Nuclear Magnetic Resonance Spectroscopy. Magnetic Nonequivalence Due to Slow Inversion in Amines¹

Parametrization of Substituents: Effects of Fluorine and Other Heteroatoms on OH, NH, and CH Acidities

“…New factors become crucial such as lone-pair/lone-pair repulsions between excess electron density and adjacent heteroatoms. [48,49] Previously, evidence was presented according to which a fluorine atom, when directly attached to a carbanionic center, exerts a destabilizing effect. [50] More recently, two striking examples of benzyl anion destabilization by a para-positioned fluorine atom have been identified.…”

“…In contrast, deprotonation of p-fluorotoluene at the a-position occurs 10 times more slowly than with the halogen-free parent compound (Table 43, k a rel 0.1). [51] The deactivation of the benzylic site in p-fluorotoluene seems to be caused by n-p electron ± electron repulsion [48,49] (Section 3.2). This has practical consequences.…”

The Reactivity of 2‐Fluoro‐ and 2‐Chloropyridines toward Sodium Ethoxide: Factors Governing the Rates of Nucleophilic (Het)Aromatic Substitutions

“…Whereas it facilitates the substitution to approximately the same extent as the other halogen atoms when accommodated at the 3-position, it diminishes the reaction rate by a factor ranging from 1.5 (Table 1) to 4.0 ( Table 2) when at the 5-position. The F-atom is known to raise the free energy of planar (and only planar) carbanions by lone-pair/lone-pair repulsion (conjugative destabilization [19]) [20 ± 22]. That this effect manifests itself at the 5-but not the 3-position must mean an unequal distribution of the electron excess, perhaps in the sense of the transient intermediate being better visualized as an azaallyl anion 15 as opposed to the azapentadienyl anion 16 (see Scheme 2).…”

Parametrisierung von Substituenten – der Einfluß von Fluor und anderen Heteroatomen auf OH-, NH- und CH-Aciditäten

“…Andere Faktoren werden dann ausschlaggebend, wie etwa die Elektronenpaar/Elektronenpaar-Abstoûung zwischen Zentren erhöhter Elektronendichte und angrenzenden Heteroatomen. [48,49] Wie bereits vor längerer Zeit gezeigt wurde, [50] kann ein Fluoratom, wenn es unmittelbar an ein carbanionisches Zentrum gebunden ist, auf dieses eine destabilisierende Wirkung ausüben. Kürzlich wurden zwei eindrucksvolle Fälle von Benzylanionendestabilisierungen durch p-ständige Fluoratome erkannt.…”

“…[51] Dabei wurden nur Näherungswerte erhalten ( Tabelle Die Desaktivierung der Benzylposition in p-Fluortoluol scheint durch n-p-Elektron-Elektron-Abstoûung verursacht zu werden [48,49] (Abschnitt 3.2). Daraus ergeben sich praktische Folgen.…”