Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis‐Azobenzene Photoisomer

Abstract

The reactivity of the geminal frustrated Lewis pair (FLP) (F5C2)3SnCH2P(tBu)2 (1) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H3CC(O)CH=CH2, Ph[C(O)]2Ph, PhN=NPh and Me3SiCHN2), featuring polar or non‐polar multiple bonds and/or represent α,β‐unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV‐induced photoisomerization, which results in the highly selective complexation of cis‐azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2‐ or 1,4‐addition products, but to the non‐stereoselective (tBu)2PCH2‐transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X‐ray diffraction experiments.

Selective 3,3‐Rearrangement of Azobenzenes upon Complexation by a Frustrated Lewis Pair

“…Most intriguingly and not unlikely to be important for the follow‐up chemistry described below, the dihedral angle C(7)−N(2)−N(1)−C(1) is 52.0(1)° in this system and much wider at 72.1(1)° in 4 . For comparison, this angle is 58.7(3)° in the PhN=NPh adduct of t Bu 2 P−CH 2 −Sn(C 2 F 5 ) 3 and 78.7(1)/79.8(1)° in the twofold PhN=NPh adduct of Ph 2 P−Bcat [16, 17] . This brings the two phenyl groups much closer together in 3 (C(1)⋅⋅⋅C(7) 2.845(1) Å) than in 4 (3.052(2) Å).…”

“…A variety of FLPs has also proven capable of binding azobenzene either by phosphinoboration or under the formation of five‐ or six‐membered heterocycles [16, 17] . Furthermore, the addition to the N=N bond of diethyl azodicarboxylate was described for a 1,2‐phenylene‐bridged phosphino‐borane [18] .…”

ortho-Phenylene-bridged phosphorus/silicon Lewis pairs

“…12 To enhance the Lewis acidity of the silyl group, we attempted to implement the more electronegative pentafluoroethyl substituents at the silicon atom, which was already successfully used in our geminal phosphorus/tetrel FLP. 11,23 However, the reaction of 1 with chlorotris-(pentafluoroethyl)silane did not afford the desired iPr 2 P( o -C 6 H 4 )Si(C 2 F 5 ) 3 but a mixture of different (decomposition) products which could not be separated.…”

Selektive 3,3‐Umlagerung von Azobenzolen nach Komplexierung durch ein frustriertes Lewis–Paar

“…Eine Reihe von FLPs hat sich als fähig erwiesen, Azobenzol entweder durch Phosphinoborierung oder unter Bildung fünf‐ oder sechsgliedriger Heterocyclen zu binden [16, 17] . Außerdem wurde die Addition an die N=N‐Bindung von Diethylazodicarboxylat für ein 1,2‐phenylenverbrücktes Phosphinoboran beschrieben [18] .…”