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Using Ylide Functionalization to Stabilize Boron Cations.

ABSTRACT: The metalated ylide YNa [Y=(Ph₃ PCSO₂ Tol)^- ] was employed as X,L-donor ligand for the preparation of a series of boron cations. Treatment of the bis-ylide functionalized borane Y₂ BH with different trityl salts or B(C₆ F₅ )₃ for hydride abstraction readily results in the formation of the bis-ylide functionalized boron cation [Y-B-Y]⁺ (2). The high donor capacity of the ylide ligands allowed the isolation of the cationic species and its characterization in solution as well as in solid state. DFT calculations demonstrate that the cation is efficiently stabilized through electrostatic effects as well as ?-donation from the ylide ligands, which results in its high stability. Despite the high stability of 2 [Y-B-Y]⁺ serves as viable source for the preparation of further borenium cations of type Y₂ B⁺ ?LB by addition of Lewis bases such as amines and amides. Primary and secondary amines react to tris(amino)boranes via N-H activation across the B-C bond.

SUBMITTER: Scherpf T

PROVIDER: S-EPMC5363341 | biostudies-literature | 2017 Mar

REPOSITORIES: biostudies-literature

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Using Ylide Functionalization to Stabilize Boron Cations.

Scherpf Thorsten T Feichtner Kai-Stephan KS Gessner Viktoria H VH

Angewandte Chemie (International ed. in English) 20170210 12

The metalated ylide YNa [Y=(Ph3 PCSO2 Tol)- ] was employed as X,L-donor ligand for the preparation of a series of boron cations. Treatment of the bis-ylide functionalized borane Y2 BH with different trityl salts or B(C6 F5 )3 for hydride abstraction readily results in the formation of the bis-ylide functionalized boron cation [Y-B-Y]+ (2). The high donor capacity of the ylide ligands allowed the isolation of ...[more]

PMID: 28185370

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Project description:Conspectus Redox-neutral methods for the functionalization of amine α-C-H bonds are inherently efficient because they avoid external oxidants and reductants and often do not generate unwanted byproducts. However, most of the current methods for amine α-C-H bond functionalization are oxidative in nature. While the most efficient variants utilize atmospheric oxygen as the terminal oxidant, many such transformations require the use of expensive or toxic oxidants, often coupled with the need for transition metal catalysts. Redox-neutral amine α-functionalizations that involve intramolecular hydride transfer steps provide viable alternatives to certain oxidative reactions. These processes have been known for some time and are particularly well suited for tertiary amine substrates. A mechanistically distinct strategy for secondary amines has emerged only recently, despite sharing common features with a range of classic organic transformations. Among those are such widely used reactions as the Strecker, Mannich, Pictet-Spengler, and Kabachnik-Fields reactions, Friedel-Crafts alkylations, and iminium alkynylations. In these classic processes, condensation of a secondary amine with an aldehyde (or a ketone) typically leads to the formation of an intermediate iminium ion, which is subsequently attacked by a nucleophile. The corresponding redox-versions of these transformations utilize identical starting materials but incorporate an isomerization step that enables α-C-H bond functionalization. Intramolecular versions of these reactions include redox-neutral amine α-amination, α-oxygenation, and α-sulfenylation. In all cases, a reductive N-alkylation is effectively combined with an oxidative α-functionalization, generating water as the only byproduct. Reactions are promoted by simple carboxylic acids and in some cases require no additives. Azomethine ylides, dipolar species whose usage is predominantly in [3 + 2] cycloadditions and other pericyclic processes, have been identified as common intermediates. Extension of this chemistry to amine α,β-difunctionalization has been shown to be possible by way of converting the intermediate azomethine ylides into transient enamines. This Account details the evolution of this general strategy and the progress made to date. Further included is a discussion of related decarboxylative reactions and transformations that result in the redox-neutral aromatization of (partially) saturated cyclic amines. These processes also involve azomethine ylides, reactive intermediates that appear to be far more prevalent in condensation chemistry of amines and carbonyl compounds than previously considered. In contrast, as exemplified by some redox transformations that have been studied in greater detail, iminium ions are not necessarily involved in all amine/aldehyde condensation reactions.

Dataset Information

Using Ylide Functionalization to Stabilize Boron Cations.

Publications

Using Ylide Functionalization to Stabilize Boron Cations.

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