Epimerization of .alpha.-chloroboronic esters by lithium and zinc chlorides

Matteson, Donald S.; Erdık, Ender

doi:10.1021/om50003a004

Cited by 42 publications

(28 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Thus, a-chloroalkyl boronic esters undergo halide exchange in nonionizing solvents at significantly higher rates than comparable boron-free compounds [137][138][139]. By analogy with a-halo ketones or electron-poor benzyl or allyl halides rate enhancement for bimolecular substitutions would be expected, and is indeed found.…”

Section: A-boron Electrophilesmentioning

confidence: 78%

Aliphatic Nucleophilic Substitutions: Problematic Electrophiles

Dörwald

2004

Side Reactions in Organic Synthesis

View full text Add to dashboard Cite

Nucleophilic substitutions at sp 3 -hybridized carbon are among the most useful synthetic transformations, and have been thoroughly investigated [1][2][3]. The success of these reactions depends mainly on the structures of the nucleophile and electrophile, their concentration, and on the solvent and reaction temperature chosen. The structure of the electrophile in nucleophilic substitutions is of critical importance, because many unwanted side reactions result from unexpected reactivity of the electrophile.In Scheme 4.1 the mechanisms of typical monomolecular (Sn1) and bimolecular (Sn2) nucleophilic substitutions at a neutral electrophile with an anionic nucleophile are sketched. Sn1 reactions usually occur when the electrophile is sterically

show abstract

Section: A-boron Electrophilesmentioning

confidence: 78%

Aliphatic Nucleophilic Substitutions: Problematic Electrophiles

Dörwald

2004

Side Reactions in Organic Synthesis

View full text Add to dashboard Cite

show abstract

“…Hence,i fc ommon ion halides are present in solution, such substitution causes racemization/epimerization of the carbenoids. [10,11] Moreover, a-haloalkyllithium compounds show high reactivity as organolithium compounds in their own right and, as such, they may enter into halogenlithium exchange reactions.T hus,f or example,i ft hese carbenoids are generated by bromine-lithium exchange from 1,1-dibromoalkanes,the carbenoids formed will interact with the precursor dibromoalkane,t hus providing am echanism for epimerization by reversible bromine-lithium exchange. [12,13] Although this may result in mixtures of epimeric carbenoids under thermodynamic control, most of the time halogen-lithium exchange on 1,1-dihaloalkanes is problematic when carbenoids of defined configuration are required.…”

Section: Generation Of Stereodefined Carbenoids and Their Configuratimentioning

confidence: 99%

Formation of Olefins by Eliminative Dimerization and Eliminative Cross‐Coupling of Carbenoids: A Stereochemical Exercise

Blakemore

Hoffmann

2017

Angew Chem Int Ed

View full text Add to dashboard Cite

Two carbenoids combine to generate an olefin by a mechanism involving formation of an ate complex, 1,2-metalate rearrangement, and β-elimination. As each stage of this eliminative coupling is stereospecific, the overall stereochemical outcome can be understood and, in principle fully controlled, providing that the absolute stereochemical configurations of the reacting carbenoid species are defined. In contrast to traditional alkene syntheses, the eliminative cross-coupling of carbenoids offers a connective approach to olefins capable of precisely targeting a given isomer regardless of the nature of the features distinguishing the isomers. The formation of olefins by the eliminative dimerization and eliminative cross-coupling of carbenoids is reviewed with a range of illustrative examples, including the reactions of α-lithiated haloalkanes, epoxides, and carbamates. An emphasis is placed on stereochemical analysis and methods to generate sp -hybridized carbenoids in stereodefined form are surveyed.

show abstract

“…However, lithium cation functions in the same capacity if zinc chloride is not added, but usually does not provide optimum stereoselection. One factor in favor of the zinc chloride is that it sequesters chloride as ZnCl 3 -or ZnCl 4 2-, thus slowing epimerization of the product 4 by free chloride ion [14]. 4.…”

Section: -Symmetrical Boronic Estersmentioning

confidence: 99%

(α‐Haloalkyl)boronic Esters in Asymmetric Synthesis

Matteson¹

2005

Boronic Acids

Self Cite

View full text Add to dashboard Cite

The utility of (α-haloalkyl)boronic esters in asymmetric synthesis results from a unique combination of several features of their chemistry. A wide variety of products can be obtained in very high stereopurity, and the reactions are compatible with a considerable variety of functional substituents, provided that OH and NH groups are masked. Stereospecific displacement of halide from an (α-haloalkyl)boronic ester with a nucleophile yields an asymmetric boronic ester, which can either be converted stereospecifically into another product such as an alcohol or put into another cycle of reaction with (dihalomethyl)lithium to install additional stereocenters. The general synthetic utility of these boronic esters can best be understood from a detailed outline of the general processes involved.The most useful biological applications of these compounds have included the synthesis of some asymmetric insect pheromones in very high stereopurity, described in Sections 8.3.1 and 8.3.3, and the proteasome inhibitor "Velcade™" (bortezomib) developed by Millennium Pharmaceuticals and recently approved by the United States FDA as well as the European Union for treatment of relapsed and refractory multiple myeloma, Section 8.3.6. General Description of (α-Haloalkyl)boronic Ester Chemistry A Brief History of Boronic Ester ChemistryFirst, simple achiral boronic acids and esters are readily available from various sources (Chapter 1). The first synthesis from an organozinc reagent preceded general acceptance of Avogadro's hypothesis [1], and was superseded nearly a century ago by the now standard preparation from Grignard reagents [2,3]. Hydroboration has Boronic Acids. Edited by Dennis G. Hall

show abstract

Epimerization of .alpha.-chloroboronic esters by lithium and zinc chlorides

Cited by 42 publications

References 0 publications

Aliphatic Nucleophilic Substitutions: Problematic Electrophiles

Aliphatic Nucleophilic Substitutions: Problematic Electrophiles

Formation of Olefins by Eliminative Dimerization and Eliminative Cross‐Coupling of Carbenoids: A Stereochemical Exercise

(α‐Haloalkyl)boronic Esters in Asymmetric Synthesis

Contact Info

Product

Resources

About