Formation and reactions of active five-membered phosphane/borane frustrated Lewis pair ring systems

Abstract

The reactive five-membered frustrated P/B Lewis pair 6a was generated from Tipp-P(vinyl)2 (Tipp: 2,4,6-triisopropylphenyl) by a series of anti-Markovnikov and Markovnikov hydroboration reactions. The in situ generated compound dimerized to give 7a under kinetic control and the dimer pair 8a/9a under thermodynamic control. The Mes*-P(vinyl)2 analogue (Mes*: 2,4,6-tri-tert-butylphenyl) reacts in a similar way, but in this case the thermodynamic dimers are observed at r.t. whereas the monomer 6b is the dominant species at 80 °C in solution. It is an active dihydrogen splitting reagent. The in situ generated monomeric five-membered P/B FLPs undergo addition reactions to various organic π-reagents. With benzaldehyde or phenylacetylene this gave the respective zwitterionic heteronorbornene or -norbornadiene type products, the latter with liberation of ethylene.

Characterization of H2‐Splitting Products of Frustrated Lewis Pairs: Benefit of Fast Magic‐Angle Spinning

“…For our model compound 1 , similar connectivity patterns for the two HP doublets centered at 5.5 and 6.6 ppm are observed (for the assignments see Figure c). The observation of two HP resonances agrees with the single‐crystal structure in which two crystallographically distinct monomers of compound 1 were found in the asymmetric unit and explains the deviations from the solution‐state chemical‐shift values for which only one species is detected (see Figure a for the solution‐state spectrum). This illustrates the potential of MAS experiments >100 kHz particularly in NMR crystallographic approaches ,,…”

Aggregation Behavior of a Six‐Membered Cyclic Frustrated Phosphane/Borane Lewis Pair: Formation of a Supramolecular Cyclooctameric Macrocyclic Ring System

“…It shows achair conformation of the six-membered heterocyclic framework with both the bulky Mes* substituents at phosphorus and the C 6 F 5 group at boron being in equatorial positions.Consequently,the proton/ hydride pair had been attached trans at the phosphorous Lewis base and the boron Lewis acid. Since this cannot be achieved in an intramolecular reaction, [3,15] we conclude that the H 2 splitting reaction at the cyclic P/B FLP has probably taken place in an intermolecular fashion involving more than one FLP molecule. [16] In solution (CD 2 Cl 2 ) 12 shows the typical broad 1 HNMR 1:1:1:1q uartet of the [B]-H moiety at d = 1.97 ppm and the [P] + -H resonance at d = 6.50 ppm ( 1 J PH = 456.0 Hz).…”

“…[2] Cyclic systems,w here the phosphorus base and the boron Lewis acid are part of aring structure are rare. [3] We have recently described the active five-membered cyclic P/B FLP 3,w hich was formed starting from Mes*P-(vinyl) 2 (1)b ym eans of as equence of hydroboration reactions,a nd we reported on its specific chemical features (Scheme 1). [3a] We also reported on the synthesis of the sixmembered P/B system 6.…”

“…Our study has shown that active cyclic six-membered P/B FLPs can readily be prepared by means of hydroboration sequences,c ombined with as pecific sequence involving formation and removal of aH B(C 6 F 5 ) 2 equivalent. The resulting system 7 is an active P/B FLP.I tu ndergoes cycloaddition-type P/B addition reactions to p systems [3] and it actively cleaves the dihydrogen molecule,a lthough this reaction deviates from the common behaviour of intramolecular P/B FLPs in the sense that it produces the respective trans-H + /H À addition product. More importantly, 7 shows ar emarkable aggregation behaviour.I tf orms the unique macrocyclic cyclooctameric ring structure [(7) 8 ]inthe crystal and in solution at low temperature.C yclic frustrated Lewis pairs have so far rarely been encountered.…”

“…More importantly, 7 shows ar emarkable aggregation behaviour.I tf orms the unique macrocyclic cyclooctameric ring structure [(7) 8 ]inthe crystal and in solution at low temperature.C yclic frustrated Lewis pairs have so far rarely been encountered. [3] Their aggregation behaviour might open new pathways in FLP chemistry and it might serve to connect FLP chemistry with the important established area of supramolecular chemistry. Figure 5.…”