Modular asymmetric synthesis of 1,2-diols by single-pot allene diboration/hydroboration/cross-coupling

Chiral allyl vinyl boronates are generated by catalytic enantioselective diboration of prochiral allenes. They may then be reacted, in situ, with a hydroborating reagent to form a novel triboron intermediate. The least hindered and most reactive C-B bond then participates in cross-coupling wherein the coupling is brought about by the same catalyst as that which catalyzed the diboration reaction. The remaining C-B bonds are then oxidized in the reaction workup, thereby allowing for the modular synthesis of chiral diols in a concise single-pot fashion.     

Platinum-catalyzed tandem diboration/asymmetric allylboration: access to nonracemic functionalized 1,3-diols

[reaction: see text] A single-pot tandem catalytic diene diboration/carbonyl allylation reaction is described that uses a commercially available chiral diboron reagent. The chirality of the intermediate diboration adduct is transferred to the product in the carbonyl allylation reaction, thereby providing access to enantioenriched chiral products. Notably, the reaction allows for construction of a quaternary stereocenter and furnishes a synthetically versatile C-B bond in the reaction product.     

DFT studies on the mechanism of the diboration of aldehydes catalyzed by copper(I) boryl complexes

The detailed mechanism for the diboration of aldehydes catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition states. The results show that the catalyzed diboration occurs through aldehyde insertion into Cu-B to give a Cu-O-C(boryl) species followed by sigma-bond metathesis with a diboron reagent. It is the "electron-richness", that is, the nucleophilicity of the Cu-boryl bond, which gives rise to a small insertion barrier and determines the direction of insertion. The results of our calculations also explain the formation of the product, observed experimentally, from the stoichiometric reaction of (IPr)Cu-Bpin (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with mesitylaldehyde. In the absence of a diboron reagent, the insertion intermediate having a Cu-O-C(boryl) linkage isomerizes to the thermodynamically preferred Cu-C-O(boryl) isomer via a boryl migration to the metal-bonded oxygen through an S(E)2-like transition state. We have also studied the catalyzed diboration of 2-pyridinecarboxaldehyde, which gives the unexpected reductive coupling product 1,2-di-2-pyridyl-1,2-bis(pinacolboroxy)ethane. The insertion intermediate, which contains a coordinated pyridyl group, isomerizes easily to a 1,2-dihydropyridine form, preventing its metathesis with a diboron reagent to give the expected diboration product as observed for other aldehyde substrates.     

Iron‐Catalyzed Diboration and Carboboration of Alkynes

An iron‐catalyzed diboration reaction of alkynes with bis(pinacolato)diboron (B₂pin₂) and external borating agents (MeOB(OR)₂) affords diverse symmetrical or unsymmetrical cis‐1,2‐diborylalkenes. The simple protocol for the diboration reaction can be extended to the iron‐catalyzed carboboration of alkynes with primary and, unprecedentedly, secondary alkyl halides, affording various tetrasubstituted monoborylalkenes in a highly stereoselective manner. DFT calculations indicate that a boryliron intermediate adds across the triple bond of an alkyne to afford an alkenyliron intermediate, which can react with the external trapping agents, borates and alkyl halides. In situ trapping experiments support the intermediacy of the alkenyl iron species using radical probe stubstrates.     

Microwave-assisted one-pot diboration/Suzuki cross-couplings. A rapid route to tetrasubstituted alkenes

Internal and terminal alkynes undergo rapid platinum(0)-catalyzed diboration with bis(pinacolato)diboron in dioxane to yield cis-1,2-bis(boryl)alkenes under sealed vessel microwave conditions. Subsequent addition of aryl bromides, base and a palladium catalyst to the reaction vial followed by resubjection to microwave conditions provides tetrasubstituted ethylenes in high yields via Suzuki cross-coupling of the boron intermediates.     

Sequential Pd-catalyzed asymmetric allene diboration/alpha-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.     

A new reaction of phosphorylated N-sulfonylimines with hydrophosphoryl agents involving C → N transfer of phosphoryl groups

Hydrophosphoryl nucleophilic agents add to the CN bond of N-sulfonyltrichloroacetimidoylphosphonates to give unstable C,C-diphosphorylated adducts, which undergo competitive 1,2-C → N phosphorotropic rearrangement and dehydrochlorination with the formation of aza-Perkow reaction products, C,N-diphosphorylated dichlorovinylsulfonamides. This is the first reliably identified case of an aza-Perkow transformation for acid phosphites and their initial nucleophilic attack at the C atom of the azomethine bond in the aza-substrates.     

One-pot synthesis of α,α-difluoroimines from alkynes through tandem catalytic diboration/fluorination/imination reaction

Tandem catalytic diboration/fluorination/imination of arylacetylenes leads to the formation of α,α-difluoroimines, where the adjacent CN and C-F₂ bonds are formed simultaneously. The convenient one-pot protocol involves a Pt(0)-catalyzed diboration of terminal or internal arylalkynes followed by electrophilic fluorination with Selectfluor in the presence of primary amines and a dehydrating agent. A plausible mechanism for the three consecutive steps (diboration/fluorination/imination) is suggested in accordance with the electronic properties of the substrates. Alkynes/catalytic diboration/alkenyl diboronate esters/Selectfluor/electrophilic fluorination/α,α-difluoroimines.     

Synthesis of cyclic hydrazines by ring-closing metathesis of dienes and enynes tethered by an N-N bond

Ring-closing metathesis of dienes and enynes tethered by an N-N bond produced 6- to 10-membered cyclic 1,2-diaza compounds. The enyne RCM adducts were further transformed by Diels-Alder reaction into aromatic compounds.     

Rh-catalyzed enantioselective diboration of simple alkenes: reaction development and substrate scope

[Reaction: see text]. The rhodium-catalyzed reaction between bis(catecholato)diboron and simple alkenes results in the syn addition of the diboron across the alkene. The resulting 1,2-bis(boronate) is subsequently oxidized to provide the 1,2-diol. In the presence of enantiomerically enriched Quinap ligand, high enantioselection in the diboration can be achieved. The reaction is highly selective for trans- and trisubstituted alkenes and can be selective for some monosubstituted alkenes as well. The development of this reaction is described as is the substrate scope and experiments that are informative about the reaction mechanism and competing pathways.     

Nucleophilic Diboration Strategy Targeting Diversified 1-Boraphenarene Architectures

1-Boraphenalenes, a class of boron-doped polyaromatic hydrocarbons, were synthesized by nucleophilic diboration reaction of alkynes. Activation of diboron reagents with a highly basic sp²-carbanion results in very fast successive C−B bond formations to construct the boracycle. This methodology is characterized by high chemoselectivity, affording a wide variety of 1-boraphenarenes with diverse polar substituents. The endocyclic boron can be arylated conveniently in one pot, and the peripheral boron is available for various chemical transformations. Highly diastereoselective diboration gives pseudo-enantio-enriched boraphenarene, which emits circularly polarized fluorescence (CPL).     

Benzocyclobutenes-x : Synthesis of cyclobuta[l]phenanthrene derivatives from [2 + 2]photoadducts of chlorinated ethenes and phenanthrene

UV irradiation of phenanthrene and 1,1-dichloro-, 1,2-dichloro-, and 1,1,2-trichloroethenes resulted m [2+2]cycloaddition to give cyclobutane derivatives. Treatment of the 1,2-dichloro-adducts with N-bromosuccinimide resulted in aromatisation to yield cis- and tras-l,2-dichloro-1,2- dihydrocyclobuta[l)phenanthrene, whereas the 1,1-dichloro-adduct resulted in aromatisation and hydrolysis to give cyclobuta[l]phenanthrene-l(2H)-one. The trichloroethene adducts gave 1,2-dichloro-2a,10b-dihydrocyclobuta [l)phenanthrene upon treatment with base, and reaction with N- bromosuccinimide resulted in bromine substitution in the cyclobutane ring. The stereochemistry of the adducts and the stereoselectivity of the bromination reactions are discussed.     

Generation of functionalized azomethine ylides and their application to stereoselective heterocycle synthesis: an equivalent process of C-unsubstituted nitrile ylide cycloaddition reaction

The synthetic process equivalent to C-unsubstituted (CH) nitrile ylides cycloaddition reaction is achieved via cycloaddition of NH-azomethine ylide and the following fission reaction of the cycloadducts under acidic conditions. Cycloaddition of NH-azomethine ylide generated by a thermal 1,2-prototropy in 4-oxo-4H-pyrido[1,2-a]pyrimidine-3-carbaldehyde system with maleimides provides proline derivatives under extremely mild conditions. Heating their adducts in AcOH at 85 °C causes a cleavage of C-C bond between the proline and heterocyclic moiety to give the parent heterocyclic system and dehydroproline derivatives, which is regarded as a cycloadduct of C-unsubstituted (CH) nitrile ylide. This cycloaddition-fission reaction sequences can be applied to one-pot three-component reaction.     

Unexpected 1,2 syn diastereoselectivity in the three-component ‘aza Sakurai-Hosomi’ reaction

The three-component ‘aza Sakurai-Hosomi’ reaction performed on (±) O-protected mandelic aldehydes provided the unexpected syn hydroxy homoallylamines 2 and 2d as the major adducts. An intramolecular chelated transition state via a hydrogen bond is proposed to explain this 1,2 syn diastereoselectivity.     

A new synthesis of β,γ-unsaturated esters from three components, aldehydes, chloromethyl p-tolyl sulfoxide, and tert-butyl acetate, via magnesium carbenoid 1,2-CH and 1,2-CC insertion as the key reaction

Addition reaction of 1-chlorovinyl p-tolyl sulfoxides, which were derived from various aldehydes, with lithium enolate of tert-butyl acetate at −78 °C in THF gave adducts in high yields. Magnesium carbenoids were generated by treatment of these adducts with Grignard reagents via the sulfoxide-magnesium exchange reaction. When the adducts were derived from alkyl aldehydes or electron-deficient aromatic aldehydes, carbenoid 1,2-CH insertion reaction took place from the magnesium carbenoids to afford β,γ-unsaturated butyric esters having a substituent at the β-position. On the contrary, when the adducts were derived from electron-rich aromatic aldehydes, carbenoid 1,2-CC insertion reaction took place from the magnesium carbenoids to give β,γ-unsaturated butyric esters having the aromatic group at the γ-position. Highly stereospecific 1,2-CC insertion reactions were observed in the latter reactions. This procedure provides a good way for a synthesis of β,γ-unsaturated esters from aldehydes with two carbon-carbon bond-formations.     

Amine Catalysis for the Organocatalytic Diboration of Challenging Alkenes

The generation of in situ sp²–sp³ diboron adducts has revolutionised the synthesis of organoboranes. Organocatalytic diboration reactions have represented a milestone in terms of unpredictable reactivity of these adducts. However, current methodologies have limitations in terms of substrate scope, selectivity and functional group tolerance. Here a new methodology based on the use of simple amines as catalyst is reported. This methodology provides a completely selective transformation overcoming current substrate scope and functional/protecting group limitations. Mechanistic studies have been included in this report.     

Mechanistic study on the reaction of pinB-BMes2 with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent

Transition metal-free direct and base-catalyzed 1,2-diborations of arylacetylenes using pinB-BMes₂ provided a syn/anti-isomeric mixture of diborylalkenes. The kinetic analysis showed that the reaction rate and isomer ratio were affected by reaction conditions and substituents on the aryl ring. DFT calculations indicated that direct addition proceeded via the interaction of acetylene-π with the BMes₂ fragment. In contrast, for the base-catalyzed diboration, the previously isolated sp²–sp³ diborane and borataallene were confirmed as stable intermediates by calculations. The whole reaction pathways can be divided into the Bpin-migration and deprotonation steps, where the borataallene should be considered as a common intermediate. It should be noted that the deprotonation step is reversible and affords the kinetically less favoured isomer under the thermodynamic conditions. As a result, the composition of isomeric products, in the base-catalyzed diboration, is attributed to the small difference of activation barriers between direct and base-catalyzed systems.

Combination of kinetic and DFT studies revealed a subtle balance for substituent effect toward the regioselectivity of the product in metal-free and base-catalyzed diboration of arylacetylenes.     

Regio‐ and Stereoselective Homologation of 1,2‐Bis(Boronic Esters): Stereocontrolled Synthesis of 1,3‐Diols and Sch 725674

1,2‐Bis(boronic esters), derived from the enantioselective diboration of terminal alkenes, can be selectively homologated at the primary boronic ester by using enantioenriched primary/secondary lithiated carbamates or benzoates to give 1,3‐bis(boronic esters), which can be subsequently oxidized to the corresponding secondary‐secondary and secondary‐tertiary 1,3‐diols with full stereocontrol. The transformation was applied to a concise total synthesis of the 14‐membered macrolactone, Sch 725674. The nine‐step synthetic route also features a novel desymmetrizing enantioselective diboration of a divinyl carbinol derivative and high‐yielding late‐stage cross‐metathesis and Yamaguchi macrolactonization reactions.     

Addition von Acetylendicarbonsäuredimethylester an Imidazole; Aminierung der Addukte: neue Synthesen von 3-(2-Imidazolyl)-2-pyridon-4,5-dicarbonsäureamiden und Pyrido[1,2-a]pyrazinen

The reaction between dimethyl acetylene dicarboxylate and 2-amino-1-methylimidazole affords dimethyl 2-amino-1-methyl-1,3-diazepine-5,6-dicarboxylate in low yield. This 1:1 – adduct was formed by addition of the acetylenic compound to the enamine double bond of the imidazole ring followed by ring enlargement. On the other hand, 2:1 – adducts to the imine bond are isolated in moderate yield when dimethyl acetylene dicarboxylate is treated with either 1-methyl-2-methylmercapto-imidazole or 1-methyl-2-methylmercapto-imidazoline. These adducts behave differently on heating with ethylamine: the adduct of the imidazole series cyclizes to the pyridone 15 with concomittant loss of one carboxamide group whereas that of the imidazoline series forms a pyrido[1,2-a]pyrazine derivative 20 , both in high yield. The possible reaction mechanisms are discussed. ¹³C–NMR.-spectroscopy and X-ray analysis were used in the determination of several structures.