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

Abstract

SummaryThe β-unsubstituted title compound dissolves in THF as a uniformly trisolvated monomer, whereas it forms exclusively disolvated monomers in tert-butyl methyl ether, Et2O, TMEDA, or toluene with TMEDA (1.4 equiv). This was established at low temperatures through the observation of separated NMR signals for free and lithium-coordinated ligands and/or through the patterns and magnitudes of 13C,6Li NMR coupling constants. An aggregated form was observed only with Et2O (2 equiv) in toluene as the solvent. The olefinic geminal interproton coupling constants of the H2C= part can be used as a secondary criterion to differentiate between these differently solvated ground-states (3, 2, or <2 coordinated ligands per Li). Due to a kinetic trisolvation privilege of THF, the cis/trans sp2-stereoinversion rates could be measured through analyses of 1H NMR line broadening and coalescence only in THF as the solvent: The pseudomonomolecular (because THF-catalyzed), ionic mechanism is initialized by a C–Li bond heterolysis with the transient immobilization of one additional THF ligand, followed by stereoinversion of the quasi-sp2-hybridized carbanionic center in cooperation with a “conducted tour” migration of Li+(THF)4 along the α-aryl group within the solvent-separated ion pair.

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

“…Internal repulsion was indeed much less of a problem in the ground states of our α‐arylvinyllithiums 1 – 6 , as shown by the slightly lower barrier (entry 5: 15.79 kcal mol −1 ) of α‐(2,6‐diisopropylphenyl)vinyllithium ( 6 ) in comparison with that of 4 (entry 8). Internal repulsion was not totally absent, however, as revealed by a substantially lower barrier of α‐(2,6‐di‐ tert ‐butylphenyl)vinyllithium. In spite of the significant differences of 1 – 7 b in their structural details and barriers, the pseudoactivation entropies in entries 1–3 and 5–8 fall into the narrow range of Δ S ψ ≠ =−23±3 cal mol −1 K −1 , which is characteristic of the ionic mechanism and includes a contribution from the transitory immobilization of the fourth THF ligand that is required for the two THF‐separated ion pair intermediates (SSIP) and the transition state in analogy with Scheme .…”

“…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.…”

“…Favorable electrostatic interactions might make it more attractive for Li + (THF) 4 to migratea cross the still charge-bearing, temporarily sp-hybridized C(a)c enter (a-route). The proute had been proposed previously [1][2][3][4][5] to proceed as a" conducted tour" [6] along the chargeg radients that resulted through delocalization of negative electric chargef rom the LiÀ C(a)b ond into the a-aryl p-electron system:S uch ad elocalization is at its maximum in the depicted transition state with a lateral overlap (Scheme1)o ft he two p x orbitals at C(a)a nd C (4), which are situated within the C(a) = C(b)d ouble-bond plane;t his confers ap artial double-bondc haracter to the C(4)-C(a)b ond (quasibenzyl-anion resonance [7] )a nd createsa ne lectronic barrier against deviation from the close to perpendicular orientation of the a-aryl plane with respect to the doublebond plane.S uch ar otationalb arrier would disfavor the possibility of at ransport of Li + (THF) 4 riding on a p face of arotating a-aryl group. The energetic stabilizationa gainst a-aryl rotation and the concomitanta ccumulation of delocalized negative pcharge in the para position( at C-9) are stronger in the linear transition state than in the nonlinearg round state 1-CIP;t his had been established [1] throught he Hammett reactionc onstant 1 =+5.2 for the stereoinversion rate constants as determined for ar elatedf amily of a-arylalkenyllithiums (para-substituted derivatives of 7a in Scheme 2).…”

“…Favorable electrostatic interactions might make it more attractive for Li + (THF) 4 to migrate across the still charge‐bearing, temporarily sp‐hybridized C(α) center (α‐route). The π‐route had been proposed previously to proceed as a “conducted tour” along the charge gradients that resulted through delocalization of negative electric charge from the Li−C(α) bond into the α‐aryl π‐electron system: Such a delocalization is at its maximum in the depicted transition state with a lateral overlap (Scheme ) of the two p x orbitals at C(α) and C(4), which are situated within the C(α)=C(β) double‐bond plane; this confers a partial double‐bond character to the C(4)‐C(α) bond (quasibenzyl‐anion resonance) and creates an electronic barrier against deviation from the close to perpendicular orientation of the α‐aryl plane with respect to the double‐bond plane. Such a rotational barrier would disfavor the possibility of a transport of Li + (THF) 4 riding on a π face of a rotating α‐aryl group.…”