A 1,1‐Carboboration Route to Bora‐Nazarov Systems

Abstract

Hydroboration of the conjugated enynes 1 a and 1 b with Piers' borane [HB(C6F5)2] gave the respective dienylboranes trans-2 c and trans-2 d. Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d. Both were converted to their pyridine adducts 5 c and 5 d, respectively. Compounds 3 c and 5 c,d were characterized by X-ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C6F5)3 resulted in the formation of the dihydroboroles 7 a and 7 b, respectively. This reaction is thought to proceed by 1,1-carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a/8 b, followed by thermally induced bora-Nazarov ring-closure and subsequent stabilizing 1,2-pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X-ray diffraction and solid-state (11)B NMR spectroscopy.

The Borole Route to Reactive Pentafluorophenyl‐Substituted Diboranes(4)

“…[6,7] Thesubsequent [4+ +2]-cycloaddition reactions with acetylenes provide av iable pathway to av ariety of 7-boranorbornadienes.T he examples 4a,b undergo adefined rearrangement reaction upon thermolysis (60 8 8C), probably by means of boryl migration to generate the respective boranorcaradiene intermediates 5.Inthe cases investigated in this study,t hese do not undergo ring enlargement to the borepines,a si so ften observed in borole Diels-Alder chemistry. [6,7] Thesubsequent [4+ +2]-cycloaddition reactions with acetylenes provide av iable pathway to av ariety of 7-boranorbornadienes.T he examples 4a,b undergo adefined rearrangement reaction upon thermolysis (60 8 8C), probably by means of boryl migration to generate the respective boranorcaradiene intermediates 5.Inthe cases investigated in this study,t hese do not undergo ring enlargement to the borepines,a si so ften observed in borole Diels-Alder chemistry.…”

“…[6,7] Bis(trimethylsilylethynyl)phenylborane (1) reacted with B(C 6 F 5 ) 3 to generate the intermediate 2 (which is shown by X-ray diffraction to exhibit a Z-configured tetracoordinated C = Cd ouble bond;s ee the Supporting Information), which then rearranged in an intramolecular 1,1-carboboration reaction [8,9] to give the borole product. [6,7] Bis(trimethylsilylethynyl)phenylborane (1) reacted with B(C 6 F 5 ) 3 to generate the intermediate 2 (which is shown by X-ray diffraction to exhibit a Z-configured tetracoordinated C = Cd ouble bond;s ee the Supporting Information), which then rearranged in an intramolecular 1,1-carboboration reaction [8,9] to give the borole product.…”

“…We had recently described a1 ,1-carboboration synthesis of the borole 3. [6,7] Bis(trimethylsilylethynyl)phenylborane (1) reacted with B(C 6 F 5 ) 3 to generate the intermediate 2 (which is shown by X-ray diffraction to exhibit a Z-configured tetracoordinated C = Cd ouble bond;s ee the Supporting Information), which then rearranged in an intramolecular 1,1-carboboration reaction [8,9] to give the borole product. Theb orole 3 underwent av ariety of [4+ +2]-cycloaddition reactions; [10,11] for example,t reatment of 3 with 1-pentyne gave the 7-boranorbornadiene derivative 4a (Scheme 1).…”

Selective N,O‐Addition of the TEMPO Radical to Conjugated Boryldienes

“…It is known that the complexation of TEMPO with boron-, aluminum-, or iron-based Lewis acids leads to an increase in the spin density at the Natom, [5] and this increase in spin density should provide aminyl radicalcation character to the nitroxide.I ndeed, it has been shown that intra-and intermolecular H-abstraction by the Natom in such LA/LB adducts is feasible. Theunusual process involves bond formation to both the TEMPO oxygen and nitrogen atom.We generated the dienyl borane 4a by hydroboration of the conjugated enyne 3a [9] with the Piers borane,H B(C 6 F 5 ) 2 (2). We assumed that, in analogy with the well-established cyclization of aminyl radicals, [8] appropriately alkene substituted TEMPO-LA adducts might undergo C À Nbond-forming radical cyclization reactions.Notably,for steric reasons,T EMPO shows very low reactivity towards alkenes in the absence of aL A, and if it does undergo addition to an alkene,the reaction occurs at the Oatom of the nitroxide (Scheme 1).…”

“…We generated the dienyl borane 4a by hydroboration of the conjugated enyne 3a [9] with the Piers borane,H B(C 6 F 5 ) 2 (2). [10] Theb oryldiene 4a was then exposed to two molar equivalents of TEMPO (1)inbenzene solution.…”