Responsive Iron Neighboring Group Participation in Amino-Substituent-Stabilized [3]Ferrocenophane α-Carbenium Ions:  A Combined Theoretical and Experimental Study

Abstract

Treatment of the dimethylamino-[3]ferrocenophane 1a with B(C6F5)3 results in hydride abstraction at the −CH(NMe2) moiety in α-position to the ferrocene nucleus to yield the corresponding cation 4a. The X-ray crystal structure analysis of the salt 4a·[HO{B(C6F5)3}2 -] has revealed a residual Fe−Cα interaction even in this case of a strongly substituent-stabilized α-ferrocenyl carbenium ion. A DFT calculation of a series of derivatives containing nitrogen substituents of a decreasing ability for carbenium ion stabilization (4a, −NMe2; 4b, −pyrrolyl; 4c, −triazolyl) revealed a markedly increased Fe−C interaction with decreasing C+ stabilization by the hetarene substituents.

Enantioselective reductions of [m] ferrocenophanones

“…The a-carbocation stabilization in ferrocene derivatives was well documented [28]. Erker suggested iron-stabilized a-carbenium ion also in [3] ferrocenophane derivative [29]. Our DFT calculations of tentative carbenium ions in a and b-position derived from [4] ferrocenophane also support this notion.…”

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

“…However, under the reaction conditions this intermediate salt is unstable towards S N 1-substitution, leading to hydride from [HB(C 6 F 5 ) 3 ] effecting displacement of HP(C 6 H 2 Me 3 ) 2 with anchimeric assistance by the iron center (Scheme 50). [65,87] The analogous reaction was also observed in the related [3] (Figure 27) features a close PH···halide contact that might make the HP(C 6 H 2 Me 3 ) 2 moiety a slightly less-favorable leaving group in this special case. [88] Scheme 49.…”

“…[64] These systems apparently represent borderline cases in which the ferrocenylstabilized iminium ions are slow to react with hydride. [65] Thus, the system 123[HB(C 6 F 6 ) 3 ] is stable and isolable, whereas treatment of 123 [BF 4 ] with the slightly more nucleophilic reagent 27 a results in the formation of the respective stoichiometric hydrogenation products 121 a and 122 a. …”

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

“…Die selektive 1,4-Hydrierung von 133 zum Produkt 134 gelang mit dem Katalysatorsystem 27 a (Schema 49). [63] Der [65,87] Die analoge Reaktion wurde auch bei einem Beispiel aus der verwandten [3] [87] Überraschend liefert die analoge Umsetzung der eng verwandten ortho-Brom-und ortho-Iod-substituierten [88] …”