Ring-Opening of THF via an Intramolecular P/Al-Based Frustrated Lewis Pair: Assistance by C₆F₅ Groups beyond Electronegativity?

Abstract

A frustrated Lewis pair composed of an acidic aluminum function (AlR 2 ) and a basic phosphine entity, linked by a xanthene spacer, is capable of cleaving THF. The rate of the ringopening reaction is higher by a factor of 10 for R = C 6 F 5 than that for R = Mes. Structural and theoretical investigations revealed for the case of C 6 F 5 aromatic interactions in the secondary coordination sphere whichin case they also exist in the transition statecould aid ring-opening through a preorientation of the reacting functions. However, in detailed computational studies these were found to occur without contributing significantly to the lowering of the activation barrier.

A strained intramolecular P/Al-FLP and its reactivity toward allene

“…Consequently, 2-Ph reacts with O-donors like Et 2 O or THF to the corresponding adducts, and evaporation/heating did not lead back to 2-Ph . Interestingly, unlike in case of the xanthene-based analogue, 10,18 no ring-opening reaction of THF was observed, which confirms the importance of the appropriate distance for a certain activation process.…”

Synthesis of Intramolecular P/Al‐Based Frustrated Lewis Pairs via Aluminum‐Tin‐Exchange and their Reactivity toward CO2

“…Similar structures to 3 which also exhibit P−Al−X−Al structural motifs (with X=H) have been investigated by Uhl et al., [4b] however, their reactivity toward substrates has not been reported [9] . To test whether 3 and 4 , respectively, can act as FLP, i. e. whether the Me 2 AlCl unit in 3 is bound weakly enough to act as a replaceable space holder, solutions of 3 and 4 in toluene‐ d 8 were treated with an excess of THF, since the THF ring‐opening reaction on a xanthene linked Al/P FLP has proved possible in our previous work [6] . In the 31 P NMR spectra recorded after both reactions a single new resonance was found at a chemical shift of δ =−18.8 ppm (for 3 ) and δ =−16.8 ppm (for 4 ), consistent with the coordination of a THF molecule to the xanthene‐bound aluminum center.…”

“…The resonance found at δ =−20.6 ppm was attributed to the reported intermediate H Xant(PPh 2 )(Al(C 6 F 5 ) 2 (THF)) and this assignment was further substantiated through the analytical data of the product of the following ring‐opening reaction, matching the ones reported for H Xant(PPh 2 (CH 2 ) 4 OAl(C 6 F 5 ) 2 ), 10 (Scheme 7). [6] In contrast to 6 , the second equivalent of MeAl(C 6 F 5 ) 2 does not coordinate to the oxygen atom of the 4‐oxybutyl bridge (see above) in 10 , probably due to steric hindrance, and is washed away during the work‐up procedure.…”

“…Our initial endeavors to synthesize an intramolecular P/Al‐based FLP on a xanthene backbone by lithiation and transmetalation resulted in the irreversible coordination of the utilized solvent (Et 2 O, THF) to the Lewis acidic Al III center. This led to interesting insights with regard to the functioning of such FLPs, exemplarily for the ring‐opening of THF, [6] but further applications were limited: the solvents are bound tightly and cannot be replaced by external substrates, so that the Lewis acid site is quenched. Consequently, we sought to develop an alternative route that leads to the corresponding solvent‐free xanthene‐linked Al/P FLP.…”

A Strained Intramolecular P/Al-FLP and its Reactivity toward Allene

“…Consequently, 2-Ph reacts with O-donors like Et2O or THF to the corresponding adducts, and evaporation/heating did not lead back to 2-Ph. Interestingly, unlike in case of the xanthene-based analogue, [10,18] no ring-opening reaction of THF was observed, which confirms the importance of the appropriate distance for a certain activation process.…”