An Unexpected “Step-Conjugated” Biphosphole via Unique P–P Bond Formation

Abstract

The synthesis of a p-conjugated organophosphorus species with bridging P-P unit is reported. Because of the pyramidal geometry of the phosphorus centers, the molecular scaffold provides intriguing electronic communication throughout the three-dimensional structure via p-s-p conjugation in stepwise fashion. The dimeric species was serendipitously found to be accessible via a reaction of the corresponding P-amino-phosphole precursor through mediation with the hard Lewis acid BF 3 . We provide detailed mechanistic studies toward a suitable reaction mechanism that was also verified via computational means. Moreover, we elaborate the utility of the biphosphole via phosphorus functionalization that lends further proof for the step conjugation provided by the unique phosphorus-based molecular architecture.

Synthesis of a Trivalent P‐Chloro‐Dithienophosphole and Its Reactivity with Organometallic Reagents

“…The ether solution was concentrated to yield 2 as a yellow solid (3.62 g, 69.9 %). Characterization data were consistent with those previously reported …”

“…The 1 H NMR spectrum exhibited only the bithiophene resonances, with no evidence of any residual pyrrolidine species. Extending the reaction time, unexpectedly resulted in the formation of the recently reported biphosphole 3 (Scheme ) as evidenced by the 31 P NMR shift δ = –37.0 ppm . While surprising at first, such dimerization is not unprecedented.…”

“…The pyrrolidinyl‐substituted dithienophosphole 2 was chosen, as we recently reported that the pyrrolidinyl phosphole allows further functionalization of the phosphorus center via phosphorus chemistry, and can be synthesized in good yields . Originally, the P ‐pyrrolidinyl‐dithienophosphole had been isolated as an oil that was 95 % pure by NMR spectroscopy . However, with a slight modification, this precursor phosphole can be isolated as a pure solid: after the solvent removal, the concentrate was taken up in pentane and the suspension filtered through a plug of Celite.…”

“…The first step for the generation of 1 was the synthesis of an aminophosphole. The pyrrolidinyl‐substituted dithienophosphole 2 was chosen, as we recently reported that the pyrrolidinyl phosphole allows further functionalization of the phosphorus center via phosphorus chemistry, and can be synthesized in good yields . Originally, the P ‐pyrrolidinyl‐dithienophosphole had been isolated as an oil that was 95 % pure by NMR spectroscopy .…”

“…To determine the nature of this apparent decomposition, an ether solution of 1 was sealed at room temperature under argon with only a rubber septum and monitored by 31 P NMR spectroscopy. After 48 hours, no trivalent chloride 1 remained in solution, instead several phosphorus resonances were present corresponding to the dithienophosphole dimer 3 ( δ = –37.0 ppm) the singly oxidized dithienophosphole dimer 4 ( δ = –50.6 ppm, d, J P‐P = 208 Hz, 35.4 ppm, d, J P‐P = 208 Hz), P ‐chloro‐dithienophosphole oxide 5 ( δ = 17.3 ppm),, and P ‐hydroxy‐dithienophosphole oxide 6 ( δ = 22.9 ppm), suggesting both a rapid and complex reactivity with even low levels of dioxygen and water (Scheme ). However, none of these oxidation or hydrolysis products were consistent with the asymmetric dimer found in the sealed NMR sample.…”