Zirconoarylation of alkynes through p-chloranil-promoted reductive elimination of arylzirconates

Summary A novel method for the zirconoarylation of alkynes was developed. TCQ-promoted reductive elimination of arylzirconate [LiCp2ZrAr(RC≡CR)], which was prepared by the reaction of zirconocene–alkyne complexes with aryllithium compounds, afforded trisubstituted alkenylzirconocenes. This reaction can afford multi-substituted olefins with high stereoselectivity.

Recently, we have reported a p-chloranil (TCQ)-promoted reductive elimination reaction of the zirconate complex Li[Cp 2 Zr(C≡CR) 3 ] toward geminal enediynes [44]. As part of our ongoing project on organozirconate chemistry [45][46][47][48], we envisioned that the use of an aryl ligand instead of one of the alkynyl ligands would provide an arylzirconation product of the alkyne. Herein we describe the TCQ-promoted reductive elimination of arylzirconate to afford an arylzirconation product of the alkyne, which can be converted to multi-substituted olefins through coupling with electrophiles (Scheme 3).

Results and Discussion
Initially, the reaction of Cp 2 Zr(PhC≡CPh)(DMAP) (1a), prepared by the reaction of Cp 2 ZrBu 2 [48] with DMAP and diphenylacetylene according to reported literature [49], with phenyllithium produced arylzirconate 2a. To this mixture, 2 equivalents of p-chloranil (TCQ) were added and the reaction mixture was stirred for 12 h at room temperature. After being quenched with HCl solution, the desired triphenylethylene (3a) was isolated in 62% yield. When the reaction mixture was quenched with DCl solution, the deuterated product 3a-D was isolated in 60% yield with >95% of deuterium incorporation (Scheme 4). The deuterium experiment revealed the formation of triphenylvinylzirconocene 4a as intermediate.
The pathway of the oxidation of zirconate 2 to vinylzirconocene 4 is not yet clear. A possible mechanism is proposed in Scheme 6. Coordination of TCQ to zirconium results in

Conclusion
We have developed a novel method for the zirconoarylation of alkynes through TCQ-promoted reductive elimination of arylzirconate. This reaction can afford multi-substituted olefins with stereoselectivity.

Experimental
General Comments. All manipulations were conducted in Schlenk tubes and under a nitrogen atmosphere with a slightly positive pressure. Unless otherwise noted, all starting materials were commercially available and were used without further purification. Tetrahydrofuran (THF) was refluxed and freshly distilled from dark purple solutions of sodium and benzophenone under nitrogen atmosphere. 1 H NMR and 13 C NMR spectra were recorded on 300 MHz and 400 MHz NMR spectrometers with TMS as internal standard. GC-MS spectra were recorded on a Hewlett Packard GC-MS system.
Typical procedure for TCQ-promoted arylzirconation of alkynes