How Microsolvation Numbers at Li Control Aggregation Modes, sp²-Stereoinversion, and NMR Coupling Constants²J_H,Hof H₂C═C in α-(2,6-Dimethylphenyl)vinyllithium

Abstract

The title compound 4 is a trisolvated monomer 4&3THF in THF solution and dimerizes endothermically to form (4&THF)2 with a strongly positive (!) dimerization entropy in toluene as the solvent. In the absence of electron-pair donor ligands, 4 aggregates (>dimer) in hydrocarbon solutions. These results followed from the (13)C-α splitting patterns and the magnitudes of the one-bond (13)C,(6)Li NMR coupling constants in combination with lithiation NMR shifts as secondary NMR criteria. The rate constants of cis/trans sp(2)-stereoinversion could be measured on the (1)H NMR time scale in THF, in which solvent the preinversion lifetime is 0.24 s at 25 °C. This inversion proceeds according to the pseudomonomolecular, ionic mechanism with the typical, strongly negative pseudoactivation entropy. In a different mechanism, the lifetimes are much longer at 25 °C for the dimer (4&t-BuOMe)2 in toluene (ca. 2.5 min) and for donor-free, aggregated 4 in hexane solution (roughly 1 min). The olefinic interproton two-bond coupling constants (2)JH,H of the H2C═CLi part are proposed as an indicator of microsolvation at Li, because they were found to increase linearly with the "explicit" solvation of α-arylvinyllithiums by 0, 1, 2, and 3 electron-pair donor ligands.

“Conducted Tour” Migration of Li+ during the cis/trans Stereoinversion of α‐Arylvinyllithiums

“…(2)] microsolvation by only d = 1 tBuOMe ligand per Li. Such upfield lithiation NMR shifts are partially [7] caused by the delocalization of electric charge from the LiÀC(a)b ond to the para position (as detailed previously [2,7] ), so that their similar magnitudesi ndicated similar interplanar anglesb etweent he a-aryl and the C(a)= C(b)d ouble-bondplanesind imeric 1 and 4.These magnitudes revealed close to perpendicular conformationst hat provide for ac lose to maximum overlap of the charge-carrying LiÀC(a) bond (quasi-sp 2 )o rbitala nd the p x orbitala tC -ipso in the C(a)=C(b)d ouble-bond plane (quasi-benzyl-anion resonance). These data and the above assignments for the "parent" alkene 14 were required for calculations of lithiation NMR shifts Dd = d(RLi)Àd(RH), which can purge d(RLi = 1)f rom special effects that are also present in d(RH = 14), making the effects of lithiation more clearly visible.…”

“…Eur.J.2017, 23,12861 -12869 www.chemeurj.org of nBu 6 Li (22.9 mmol) in pentane (18.4 mL). The Br/Li interchange reaction was almost complete after 25 min at RT,w hereupon the product 2 began to precipitate completely within 4h.T he supernatant was withdrawn by pipette, and the precipitate was washed with anhydrous pentane (2 ), then evaporated to dryness at RT in vacuo:Y ield:2 .0 g( 66 %); 1 HNMR (THF,8 134.2 (quat, C-4a), 163.2 (quat, C-1), 210.5 ppm (quat, C-a), assigned through gated and off-resonance decoupling and comparison with 1;n ot emperature and concentration dependence detected between À38 and À96 8C.…”

“…The whole process is illustrated in Scheme 1f or the imagined 4-(a-lithiovinyl)-2,2-dimethylbenz [f]indane( 1), the stereoinversion ("diastereotopomerization") of which would interconvert the NMR chemical shifts d of the two diastereotopic b-H atoms. The ensuing descent from the transition state would transform p x into the quasi-sp 2 orbital of the stereoinverted intermediate 1'-SSIP.H owever, 1'-SSIP cannot be formed without as uitable migration of Li + (THF) 4 :G iven that ionic dissociation of an SSIP intermediate into the free ions had been excluded for related a-arylalkenyllithiumst hrough the observation of concentration-independent rate constants k y , [1][2][3][4][5] such am igration must occur within the THF-solvatedi on pairs on the way from 1-SSIP to 1'-SSIP.T he subsequentl oss of one THF ligand from 1'-SSIP would form the final product 1'-CIP,s ot hat the transitory immobilization of one additional THFl igand at Li implies the whole process is catalyzed by THF and hence runs with ap seudomonomolecular rate constant k y that depends on the THF concentration. The increased distance of Li + (THF) 4 from the carbanionic centerC ( a)i n1-SSIP would enable the charge-carrying, quasi-sp 2 orbitala tC ( a)t oc hange into at ransitory p x orbital (Scheme1)i nt he proposed [1] transition state of stereoinversion.…”

“…The increased distance of Li + (THF) 4 from the carbanionic centerC ( a)i n1-SSIP would enable the charge-carrying, quasi-sp 2 orbitala tC ( a)t oc hange into at ransitory p x orbital (Scheme1)i nt he proposed [1] transition state of stereoinversion. The ensuing descent from the transition state would transform p x into the quasi-sp 2 orbital of the stereoinverted intermediate 1'-SSIP.H owever, 1'-SSIP cannot be formed without as uitable migration of Li + (THF) 4 :G iven that ionic dissociation of an SSIP intermediate into the free ions had been excluded for related a-arylalkenyllithiumst hrough the observation of concentration-independent rate constants k y , [1][2][3][4][5] such am igration must occur within the THF-solvatedi on pairs on the way from 1-SSIP to 1'-SSIP.T he subsequentl oss of one THF ligand from 1'-SSIP would form the final product 1'-CIP,s ot hat the transitory immobilization of one additional THFl igand at Li implies the whole process is catalyzed by THF and hence runs with ap seudomonomolecular rate constant k y that depends on the THF concentration.…”

“…AB system, 2 d,2 J = 14 Hz, 1 + 1H;C H 2 -1), 5.32 (d, 2 J = 8.9 Hz, 1H; b-H trans to aryl), 5.75 (d, 2 J = 8.9 Hz, 1H; b-H cis), 6.75 (s, 1H;9 -H), 6.93 (t, 3 J = 7.4 Hz, 1H;6 -H), 7.04 (t, 3 J = 7.4 Hz, 1H;7 -H), 7.40 (d, 3 J = 7.5 Hz, 1H;8 -H), 8.00 ppm (d, 3 J = 7.6 Hz, 1H;5 -H), compare Ta ble S5 b 6-H as adca.…”