Iminophosphoranes (R3P NR′): From terminal to multidentate ligands in organometallic chemistry

Enantioselective Synthesis of Quaternary Oxindoles: Desymmetrizing Staudinger–Aza-Wittig Reaction Enabled by a Bespoke HypPhos Oxide Catalyst

“…An attempt to lower the reaction temperature (from 70 to 40 °C) to increase the enantioselectivity produced only a trace amount of product (entry 3). Hypothesizing that a Lewis acid might enhance the electrophilicity of the malonate ester , and promote the silane-mediated reduction of the phosphine oxide, , we surveyed a variety of metal salts for the reactions involving the HypPhos oxide 1•[O] (air-stable and, therefore, easier to handle than its tertiary phosphine counterpart) and phenylsilane . Because both the phosphine ligand and metal ion were present in the reaction, we were cognizant that “self-quenching” might possibly preclude the Staudinger–aza-Wittig reaction .…”

“…The resulting imidate would readily hydrolyze to give a lactam 4 with the quaternary stereogenic center unchanged. Previously reported Staudinger–aza-Wittig reactions of esters have been performed under harsh conditions (e.g., reflux in toluene). ,, In terms of the relatively low reactivity of esters and the challenges of asymmetric induction, we surmised that coordination of a Lewis acid and a chiral iminophosphorane might facilitate the aza-Wittig reaction of unactivated esters, considering that both 1,3-dicarbonyl groups and iminophosphoranes are good ligands for Lewis acids. , Moreover, Lewis acids can facilitate silane-mediated reductions of phosphine oxides, potentially assisting the P III /P V O redox cycling in this present case. , Accordingly, we applied a custom-made HypPhos oxide catalyst, 2•[O] , to the synthesis of valuable quaternary oxindoles, with the aid of cooperative Ir I –P III /P V O redox cycling.…”

Phosphonium Ylides vs Iminophosphoranes: The Role of the Coordinating Ylidic Atom in cis-[Phosphine-Ylide Rh(CO)2] Complexes

“…In the case of phosphonium ylides, the ylidic bond was alternatively described in terms of donor–acceptor interactions between closed-shell R 3 P: and :CR 2 fragments with the contribution of two pairs of electrons . Beyond the difference in the electronegativity of the ylidic-type atom (C: 2.5 vs N: 3.0), phosphonium ylides and iminophosphoranes behave mainly as η 1 -ligands with strong σ-donor and relatively weak π-acceptor character either in main group or coordination chemistry . Moreover and especially compared to NHCs, while ylide-based ligands act as harder donors which, a priori, should be more prone to coordinate electropositive metals, their overall neutral character allows accommodating late metal elements, similarly to NHCs. , For catalytic purposes, examples involving P + –C – ylide ligands remain scarce, certainly due to synthetic and stability issues of the corresponding metal complexes .…”

“…Beyond the difference in the electronegativity of the ylidic-type atom (C: 2.5 vs N: 3.0), phosphonium ylides and iminophosphoranes behave mainly as η 1 -ligands with strong σ-donor and relatively weak π-acceptor character either in main group or coordination chemistry . Moreover and especially compared to NHCs, while ylide-based ligands act as harder donors which, a priori, should be more prone to coordinate electropositive metals, their overall neutral character allows accommodating late metal elements, similarly to NHCs. , For catalytic purposes, examples involving P + –C – ylide ligands remain scarce, certainly due to synthetic and stability issues of the corresponding metal complexes . However, the pioneering report of a diylide rhodium complex active for olefin hydrogenation is worth mentioning, as well as the more recent study concerning an NHC core palladium pincer exhibiting two side ylide ends for the allylation of aldehydes .…”

Introducing N-Heterocyclic Iminophosphoranes (NHIPs): Synthesis by [3 + 2] Cycloaddition of Azophosphines with Alkynes and Reactivity Studies

“…Iminophosphoranes (R 3 PNR) not only are versatile intermediates for functional group manipulations − but also are used as ligands − and organocatalysts because of the basic nitrogen center, which can be rationalized by the resonance structures (R 3 PNR ↔ R 3 P + –N – R). , Cyclic iminophosphoranes, where all of the substituents are bonded to phosphorus atoms through nitrogen atoms, have been prepared by condensing amines with PCl 5 and subsequent deprotonation. The rigid cyclic structure enabled selective anion recognition via hydrogen bonding, which was essential for achieving high enantioselectivity. − A rigid cyclic structure is also advantageous for the stabilization of reactive main-group or metal species by preventing decomposition by steric shielding.…”