What can 13C and 1H NMR lithiation shifts tell us about the charge distribution in α-arylvinyllithium compounds?
作者:Rudolf Knorr、Ernst Lattke、Jakob Ruhdorfer、Ulrich von Roman、Joachim Firl、Petra Böhrer
DOI:10.1016/j.jorganchem.2016.09.029
日期:2016.12
on the other hand, the α-aryl group is conformationally fixed in a coplanar relationship with the CC double bond; this prevents a direct π charge delocalization from the LiC(α) bond into the adjacent aryl ring with the expected result of Δδ(C-para) ≈ 0 in the absence of electron donor ligands. However, the disolvated dimer of this conformationally flattened α-arylvinyllithium exhibited a Δδ(C-para)
能量稳定地从Li C(α)键到α-芳基乙烯基锂的芳基π系统Ar C(Li)CH 2的负电荷离域化最有效,且芳基环平面与C C呈正交关系双键平面。该芳基构象仍被α-芳基中的至少一个邻位取代基所偏爱。远程,对位13 C和1 H原子核的锂化NMR位移Δδ= δ(R–Li) -δ(RH)似乎由离域π电荷(准苄基锂)控制,通过比较C- para / para来判断-H锂化位移商与苄基锂的商。对于THF中的三溶剂化单体,在两个α-(2-烷基苯基)乙烯基锂上观察到的magnitudeΔδ(C-对位)∣最高,并且随着溶剂化的减少和聚集的增加而降低。另一方面,在3-硫代-1,1-二甲基茚中,α-芳基与CC双键共面地构象固定。这可以防止在没有电子供体配体的情况下,从Li C(α)键直接π电荷离域到相邻的芳基环,而预期结果为Δδ(C- para)≈0。然而,这种构象变平的α-芳基乙烯基锂的溶剂化二聚体表现出