectivity. Mechanistic studies revealed in situ formation of the allylic borane, allylation of the imine tautomer of the indole and B-N/B-H transborylation for catalytic turnover.

Toward the Briarane Core via 1,3-Dipolar Cycloaddition

The complete C20 framework of brianthein W was established, featuring hydroboration/allylation, to provide the C1-C2 quaternary/tertiary stereoarray with excellent stereocontrol. Intramolecular nitrile oxide cycloaddition (INOC) was adopted as the key transformation to establish the trans-fused 6/10-bicyclic ring system. Evolution of the second INOC event revealed the intricacies governing regioselectivity, which ultimately led to construction of the highly strained 10-membered carbocycle.

Total Synthesis of Lobatamides A and C

The total synthesis of lobatamides A (1 a) and C (1 c) via a common bislactone intermediate is reported. The allylic aryl moiety including a trisubstituted Z-olefin was constructed by hydroboration of a 1,1-disubstituted allene and subsequent Migita-Kosugi-Stille coupling. Although the seco acid proved to be highly unstable even in the presence of weak bases, Zhao macrolactonization under acidic conditions via the α-acyloxyenamide successfully provided the common bislactone intermediate. Hydrozirconation-iodination of the terminal alkyne and subsequent copper-mediated coupling with primary amides proceeded successfully in the presence of the sensitive bislactone framework. The developed synthetic route enables the late-stage installation of enamide side chains, which are crucial structures for V-ATPase inhibition.

Construction of C2-indolyl-quaternary centers by branch-selective allylation: enabling concise total synthesis of the (±)-mersicarpine alkaloid

Herein we report a branch-selective allylation strategy for accessing C2-indolyl-all-carbon quaternary centers using allylboronic acids. This approach boasts broad functional group tolerance, scalability, and relies on easily accessible allyl alcohol precursors. Importantly, the C3-position of the indole remains free, offering a handle for further synthetic refinement. Mechanistic pathways, corroborated by density functional theory (DFT), suggest the involvement of an indolenine intermediate and a Zimmerman-Traxler-like transition state during allylboration. Demonstrating its efficacy, the method was applied to the total synthesis of the (±)-mersicarpine alkaloid and enabled formal synthesis of additional alkaloids, such as (±)-scholarisine G, (±)-melodinine E, and (±)-leuconoxine.

Synthesis of 1,1-Disubstituted Allenylic Silyl Ethers Through Iron-Catalyzed Regioselective C(sp2)─H Functionalization of Allenes

We report a synthesis of allenylic silyl ethers through iron-catalyzed functionalization of the C(sp2)─H bonds of monosubstituted alkylallenes. In the presence of a cyclopentadienyliron dicarbonyl based catalyst and triisopropylsilyl triflate as a silylation agent, a variety of aryl aldehydes were suitable coupling partners in this transformation, furnishing a collection of 1,1-disubstituted allenylic triisopropylsilyl ethers as products in moderate to excellent yields as a single regioisomer. Lithium bistriflimide was identified as a critical additive in this transformation. The optimized protocol was scalable, and the products were amenable to further transformation to give a number of unsaturated, polyfunctional derivatives.

Regio- and stereoselective copper-catalyzed α,β-protoboration of allenoates: access to Z-β,γ-unsaturated β-boryl esters

A highly efficient regio- and stereoselective method for allenoate borylation has been developed. Using CuCl and bis(pinacolato)diboron in methanol, a variety of allenoates underwent β-boration and α-protonation to afford the corresponding Z-β,γ-unsaturated β-boryl esters under mild conditions with up to 81% yields.

Contrasteric coupling of allenes and tetrahydroisoquinolines by iron-catalysed allenic C(sp2)-H functionalisation

An iron-catalysed C-H functionalisation of simple monosubstituted allenes for the synthesis of 1-tetrahydroisoquinolinyl 1,1-disubstituted allenes is reported. This transformation represents the first example of a direct conversion of allenic C-H bonds to C-C bonds through cross dehydrogenative coupling. The optimized protocol features broad scope and employs mild, functional group tolerant conditions.

Iron-Catalyzed Contrasteric Functionalization of Allenic C(sp2)-H Bonds: Synthesis of α-Aminoalkyl 1,1-Disubstituted Allenes

An iron-catalyzed C-H functionalization of simple monosubstituted allenes is reported. An efficient protocol for this process was made possible by the use of a newly developed electron-rich and sterically hindered cationic cyclopentadienyliron dicarbonyl complex as the catalyst and N-sulfonyl hemiaminal ether reagents as precursors to iminium ion electrophiles. Under optimized conditions, the use of a mild, functional-group-tolerant base enabled the conversion of a range of monoalkyl allenes to their allenylic sulfonamido 1,1-disubstituted derivatives, a previously unreported and contrasteric regiochemical outcome for the C-H functionalization of electronically unbiased and directing-group-free allenes.

Zinc Iodide Catalyzed Synthesis of Trisubstituted Allenes from Terminal Alkynes and Ketones

A straightforward, user-friendly, efficient protocol for the one pot, ZnI₂-catalyzed allenylation of terminal alkynes with pyrrolidine and ketones, toward trisubstituted allenes, is described. Trisubstituted allenes can be obtained under either conventional heating or microwave irradiation conditions, which significantly reduces the reaction time. A sustainable, widely available, and low-cost metal salt catalyst is employed, and the reactions are carried out under solvent-free conditions. Among others, synthetically valuable allenes bearing functionalities such as amide, hydroxyl, or phthalimide can be efficiently prepared. Mechanistic experiments, including kinetic isotope effect measurements and density functional theory (DFT) calculations, suggest a rate-determining [1,5]-hydride transfer during the transformation of the intermediate propargylamine to the final allene.

Polymerization of Allenes by Using an Iron(II) β-Diketiminate Pre-Catalyst to Generate High Mn Polymers

Herein, we report an iron(II)-catalyzed polymerization of arylallenes. This reaction proceeds rapidly at room temperature in the presence of a hydride co-catalyst to generate polymers of weight up to Mn =189 000 Da. We have determined the polymer structure and chain length for a range of monomers through a combination of NMR, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) analysis. Mechanistically, we postulate that the co-catalyst does not react to form an iron(II) hydride in situ, but instead the chain growth is proceeding via a reactive Fe(III) species. We have also performed kinetic and isotopic experiments to further our understanding. The formation of a highly unusual 1,3-substituted cyclobutane side-product is also investigated.

Stereodivergent routes in organic synthesis: marine natural products, lactones, other natural products, heterocycles and unnatural compounds

Enantio- and diastereodivergent routes to marine-origin natural products with different sizes of cyclic ethers and lactones have been used in order to assign stereochemical features. Kainoid amino acids such as isodomoic acids have been synthesized using diastereodivergent routes. The bis(indole) alkaloid dragmacidin F has been prepared by enantiodivergent strategies as well as furanoterpenes and the tetracyclic agelastatin A. Natural products containing five-membered lactones like quercus lactones, muricatacins, goniofufuranones, methylenolactocins and frenolicin B have been synthesized using stereodivergent routes. Macrolides are very abundant lactones and have been mainly prepared from the corresponding seco-acids by lactonization, such as lasiodiplodin, zaeralanes, macrosphelides and haloprins, or by ring-closing metathesis, such as aspercyclides, microcarpalides, macrolides FD-891 and 892, and tetradic-5-en-9-olides. Other natural products including cyclic ethers (such as sesamin, asarinin, acetogenins, centrolobines and nabilones), alcohols (such as sulcatol), esters (such as methyl jasmonates), polycyclic precursors of fredericamycin, amino alcohols (such as ambroxol and sphingosines), isoprostanes, isofurans, polyketide precursors of anachelins, brevicomins, gummiferol, shikimic acid and the related compounds, and the pheromone disparlure have been synthesized stereodivergently. Heterocyclic systems such as epoxides, theobroxides and bromoxones, oxetan-3-ones, 5- to 8-membered cyclic ethers, azetidones, γ-lactams, oxazolidinones, bis(oxazolines), dihydropyridoisoindolines and octahydroisoquinolines have been prepared following stereodivergent routes. Stereodivergent routes to unnatural compounds such as alkenes, dienes, allenes, cyclopropanes, alcohols, aldols, amines, amino alcohols, β-amino acids, carboxylic acids, lactones, nitriles and α-amino nitriles have been considered as well.

Ligand-controlled cobalt-catalyzed regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes

We report a stereoselective regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes with pinacolborane (HBpin) in the presence of cobalt catalysts generated from bench-stable Co(acac)2 and bisphosphine ligands. An interesting correlation between the regioselectivity of this hydroboration and the bite angles of bisphosphine ligands was identified. When hydroboration was conducted with cobalt catalysts containing bisphosphines with medium bite angles (e.g. 98° for dppb and 96° for dppf), HBpin was selectively added to the alkyl-substituted double bond. However, HBpin was selectively added to the aryl-substituted double bond when the reactions were conducted with cobalt catalysts containing bisphosphines with large bite angles (e.g. 111° for xantphos and 114° for Nixantphos). A range of internal allenes underwent these Co-catalyzed hydroboration reactions in a regiodivergent manner to yield the corresponding (Z)-alkenylboronates in high isolated yields and with high regioselectivity. These reactions show good functional group compatibility and can be readily scaled up to gram scales without using a dry box. In addition, the comparison of regioselectivity between the Co-catalyzed hydrosilylation and hydroboration reactions of the same allene substrate suggests that this Co-catalyzed regiodivergent hydroboration of allenes proceeds through a Co-Bpin intermediate. Deuterium-labeling experiments suggest that the Co-Bpin intermediates react with allenes to form allylcobalt species which then react with HBpin to release (Z)-alkenylboronate products.

Palladium-catalyzed intramolecular aerobic alkenylhydroxylation of allenamides with alkenyl iodides

An efficient palladium-catalyzed aerobic alkenylhydroxylation cyclization of allenamide derivatives was developed. Mechanistic studies indicated that the reaction might undergo a radical process.

Caesium fluoride-mediated hydrocarboxylation of alkenes and allenes: scope and mechanistic insights

A caesium fluoride-mediated hydrocarboxylation of olefins is disclosed that does not rely on precious transition metal catalysts and ligands. The reaction occurs at atmospheric pressures of CO2 in the presence of 9-BBN as a stoichiometric reductant. Stilbenes, β-substituted styrenes and allenes could be carboxylated in good yields. The developed methodology can be used for preparation of commercial drugs as well as for gram scale hydrocarboxylation. Computational studies indicate that the reaction occurs via formation of an organocaesium intermediate.

An Enantioconvergent and Concise Synthesis of Lasonolide A

Efficient access to medicinally significant natural products is an essential basis for the development of pharmaceuticals. The limited availability of marine natural products impedes broad biological evaluation. Despite several elegant syntheses of (-)-lasonolide A having been reported, a practical synthesis of this potent anticancer polyketide remains elusive. Based on the application of borane as a traceless protecting group and the development of an unprecedented bissulfone reagent for Julia olefination, (-)-lasonolide A was assembled in an enantioconvergent manner through the application of stereoselective hydroboration, allylation, and oxidation. This concise route may provide a realistic solution for accessing derivatives and analogues.

Borane-Induced Dimerization of Arylallenes

A series of arylallenes react with HB(C₆ F₅ )₂ in a 2:1 molar ratio to give the tail-to-tail 1,6-diaryl-2-boryl-hexa-1,5-diene coupling products. The reaction of the phenylallene substrate with HB(C₆ F₅ )₂ was shown to initially give the 2-boryl-3,4-diphenyl-1,5-hexadiene head-to-head coupling product which then rearranged, by a thermally induced Cope rearrangement, into the final product.

The regulation of hydroboration of olefins by oriented external electric field

By employing an oriented external electric field as a catalyst or inhibitor, the model reactions of the hydroboration of terminal olefins with the simplest borane were studied for the first time.