Phosphorane-Promoted C-C Coupling during Aryne Annulations

Abstract

Arynes are fleeting, high-energy intermediates that undergo myriad trapping reactions by nucleophiles. Their unusual reactivity compared to other electrophiles can spur unexpected mechanistic pathways enroute to the formation of benzenoid products. Herein we explore a particularly unique case of thermally generated arynes reacting with phosphoranes to form helical dibenzothiophenes and -selenophenes. Multiple new helical polycyclic aromatic products are reported. DP4+ and X-ray crystallographic analysis were used in tandem to confirm the structural topologies of selected products and to demonstrate the utility of DP4+ for distinguishing between isomeric polycyclic aromatic compounds. Lastly, we discuss a plausible mechanism consistent with DFT computations that accounts for the product formation; namely, ligand coupling (i.e., reductive elimination) within a hypervalent, pentacarbon-ligated σ-phosphorane furnishes the dibenzothio- or dibenzoselenophene.

Keywords: DP4+; HDDA; arynes; hypervalent compounds; phosphorane.

Figure 1.

a) Divergent selectivity in σ-sulfurane ligand-coupling. b) Trapping of “Kobayashi” benzynes with π-phosphoranes yields phosphonium salts. c) Hypothesized intermediate upon thermal generation of arynes in the presence of phosphine sulfides(selenides). d) First report of C(sp²)–C(sp²) ligand coupling within a σ-phosphorane.

Figure 2.

a) Hu’s reported outcome for trapping of a HDDA benzyne derived from 10 with Ph₃P=S. b) Our initial observation of desired dibenzothiophene formation. c) HDDA benzyne intermediates formed enroute to dibenzothiophene products.

Figure 3.

Various dibenzothiophenes and -selenophenes synthesized through phosphorane trapping of HDDA arynes from a) aryl triynone precursors, or b) the tetrayne 23 leads to the S-shaped product 24-S.

Figure 4.

a) DP4+ analysis of U-shaped dimers based on ¹H and ¹³C NMR spectra to discern between narrow bay region (left) and wide bay region (right) topologies. b) PyMOL renderings of the X-ray crystal structures of 17-S and 22-Se (hydrogens are omitted).

Figure 5.

DFT modeling of key, late-stage events converting VIII via IX to products during the overall reaction of two molecules of 25 + 4d-S → 26-S + Ph₂PMe. For DFT calculations of all steps, see Figure S5 (SI). Calculations performed at the [(SMD:PhCF₃)/MN15/6–311+G**//(SMD:PhCF₃)/MN15/6–31G*] level of theory.