Allylic and Allenic Halide Synthesis via NbCl₅- and NbBr₅-Mediated Alkoxide Rearrangements

Abstract

Addition of NbCl 5 , or NbBr 5 , to a series of magnesium, lithium, or potassium allylic or propargylic alkoxides directly provides allylic or allenic halides. Halogenation formally occurs through a metallahalo-[3,3] rearrangement although concerted, ionic, and direct displacement mechanisms appear to operate competitively. Transposition of the olefin is equally effective for allylic alkoxides prepared by nucleophilic addition, deprotonation, or reduction. Experimentally, the niobium pentahalide halogenations are rapid, afford essentially pure E-allylic or allenic halides after extraction, and are applicable to a range of aliphatic and aromatic alcohols, aldehydes, and ketones.

Further insights into the chemistry of niobium and tantalum pentahalides with 1,2-dialkoxyalkanes: Synthesis of bromo- and iodoalkoxides, spectroscopic and computational studies

“…CH2Cl2 16, MeI 12, CH2ICH2I (4), CH2ClI 2, ClCH2OCH2OCH2Cl (4), MeO(CH2)2OMe 11, Me2O 2 1.022 R1 (I > 2σ(I)) 0.0393 wR2 (all data) 0.0850 Largest diff. peak and hole, e.Å - 3 1.460 and -1.476…”

“…Nevertheless, an increasing interest in the use of 1 in metal-mediated synthesis has grown up in the recent years [2]; this is consequence of the unusual and striking behaviour exhibited by such complexes, in comparison with other early transition metal halides [3].…”

NbCl5 as an efficient catalyst for rapid synthesis of quinoxaline derivatives

“…In recent years, niobium pentachloride has been considered as mild Lewis acid catalyst for a variety of organic transformations [31], such as one-pot Mannichtype reaction [32], alkoxide rearrangements [33], the intramolecular Friedel-Crafts acylation reaction [34], conversion of aldehydes and ketones to allylic halides [35], cyanosilylation of aldehydes [36], synthesis of aaminonitriles [37], 1,1-diacetates [38], bis(indol)alkanes [39], and 1,5-benzodiazepine derivatives [40]. However, to the best of our knowledge, there is no report on the synthesis of quinoxaline derivatives using niobium pentachloride as a reagent.…”

Niobium Pentachloride Mediated (Hetero)aromatic Aldehyde Friedel–Crafts Hydroxyalkylation with Arenes: An Efficient Strategy to Synthesize Triarylmethanes

“…In this context, the use of niobium(V) chloride (NbCl 5 ) offers many advantages: it is easy to handle and can act as an excellent acid system for numerous reactions under mild conditions, including allylic halide synthesis, 18 intramolecular alkene hydrofunctionalization, 19 benzylic alcohol nucleophilic substitution reactions, 20 dithioacetal deprotec-tion, 21 Mannich reactions, 22 multicomponent reactions, 23 and Diels-Alder reactions, 24 among others. 25 As part of our interest in developing new synthetic methodologies, we report an efficient and simple procedure to obtain triarylmethanes through Friedel-Crafts-type hydroxyalkylation mediated by NbCl 5 under mild conditions.…”