First EPR characterisation of the structures and reactivities of α- and β-boryl radicals derived from boronic esters

Abstract

The first EPR observations of carbon-centred radicals with aand b-boronic ester substitutents indicated moderate thermodynamic stabilisation for the former whereas the latter had an inclination towards b-scission.Boronic acids and esters are convenient synthetic intermediates which have been widely applied in stereocontrolled reactions. 1 Recent research has shown that a-boryl radicals, generated by addition of carbon-centred radicals to alkenyl-2 and alkynylboronic esters, 3 can take part in inter-and intra-molecular chain addition reactions. a-Boryl radicals have also been generated by iodine abstraction from a-iodoboronic esters 4 and high yields of useful homoallylboronic esters were obtained in allylation reactions. These developments led us to apply EPR spectroscopy to identify the radical intermediates and examine key reaction rates for several types of carbon-centred radicals possessing a boronic ester functionality. Little is known about the EPR spectra of boron-containing species in solution, apart from the impressive spectroscopic experiments of Roberts and co-workers with amine-boryl and other ligated-boryl radicals, 5 a report on the Me 2 BCH 2. radical in the solid state 6 and some aminoxyl species. 7 The a-bromoboronic esters 1a,b were prepared according to the protocols developed by Matteson and co-workers from B(OPr i ) 3 , CH 2 Br 2 and Bu n Li for 1a, 8 from pinacol n-hexyl boronate, CH 2 Br 2 and Pr i 2 NLi for 1b. 9 EPR spectra, recorded during abstraction of the bromine atoms of these boronic esters by photochemically generated Me 3 Sn . radicals (or Et 3 Si . radicals) in tert-butylbenzene, cyclopropane or propane could be positively assigned to the corresponding a-boryl radicals 2a,b (EPR parameters in Table 1). The hyperfine splittings (hfs) from the a-Hs are somewhat smaller than in analogous unsubstituted primary and secondary alkyl radicals [e.g. for . CH 2 CH(OEt) 2 a(H a ) = 22.2 G] 10 which suggests there is some electron delocalisation into the vacant boron p-orbital (3). This is also consistent with the comparatively large 11 B hfs.If 3 makes a significant contribution to the ground state there should be an appreciable barrier to rotation about the C a -B bond. To try and detect this we examined the EPR spectra of radicals generated from the unsymmetrical boronic esters 4a,b prepared by transesterification of 1a,b with (+)-pinanediol.

The Influence of Boryl Substituents on the Formation and Reactivity of Adjacent and Vicinal Free Radical Centers

“…In agreement with this, product analyses showed that at higher temperatures the dimer 22 , derived from 19a , lessened in importance and that the major product was 1-hexene. A mechanism involving direct β-scission of 19a to produce 1-hexene and bisalkoxyboryl radical 20 was considered (Scheme ) . However, radical 20 could not be spectroscopically detected directly, by spin trapping, or on ligation with pyridine.…”

“…The objectives of the present research were to investigate the ease of formation of α- and β-boronate radicals, to ascertain their modes of reaction, to assess the stabilization of these radicals by experimental and by ab initio theoretical methods, and to examine the kinetics of hydrogen abstraction adjacent to boronate functional groups. We recently reported the first EPR spectroscopic observations of transient radicals derived from boronic esters in solution and have used this technique, integrated with product analyses, to address the above aims.…”

Cyclopropanation of Terminal Alkenes through Sequential Atom‐Transfer Radical Addition/1,3‐Elimination

“…Our long‐standing interest in developing mild methods for the functionalization of alkenes through radical pathways inspired us to investigate iodoalkylation involving 1‐borylated alkyl radicals to give γ‐iodoalkylboronates. Based on EPR studies and calculations, Walton, Carboni, and co‐workers reported that 1‐borylated radicals are stabilized when the boron is sp 2 ‐hybridized, however the extent of stabilization is smaller for radicals with a boronic ester substituent relative to the corresponding borinic ester or borane (Figure A) . This stabilization is the first key feature for designing an iodine‐atom‐transfer process according to the pioneering work of Kharasch, Curran, and co‐workers .…”

Cyclopropanation of Terminal Alkenes through Sequential Atom‐Transfer Radical Addition/1,3‐Elimination

“…[16] Then ature of the alkyl chain at boron can be easily modified with ahigh level of stereocontrol through homologation reactions upon treatment with carbenoid-type reagents. [33,34] This stabilization is the first key feature for designing an iodine-atom-transfer process according to the pioneering work of Kharasch, Curran, and co-workers. [33,34] This stabilization is the first key feature for designing an iodine-atom-transfer process according to the pioneering work of Kharasch, Curran, and co-workers.…”

“…[17][18][19] Our long-standing interest in developing mild methods for the functionalization of alkenes through radical pathways [20][21][22][23][24][25][26][27][28][29][30][31][32] inspired us to investigate iodoalkylation involving 1-borylated alkyl radicals to give g-iodoalkylboronates.B ased on EPR studies and calculations,W alton, Carboni, and co-workers reported that 1-borylated radicals are stabilized when the boron is sp 2 -hybridized, however the extent of stabilization is smaller for radicals with aboronic ester substituent relative to the corresponding borinic ester or borane ( Figure 1A). [33,34] This stabilization is the first key feature for designing an iodine-atom-transfer process according to the pioneering work of Kharasch, Curran, and co-workers. [35][36][37][38][39][40][41] Matching the philicities of the radicals and alkenes involved in the process is the second key feature for the success of the reaction.…”