The Propargyl Rearrangement to Functionalised Allyl‐Boron and Borocation Compounds

Abstract

A diverse range of Lewis acidic alkyl, vinyl and aryl boranes and borenium compounds that are capable of new carbon-carbon bond formation through selective migratory group transfer have been synthesised. Utilising a series of heteroleptic boranes [PhB(C6 F5 )2 (1), PhCH2 CH2 B(C6 F5 )2 (2), and E-B(C6 F5 )2 (C6 F5 )C=C(I)R (R=Ph 3 a, nBu 3 b)] and borenium cations [phenylquinolatoborenium cation ([QOBPh][AlCl4 ], 4)], it has been shown that these boron-based compounds are capable of producing novel allyl- boron and boronium compounds through complex rearrangement reactions with various propargyl esters and carbamates. These reactions yield highly functionalised, synthetically useful boron substituted organic compounds with substantial molecular complexity in a one-pot reaction.

Divergent Elementoboration: 1,3‐Haloboration versus 1,1‐Carboboration of Propargyl Esters

“…Of particular note is the regioselectivity of this reaction with the product predominating as the selective transfero ft he aryl group over the chloride fragment. [18] This migration pattern could be confirmed once again through detailed 2D NMR spectroscopy to affirmt he molecular connectivity revealed in the solid-state structure (see the Supporting Information).…”

“…Storing 4 a – c as a saturated CH 2 Cl 2 /hexane solution produced a number of colorless crystals suitable for X‐ray diffraction, which indeed determined the molecular structure to be the product of a formal 1,1‐carboboration reaction (Figure ). Of particular note is the regioselectivity of this reaction with the product predominating as the selective transfer of the aryl group over the chloride fragment . This migration pattern could be confirmed once again through detailed 2D NMR spectroscopy to affirm the molecular connectivity revealed in the solid‐state structure (see the Supporting Information).…”

A Comparative Assessment of Modern Cyclization Methods of Substituted Alkynyl Esters, Ethers, and Acids

“…Our group, amongst others, have also used the strong Lewis acid B(C6F5)3 as a π-Lewis acid similar to those described earlier as well as other related transformations such as cycloaminations, [31] trans-oxyborations, [32] annulations [33] amongst many others (Scheme 10). [34] Due to the strong oxophilic character of the hard Lewis acid, it is perhaps interesting to see such high reactivity with softer π-Lewis bases.…”

Lewis Acid Catalyzed Transfer Hydromethallylation for the Construction of Quaternary Carbon Centers

“…[17] In seeking ap lausible hydrocarbon unit, we were interested in ar eport by MØnard und Stephan who had achieved the stoichiometric CÀH activation of isobutene with the frustrated Lewis pair tBu 3 P/ B(C 6 F 5 ) 3 (Scheme 1, middle). [18] Them ethallyl borate 8 À formed represents ap otential nucleophilic source of the methallyl group, [19,20] and we hoped that acyclohexa-1,4-diene surrogate of isobutene gas could be developed to allow for the abstraction of the methallyl nucleofuge by B(C 6 F 5 ) 3 to give 8 À and aBrønsted acidic Wheland intermediate.Interception of this complex by an alkene might then allow for transfer Scheme 1. Cyclohexa-1,4-diene-based surrogates of isobutane and isobutene gas for metal-free transfer hydroalkylation and -allylation, respectively.…”