Chiral Thiahelicene-Based Alkyl Phosphine–Borane Complexes: Synthesis, X-ray Characterization, and Theoretical and Experimental Investigations of Optical Properties

Abstract

Chiral helical-based phosphanes are challenging and promising ligands, with a great potential for the generation of both organic and organometallic catalysts. We report here the preparation of novel chiral thiahelicene-based alkyl phosphanes, isolated and characterized as air-stable borane adducts, and the investigation of their experimental and theoretical (chir)optical properties. X-ray characterization of a mono- and a disubstituted derivative as a racemic mixture has been performed, which confirms the influence of the number and nature of substituents on the flexibility of the helix. In addition, the absolute configuration inferred from CD spectra of the two enantiomers of a diborane complex has been established from X-ray analysis. State-of-the-art quantum chemical calculations of vibrationally resolved spectra allow, for the first time, for an unambiguous assignment of the experimentally observed peaks in linear absorption and circular dichroism spectra to excited electronic states of this class of thiahelicene phosphorus derivatives.

Tetrathia[7]helicene Phosphorus Derivatives: Experimental and Theoretical Investigations of Electronic Properties, and Preliminary Applications as Organocatalysts

“…31 P NMR spectroscopic analysis was used to confirm the successful deprotection of 1 and 2 , and the signals related to the free phosphanes 3 and 4 were found to be sharp singlet peaks at more negative chemical shift values (Table ) than those of the corresponding borane adducts . Notably, phosphanes 3 b – d and 4 b – d were found to be highly sensitive toward oxidation, thus demonstrating the usefulness of the procedure to prepare and isolate them as air‐stable borane adducts.…”

“…Tetrathiahelicene phosphine–borane complexes 1 a–d and 2 a–d represent a convenient and versatile platform of stable helical phosphorus precursors, from which the corresponding 7‐TH mono and diphosphanes 3 a–d and 4 a–d could be obtained. We then investigated the most suitable conditions for the deprotection of adducts 1 and 2 , taking into account the well‐known procedures reported in the literature, including treatment with alcohols or amines …”

“…Among the plethora of chiral phosphorus ligands characterized by different stereogenic elements, helically chiral phosphorus compounds have proven to be a very promising class of ligands for many transition‐metal‐catalyzed asymmetric reactions, and, more recently, also as Lewis base organocatalysts, although helicene chemistry is still underdeveloped in the field of organocatalyzed transformations. For several years, our efforts in this field have been mainly devoted to developing thiahelicene phosphorus compounds, especially tetrathiahelicene (7‐TH) derivatives, as ligands in catalysis, which has been possible thanks to the notable improvements achieved in their synthesis and functionalization . Recently, Stephenson and Doulcet have reported a double kinetic resolution of the racemic 7‐TH to obtain homochiral 7‐TH derivatives, which are potentially useful as ligands for enantioselective transformations .…”

“…Moreover, 7‐TH aryl phosphine oxides have been preliminary tested as Lewis base organocatalysts in polychloro‐mediated aldol‐type reactions and ketoimine reduction; they demonstrated good efficiency and diastereoselectivity, although low enantioselectivity . More recently, we have also developed a selective procedure to synthesize a set of 7‐TH alkyl mono and diphosphine–borane complexes, the chiroptical properties of which have been fully elucidated by theoretical and experimental CD spectroscopy . However, no systematic study on the electronic properties of this class of thiahelicene‐based phosphorus compounds has been done so far.…”

“…As part of our continuing investigations on these 7‐TH phosphorus derivatives, in this article we report the synthesis of 7‐TH alkyl and aryl mono and diphosphanes by means of the removal of the borane group from the corresponding mono and diphosphine–borane adducts, the preparation of which has been previously reported by us . The electron‐donating ability of these alkyl phosphanes has then been evaluated on the basis of their first‐order phosphorous–selenium coupling constants in the corresponding alkyl 7‐TH phosphine–selenides.…”