Highly syn selective addition of aqueous HBr to hydrophobically shielded arylalkynes

Microsolvation and sp2-stereoinversion of monomeric α-(2,6-di-tert-butylphenyl)vinyllithium as measured by NMR

“…Obviously, 4 had not reacted with its coproduct n- BuBr to produce 7 ; instead, it had reacted faster than the concomitant n- BuLi to eliminate HBr from 3 and/or subsequently to deprotonate the generated arylalkyne [14] with formation of 6 and 8a . After carboxylation and aqueous work-up, the acidic product fraction contained only the arylpropiolic acid (δ H = 1.56 and 7.33 ppm in CDCl 3 ) derived from 6 but not the α-arylacrylic acid to be expected from 4 .…”

“…The rapid cis/trans stereoinversion of monomeric 4 in THF provides a possible explanation of the stereorandom formation [14] of [β-D] 5 through Br/Sn interchange and is partially due to trisolvation of the ground-state: The necessary C–Li bond heterolysis is achieved through immobilization of only one further THF ligand at lithium with a correspondingly low entropic penalty (ca. 44% of Δ S ψ ‡ ).…”

“…The Br/Li interchange reaction ( Scheme 2 ) between n- BuLi and the bromoalkene [ 14 ] 3 that generates the title compound α-(2,6-di- tert -butylphenyl)vinyllithium ( 4 ) in cyclopentane as the solvent was almost finished after 70 min at room temperature (rt), affording 1-bromobutane ( n- BuBr), the olefin [ 14 ] 8a , the known [ 14 ] lithium arylacetylide 6 , and residual n- BuLi (by 1 H NMR in situ analysis). Obviously, 4 had not reacted with its coproduct n- BuBr to produce 7 ; instead, it had reacted faster than the concomitant n- BuLi to eliminate HBr from 3 and/or subsequently to deprotonate the generated arylalkyne [ 14 ] with formation of 6 and 8a . After carboxylation and aqueous work-up, the acidic product fraction contained only the arylpropiolic acid (δ H = 1.56 and 7.33 ppm in CDCl 3 ) derived from 6 but not the α-arylacrylic acid to be expected from 4 .…”

“…The successful detour preparation of more stable solutions of 4 made use of the trimethylstannyl precursor 5 that had been obtained [ 14 ] from bromoalkene 3 with LiSnMe 3 . A cyclopentane solution of 5 and n- BuLi did not react during several days at rt; on addition of TMEDA (ca.…”

“…2,6-Di- tert -butylstyrene (8a) [14]. 2 J H,H = 2.2 Hz in CDCl 3 and THF; 2.3 Hz in cyclopentane, TMEDA/toluene, and TMEDA/ t- BuOMe; 2.4 Hz in Et 2 O and toluene.…”