1,2,5-Trimethylpyrrolyl Phosphines: A Class of Strongly Donating Arylphosphines

Abstract

Arylphosphines are among the most important ligands in coordination chemistry and catalysis but are generally inferior to alkylphosphines when strong electron donor properties are required. Herein, we report the synthesis and properties of 1,2,5-trimethylpyrrolyl phosphines and show that their electrondonating ability can exceed that of alkylphosphines. The new phosphines can be prepared on a multi-gram scale in one-pot procedures starting from commercially available reagents. Tris-(1,2,5-trimethyl)pyrrolylphosphine P(tmp) 3 is a crystalline solid, displays a Tolman electronic parameter (TEP) of 2055.0 cm −1 and can be handled in air. Nickel(0), palladium(II), gold(I), and ruthenium(II) complexes with 1,2,5-trimethylpyrrolyl-based phosphines are reported in addition to preliminary investigations on the use of P(tmp) 3 in ruthenium-catalyzed olefin metathesis.

Facile One‐Step Access to Pyrrole‐Based 1,4‐Diphosphabarrelenes

“…With a TEP value of 2069.7 cm À 1 the electron donor ability of 1 b falls in the typical range of aryl phosphines (Table 1), yet the value is increased by 10 cm À 1 compared to the structural related bis(1,2,5-trimethylpyrrolyl)phenylphosphine (P(tmp) 2 Ph). [19] We also attempted the synthesis of complex ] and decreased the amount of 2 b, confirming that the phosphorus species are in an equilibrium (see the SI for details). By IR analysis of this mixture, a tentative TEP value of 2069.0 cm À 1 was assigned for the first coordination of 1 b.…”

“…[19] Moreover, the CÀ PÀ C angles in the 1,4-diphosphabarrelenes increase less upon coordination (sum of CÀ PÀ C angles: 3 a: 297.7°, 4 b: 293.7°) than for P(tmp) 2 Ph (sum of CÀ PÀ C angles of [AuClP(tmp) 2 Ph]: 317.7°), underlining the rigid structure of the caged phosphines. [19] The pyrrolyl PÀ C bond length decreases upon coordination (1 a: ØPÀ C tmp = 1.811 Å, 3 a: ØPÀ C tmp = 1.784 Å). As a result, the nonbonding P•••P distances shorten upon coordination (1 a: 3.317 Å, 3 a: 3.187 Å, 1 b: 3.316 Å, 4 b: 3.211 Å).…”

“…The pyramidalization of the 1,4-diphosphabarrelene phosphorus atoms (sum of CÀ PÀ C angles: 1 a: 290.0°, 289.6°, 1 b: 284.1°, 290.2°) is more pronounced than in the acyclic phosphine P(tmp) 2 Ph (306.2°). [19] Moreover, the CÀ PÀ C angles in the 1,4-diphosphabarrelenes increase less upon coordination (sum of CÀ PÀ C angles: 3 a: 297.7°, 4 b: 293.7°) than for P(tmp) 2 Ph (sum of CÀ PÀ C angles of [AuClP(tmp) 2 Ph]: 317.7°), underlining the rigid structure of the caged phosphines. [19] The pyrrolyl PÀ C bond length decreases upon coordination (1 a: ØPÀ C tmp = 1.811 Å, 3 a: ØPÀ C tmp = 1.784 Å).…”

“…2069.0 [b] /2069.7 PPh 3 2068.9 [a] Literature values. [21,19,23] Having a rigid diphosphine ligand with opposite orientation of the two lone pairs in hand, we chose to investigate the ability of 1 a to form coordination frameworks. [24] Compound 5 a was formed by a diffusion reaction of 1 a with copper(I) chloride at room temperature and crystallizes as colorless plates.…”

“…[17] As part of our program to increase the electron-donating capacity of phosphines, we developed a series of superbasic phosphines using N-heterocyclic π-donor substituents [18] and more recently also pyrrolyl substituents. [19] For example, bis(1,2,5-trimethylpyrrolyl)phenylphosphine was synthesized by reacting dichlorophenylphosphine with 1,2,5-trimethylpyrrole (Htmp) using triethylamine as a base. The observation that 1,2,5-trimethylpyrrolyl substituted phosphines are stronger donors than alkylphosphines motivated us to synthesize bidentate phosphines, which, as disclosed herein, led instead to the discovery of a facile one-step approach to 1,4-diphosphabarrelenes.…”