Umpolung of Methylenephosphonium Ions in Their Manganese Half‐Sandwich Complexes and Application to the Synthesis of Chiral Phosphorus‐Containing Ligand Scaffolds

Abstract

Half-sandwich manganese methylenephosphonium complexes [Cp(CO) 2 Mn(h 2 -R 2 P=C(H)Ph)]BF 4 were obtained in high yield through as traightforwardr eaction sequence involving ac lassical Fischer-type manganese complex and asecondary phosphine as key starting materials.The addition of various nucleophiles (Nu) to these species took place regioselectively at the double-bonded carbon center of the coordinated methylenephosphonium ligand R 2 P + =C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO) 2 Mn(k 1 -R 2 PÀC(H)(Ph)Nu)],f rom which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures.Iminium salts are of utmost importance in organic synthesis, [1] owing in particular to their key role as intermediates in many enantioselective organocatalytic transformations of carbonyl compounds. [2] However,the synthetic utility of their phosphorus analogues,m ethylenephosphonium ions, [3, 4] remains limited to af ew examples of [2+ +2] [4h,j] and [2+ +4] [4b,e,g] cycloaddition, the ene reaction, [4b,e] electrocyclic cyclization with an arene ring, [4e,f] stereoselectiver earrangement to give phosphaalkenes, [4d] and the phospha-Stork reaction. [4l] Whereas the reactivity of iminium salts is governed by the strong electrophilicity of the carbon atom (Scheme 1), the methylenephosphonium salts undergo regioselective addition of nucleophiles (Nu = F À ,C l À ,a nd MeO À ) [3a,b, 4a] at the electropositivep hosphorus center (Scheme 1). In attempts to explore the reactivity of manganese methylenephosphonium complexes [Cp(CO) 2 Mn-(h 2 -R 2 P = C(H)Ph)] BF 4 ,r eadily available from half-sandwich Mn I Fischer carbenes and secondary phosphines as key starting materials,w eo bserved that the coordinated methylenephosphonium moiety undergoes regioselective addition of nucleophiles at the carbon center, thus mimicking the reactivity mode of iminium salts (Scheme 1; in this scheme and thereafter {Mp} = Cp(CO) 2 Mn). This key observation inspired the development of an unconventional synthetic methodology toward aw ide variety of organophosphorous derivatives,a si llustrated herein by as imple route to an ovel family of chiral pincer-type phosphine-NHC-phosphine ligands (NHC = N-heterocyclic carbene).Earlier investigations in our laboratories had previously established that the cationic manganese carbyne [{Mp} + C À Ph] BPh 4 ([2]BPh 4 )r eacts sequentially with diphenylphosphine and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give the h 1 -phosphinocarbene complex 3a,which undergoes facile CO insertion to form the h 3 -phosphinoketene complex 4a (see structures in Scheme 2). [5] We have now found that further protonation of 4a with HBF 4 ·OEt 2 leads to the quantitative formation of the cationic complex [5a] + with a h 2 -methylenephosphonium ligand (Scheme 2). [6,7] This reaction is ar are example of c...

Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional P-Coordination of the Ylide Moiety

“…21 The comparison of the experimental ν C�N frequency value for [6](OTf) 2 with those of complexes [7](OTf) 2 and [8](OTf) (2206−2207 cm −1 ) revealed that the new 5,6-P,C,Cpincer ligand has similar donor character to the related 6,6membered phosphonium ylide-based analogues exhibiting NHC or phenolate donor extremities (Scheme 4). 17 These values also indicate that only pincer ligands in complexes [9](OTf) 2 (2194 cm −1 ) and [10](OTf) (2187 cm −1 ) featuring, respectively, two peripheral ylides 16a and a central ylide 23 exhibit greater electronic donation. Given that complexes [6](OTf) 2 and [7](OTf) 2 bearing neutral P,C,Cand C,C,C-pincer ligands, respectively, have the same overall charge (+2), 17 the similarity of their electronic properties is rather unexpected, taking into account the generally higher donation of NHCs vs aryl-substituted phosphines.…”

“…This specific arrangement results in the formation of highly constraint structures composed of two five-membered and six-membered metallacycles. In both cases, the NHC and phosphine moieties are situated in a trans position to the σ-aryl and ylide fragments, respectively [C1−Pd1−C13: [4](OTf) 173.88(10)°, [14] 16,17,23 Theoretical Study of the Electronic Structure of Pd(II) Complex 15 Bearing Two types of Phosphonium Ylide Ligands by QTAIM and ELF Topological Analyses. Since all our efforts to grow single crystals of Pd(II) complex 15 suitable for X-ray diffraction were unsuccessful, we decided to investigate its structure by DFT methods.…”