Alkynyl Moiety for Triggering 1,2‐Metallate Shifts: Enantiospecific sp²–sp³ Coupling of Boronic Esters with p‐Arylacetylenes

Abstract

The enantiospecific coupling of secondary and tertiary boronic esters to aromatics has been investigated. Using p‐lithiated phenylacetylenes and a range of boronic esters coupling has been achieved by the addition of N‐bromosuccinimide (NBS). The alkyne functionality of the intermediate boronate complex reacts with NBS triggering the 1,2‐migration of the group on boron to carbon giving a dearomatized bromoallene intermediate. At this point elimination and rearomatization occurs with neopentyl boronic esters, giving the coupled products. However, using pinacol boronic esters, the boron moiety migrates to the adjacent carbon resulting in formation of ortho boron‐incorporated coupled products. The synthetic utility of the boron incorporated product has been demonstrated by orthogonal transformation of both the alkyne and boronic ester functionalities.

Stereoselective Base‐Catalyzed 1,1‐Silaboration of Terminal Alkynes

“…Specifically, a significant erosion in stereoselectivity was found upon exposing 1 v or 1 w to 2 a in DME ( top left ), whereas Z ‐configured 3 v and 3 w were exclusively obtained under a toluene regime (Scheme , top right). The former result can tentatively be interpreted on the basis of the strong coordination of DME to the escorting potassium counterion, thereby generating a separated silylboronated ion pair that precedes the formation of allene‐type intermediates via [1,2]‐shift from I . This notion was corroborated by the formal defluorosilylation of 1 x observed under our optimized reaction conditions (Scheme , bottom) as well as by the E / Z mixtures obtained for 3 u (Scheme ).…”

Stereoselective Base‐Catalyzed 1,1‐Silaboration of Terminal Alkynes

“…Thef ormer result can tentatively be interpreted on the basis of the strong coordination of DME to the escorting potassium counterion, [32] thereby generating aseparated silylboronated ion pair that precedes the formation of allene-type intermediates via [1,2]-shift from I. [8,33] This notion was corroborated by the formal defluorosilylation of 1x observed under our optimized reaction conditions (Scheme 6, bottom) as well as by the E/Z mixtures obtained for 3u (Scheme 3). In toluene as solvent, however, ac ontacted ion pair is more likely,w hich would attenuate the reactivity of I while forming the targeted 3v-3w in ahighly stereoselective fashion.…”

Stereospecific 1,2‐Migrations of Boronate Complexes Induced by Electrophiles

“…Minireviews acetylene products 95 and 96 could be accessed by coupling between p-lithiated phenylacetylenes (generated by halogenlithium exchange of the corresponding bromide 94)a nd arange of chiral boronic esters 18 (Scheme 18). [45] Treatment of the intermediate TMS-phenylacetylene-derived boronate complex with NBS results in bromination of the alkyne motif, which triggered as tereospecific 1,2-migration leading to dearomatized bromoallene intermediate 97.U sing unhindered neopentyl glycol boronic esters and MeOH as solvent, subsequent nucleophile-promoted elimination and rearomatization of 97 a occurred, resulting in the formation of coupled product 95.Incontrast, the use of the more hindered pinacol boronic esters and i PrOH as the solvent prevented nucleophile-promoted elimination, therefore 1,2-Wagner-Meerwein shift of the Bpin moiety occurred instead. This led to carbocation 98,w hich, after loss of ap roton, furnished the ortho Bpin-incorporated product 96.…”