Structural and Electronic Effects on Phosphine Chalcogenide Stabilized Silicon Centers in Four-Membered Heterocyclic Cations

Abstract

Understanding the interplay of structural and electronic parameters in the stabilization of Lewis acidic silicon centers is crucial for stereochemical questions and applications in bond activation and catalytic transformations. Phosphine chalcogenide functionalized (Ch = O, S, and Se) hydrosilanes having tert-butyl and 2,4,6-trimethoxyphenyl (TMP) substituents on the silicon atom were synthesized, and the ring-closing reactions to afford the heterocyclic four-membered CPChSi cations were investigated. Synthetic access was only achieved for the sulfur-and selenium-based cations. A thorough study by means of single-crystal X-ray structure determination, NMR spectroscopic data, and density functional theory (DFT) calculations provided insight into important electronic and structural parameters affecting the stability of the intramolecularly stabilized cations. Detailed structural considerations were made on the contributions to the ring strain (angular strain and steric repulsion). Thermochemical investigations showed that the substituents on the silicon and phosphorus atoms play an important role for the stability of the cationic heterocycles. In the absence of large steric repulsions through bulky substituents (methyl groups on silicon and tert-butyl groups on phosphorus), an intrinsic stability sequence of the intramolecular Ch−Si coordination depending on the chalcogen atom in the direction Se ≤ S < O can be observed. However, the order is reversed (O < S < Se) in the case of strong repulsions between sterically demanding substituents (tert-butyl groups on both silicon and phosphorus atoms). Natural bond orbital (NBO) analysis supported the explanations for the observed deshielding trends in 31 P NMR spectroscopy and revealed that the O−Si bond is more ionic in nature compared to the S−Si and Se−Si bonds, with the latter exhibiting higher covalent character due to a more efficient charge transfer through a σ-type n Ch → p Si interaction.

A Cyclic Phosphonium Ion with an Asymmetrically Substituted Selenide‐Stabilized Silicon Center: Synthesis, Structure, and Substituent Effects

“…With an angular sum of 358.7°, the four‐membered cycle deviates slightly from the ideal planar geometry, which is indeed found in the symmetrically substituted all‐ tert ‐butyl analogue [5c] . This ring distortion is designed to counteract an unfavorable intramolecular steric repulsion between the two ecliptically arranged tert ‐butyl groups on the silicon and phosphorus atom, thereby reducing the ring strain in the Si( t Bu)Ph‐structured cation 4 (Figure 2).…”

“…This is already indicative of a slightly stronger intramolecular stabilization in the selenium‐based cation 4 . The influence of the newly formed selenium−silicon bond on the 31 P NMR chemical shift is negligible, changing from 74.2 ppm ( 3 ) to 77.8 ppm ( 4 ), which reflects the observed deshielding properties of the phosphorus atom in phosphane chalcogenide‐based cyclic cations [5c] …”

“…The synthesis of the selenium‐based cyclic phosphonium cation 4 was achieved in three steps following a previously reported synthetic route (Scheme 1). [5c] First, (di‐ tert ‐butylphosphanyl)methyllithium was reacted with tert ‐butylchlorophenylsilane ( 1 ) to give the phosphorus(III) precursor 2 in a moderate yield of 48 % [7] . Oxidation with red selenium afforded phosphane selenide 3 in 24 % yield of pure crystalline material.…”

“…Ring formation upon hydride abstraction led to a significant downfield shift of the 77 Se NMR signal from −388.3 ppm in compound 3 to −139.1 ppm in the ion pair 4 [HB(C 6 F 5 ) 3 ] [Δ δ ( 77 Se)=249.2 ppm]. At the same time, the 1 J Se–P coupling constant decreases by 56 % from 695.7 Hz to 306.4 Hz, which is typical for cationic four‐membered CPSeSi ring systems [5c] . The somewhat stronger highfield shift of the 77 Se NMR signal of the tert ‐butylphenyl‐patterned cation 4 compared to the di‐ tert ‐butyl derivatives [5c] indicates slightly different electronic situations around the silicon atom.…”

“…The P + −Ch − group has been shown to guarantee the configurational integrity of the stereogenic Lewis acidic silicon center during the sequence of ring‐closure to a four‐membered cyclic cation and subsequent ring‐opening [5a] . Since the stereochemical consistency at the stereogenic Lewis acidic silicon center is a mandatory criterion for applications of optically pure chiral cations in asymmetric cation‐directed catalysis, [6] knowledge of electronic and steric effects, and the influence of the chalcogen atom on the strength of the intramolecular stabilization of the silicon atom in chalcogen‐based cyclic cations is needed [5c] …”