ABSTRACT
Invited for the cover of this issue are the groups of Ruthâ M. Gschwind and Robert Wolf (University of Regensburg), Christian Müller (Freie Universität Berlin), and J.â Chris Slootweg (University of Amsterdam). The image depicts playing cards representing the reported reactions involving 1-phospha-7-bora-norbornadiene. Read the full text of the article at 10.1002/chem.202000266.
ABSTRACT
Salt metathesis of 1-methyl-2,4,6-triphenylphosphacyclohexadienyl lithium and chlorobis(pentafluorophenyl)borane affords a 1-phospha-7-bora-norbornadiene derivative 2. The C≡N triple bonds of nitriles insert into the P-B bond of 2 with concomitant C-B bond cleavage, whereas the C≡C bonds of phenylacetylenes react with 2 to form λ4 -phosphabarrelenes. Even though 2 must formally be regarded as a classical Lewis adduct, the C≡N and C≡C activation processes observed (and the mild conditions under which they occur) are reminiscent of the reactivity of frustrated Lewis pairs. Indeed, NMR and computational studies give insight into the mechanism of the reactions and reveal the labile nature of the phosphorus-boron bond in 2, which is also suggested by detailed NMR spectroscopic studies on this compound. Nitrile insertion is thus preceded by ring opening of the bicycle of 2 through P-B bond splitting with a low energy barrier. By contrast, the reaction with alkynes involves formation of a reactive zwitterionic methylphosphininium borate intermediate, which readily undergoes alkyne 1,4-addition.
ABSTRACT
Metal ligand cooperativity (MLC) and frustrated Lewis pair (FLP) chemistry both feature the cooperative action of a Lewis acidic and a Lewis basic site on a substrate. A lot of work has been carried out in the field of FLPs to prevent Lewis adduct formation, which often reduces the FLP reactivity. Parallels are drawn between the two systems by looking at their reactivity with CO2, and we explore the role of steric bulk in preventing dimer formation in MLC systems.
ABSTRACT
A series of substituted phosphinines, 1-phosphabarrelenes and 5-phosphasemibullvalenes were synthesized and evaluated for their potential application as ligands in homogeneous catalytic reactions. While their buried volume (%Vbur ) was calculated to get insight into the steric properties, [LNi(CO)3 ] complexes were prepared in order to determine the corresponding Tolman electronic parameter. ETS-NOCV (extended-transition-state natural orbital for chemical valence) calculations on [LAuCl] complexes further allowed an estimation of the σ- and π-contributions to the L-M interaction. AuI coordination compounds of selected examples were prepared and characterized by single crystal X-ray diffraction. Finally, the three classes of PIII compounds were successfully used in the AuI -catalyzed cycloisomerization of N-2-propyn-1-ylbenzamide, showing very good activities and selectivities, which are comparable with the reported data of cationic phosphorus-based gold catalysts.
ABSTRACT
Inspired by the commercially available azoimidazolium dyes (e.g., Basic Redâ 51) that can be obtained from aryldiazonium salts and N-heterocyclic carbenes, we developed the synthesis of a unique set of arylazophosphonium salts. A range of colours were obtained by applying readily tuneable phosphine donor ligands and para-substituted aryldiazonium salts as nitrogen-based Lewis acids. With cyclic voltammetry, a general procedure was designed to establish whether the reaction between a Lewis acid and a Lewis base occurs by single-electron transfer or electron-pair transfer.
ABSTRACT
The reactivity of the geminal phosphinoborane tBu2PCH2BPh2 towards terminal alkynes, nitriles and nitrilium salts is investigated. Terminal alkynes react via C-H bond splitting (deprotonation) resulting in the formation of phosphonium borates. In contrast, both nitriles and nitrilium salts undergo addition reactions resulting in the formation of five-membered heterocycles. All compounds were characterized by multinuclear NMR spectroscopy, and single-crystal X-ray structure determinations. Insight into the reaction mechanisms was gained by DFT calculations.
