Formation Of Quaternary Carbon Center Research Articles

Transition-Metal-Free Dehydrogenative Cyclization via α-Csp3–H Activation of Ethers and Thioethers

AbstractWe report herein the tetraethylammonium bromide catalyzed intramolecular oxidative cyclization of O- or S-alkylated salicylic or thiosalicylic acid derivatives to access 4H-benzo[d][1,3]dioxin-4-ones or 4H-benzo[d][1,3]oxathiin-4-ones, respectively. The oxidative cyclization of salicylic acid derivatives proceeds through a radical pathway at 110 °C. In contrast, the cyclization of the thiosalicylic acids proceeds smoothly at room temperature via an ionic pathway. Notably, the overall reactions are fast, completed within short reaction times, furnishing the products in high yields and with smooth formation of quaternary carbon centers.

Use of Strain-Release for the Diastereoselective Construction of Quaternary Carbon Centers.

Herein, we describe the formation of quaternary carbon centers with excellent diastereoselectivity via a strain-release protocol. An organometallic species is generated by Cp*Rh(III)-catalyzed C-H activation, which is then coupled with strained bicyclobutanes (BCBs) and a prochiral carbon electrophile in a three-component reaction. This work illustrates a rare example of BCBs in transition metal catalysis and demonstrates their broad potential to access novel reaction pathways. The method developed exhibits ample functional group tolerance, and the products can be further transformed into valuable α-quaternary β-lactones. Preliminary mechanistic investigations suggest a twofold C-C bond cleavage sequence involving σ-bond insertion and an ensuing β-carbon elimination event.

Nickel-catalyzed formation of quaternary carbon centers using tertiary alkyl electrophiles.

Transformation of sterically hindered tertiary alkyl electrophiles under nickel-catalyzed conditions to forge C(sp3)-C bonds and simultaneously create challenging all-carbon quaternary centers has received growing attention in the recent years. The unique nature of nickel featuring flexible oxidation states ranging from Ni0 to NiIV, allows the effective activation of tertiary alkyl electrophiles through ionic (2e) or radical pathways. In nickel-catalyzed coupling of tertiary alkyl electrophiles, the competitive β-H elimination upon the resulting alkyl-Ni intermediate is relatively slow, thus benefiting the C-C bond forming process. Meanwhile, nickel-catalyzed radical addition of tertiary alkyl electrophiles to unsaturated C-C bonds has also advanced rapidly due to the successful incorporation of carboxylic acid and alcohol derivatives as radical precursors, and more importantly due to further interception of the intermediate radical adducts with nucleophiles and electrophiles to accomplish three-component cascade reactions. This review highlights these state-of-the-art nickel-catalyzed transformations of tertiary electrophiles, organized by reaction types with emphasis on the reaction mechanisms.

Chromium-catalyzed para-selective formation of quaternary carbon centers by alkylation of benzamide derivatives

Selective creation of quaternary carbon centers has been a long-standing challenge in synthetic chemistry. We report here the chromium-catalyzed, para-selective formation of arylated quaternary carbon centers by alkylative reactions of benzamide derivatives with tertiary alkylmagnesium bromides at room temperature. The reaction, which was enabled by a low-cost chromium(III) salt combined with trimethylsilyl bromide, introduces a sterically bulky tertiary alkyl scaffold on the para-position of benzamide derivatives in a highly selective fashion without either isomerization of the tertiary alkyl group or formation of ortho-alkylated byproducts. Forming low-valent Cr species in situ by reaction of CrCl3 with t-BuMgBr accompanied by evolution of hydrogen can be considered, which serves as reactive species to promote the reaction. The para-alkylation likely occurs via a radical-type nucleophilic substitution of imino-coordination benzimidate intermediate.

Regioselective Arylboration of Isoprene and Its Derivatives by Pd/Cu Cooperative Catalysis.

A method for the regioselective arylboration of isoprene and its derivatives is presented. These reactions allow for the synthesis of useful building blocks from simple components. Through these studies, an unusual additive effect with DMAP has been uncovered that allows for altered reactivity and the formation of quaternary carbon centers. The utility of this method is demonstrated toward the formal synthesis of mesembrine.

ChemInform Abstract: Stereodefined Acyclic Polysubstituted Silyl Ketene Aminals: Asymmetric Formation of Aldol Products with Quaternary Carbon Stereocenters.

AbstractStereodefined polysubstituted silyl ketene aminals are easily regioselectively and stereoselectively obtained and utilized in Mukaiyama aldol reactions with various aliphatic aldehydes involving the formation of quaternary carbon centers in α‐position to the carbonyl groups.

Preparation of the Core Structure of Aspidosperma and Strychnos Alkaloids from Aryl Azides by a Cascade Radical Cyclization.

A novel approach to prepare the core structure of Aspidosperma and Strychnos alkaloids is described. The strategy is based on a cyclization cascade involving the formation of quaternary carbon center followed by trapping of the radical intermediate by an aryl azide to build the 5-membered ring of the pyrrolocarbazole system. This reaction is run with triethylborane without the need for any hydrogen atom donor such as a tin hydride or tris(trimethylsilyl)silane, and it furnishes the tetracyclic framework as a single diastereomer. The influence of different N-protecting groups on the starting iodoacetamide has been examined.

Factors Affecting Migration of Tertiary Alkyl Groups in Reactions of Alkylboronic Esters with Bromomethyllithium

The reactions of bromomethyllithium with tert-alkylboronic esters could be of great potential for the formation of quaternary carbon centers but often give poor yields/conversions. Calculations and experimental evidence show that tert-alkyl groups migrate less effectively than other types of alkyl group in such reactions and that O-migration competes. Furthermore, slow/incomplete capture of the bromomethyl reagent by the boronic ester is a problem in more hindered systems, and an additional competing reaction, possibly Li-Br exchange on the bromomethylborate species, also leads to lower yields of migrated products. Based on this, experimental protocols have been devised in which the competing reactions are largely suppressed, leading to higher conversions to migrated product for several substrates.

ChemInform Abstract: Formation of Quaternary Carbon Centers by Highly Regioselective Hydroformylation with Catalytic Amounts of a Reversibly Bound Directing Group.

AbstractDiphenylphosphinite is applied as a catalytic directing group in the selective hydroformylation reaction leading to synthetically valuable α,α‐disubstituted butyrolactones.

Formation of Quaternary Carbon Centers by Highly Regioselective Hydroformylation with Catalytic Amounts of a Reversibly Bound Directing Group

Diphenylphosphinite is applied as a catalytic directing group in the selective hydroformylation reaction leading to synthetically valuable α,α-disubstituted butyrolactones.

ChemInform Abstract: Catalytic Asymmetric Synthesis of Either Enantiomer of Physostigmine. Formation of Quaternary Carbon Centers with High Enantioselection by Intramolecular Heck Reactions of (Z)-2-Butenanilides.

A practical method for preparing either enantiomer of physostigmine and congeners is reported. The Z stereochemistry of the butenanilide cyclization substrate is required to obtain high enantioselection (95% ee) in the key asymmetric Heck insertion

ChemInform Abstract: Formation of Quaternary Carbon Center with a Trifluoromethyl Group Using a Pd‐Catalyzed Allylation Reaction.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Formation of Quaternary Carbon Center with a Trifluoromethyl Group Using a Pd-Catalyzed Allylation Reaction

Synthesis of compounds with quaternary carbons is one of the most attractive reactions in the synthetic chemistry. However, there are only a few reports on synthesis of the compounds with a fluoroalkyl group at a quaternary carbon center. Recently, we reported the synthesis of alpha-trifluoromethylated ketones by the reaction of alpha,beta-unsaturated ketones with CF(3)-I using a Rh catalyst. When the alpha-trifluoromethylated ketones and allyl carbonates were treated with a Pd catalyst, the allylation reaction proceeded smoothly at the trifluoromethylated carbon to give the desired compounds with a trifluoromethylated quaternary carbon center in good to excellent yields.

Stereoselective Formation of Quaternary Carbon Centers and Related Functions

Stereoselective Formation of Quaternary Carbon Centers and Related Functions

Stereoselective Formation of Quaternary Carbon Centers: Alkylation of α,α‐Disubstituted Amide Enolates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereoselective Formation of Quaternary Carbon Centers: Alkylation ofα,α-Disubstituted Amide Enolates

Mit hoher Diastereoselektivität gelingt die Alkylierung von α,α-disubstituierten Amidenolaten zu Produkten mit quartären Kohlenstoffatomen (siehe Schema). Dabei werden weder für die Stereoselektivität der Enolatbildung noch für die der Alkylierungen chelatisierende Gruppen benötigt. Oft ist die Diastereoselektivität der Alkylierung höher als das Isomerenverhältnis der Enolat-Zwischenprodukte. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2002/z18123_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Stereoselective formation of quaternary carbon centers: alkylation of alpha,alpha-disubstituted amide enolates.

High diastereoselectivity is achieved in the alkylation of α,α-disubstituted amide enolates to form quaternary carbon products (see scheme; LiDBB=di-tert-butyldiphenyllithium). No chelating groups are necessary for stereocontrol in either the enolate-formation or alkylation steps. In many cases, amplification of the alkylation diastereoselectivity above the isomeric ratio of the intermediate enolates is observed. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2002/z18123_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Carboxalkylation of Enolates with Menthyl Chloroformate. Diastereoselective Formation of Quaternary Carbon Centers

Diastereomeric rations of 90 : 10 to > 95 : 5 are reached when carboxylic acid, ketone, and ester-derived enolates 2, 7a/7b, 10, and 13 are allowed to react with (-)-menthyl chloroformate 1 to deliver nonracemic compounds which contain stereogenic quaternary carbon centers.

Formation of Quaternary Carbon Centers in Ethylene Polymerization with meso-Isopropylidenebis(1-indenyl)zirconium Dichloride Activated by MAO

Formation of Quaternary Carbon Centers in Ethylene Polymerization with <i>m</i><i>eso-</i>Isopropylidenebis(1-indenyl)zirconium Dichloride Activated by MAO

ChemInform Abstract: Asymmetric Formation of Quaternary Carbon Centers Catalyzed by Novel Chiral 2,5‐Dialkyl‐7‐phenyl‐7‐phosphabicyclo[2.2.1]heptanes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.