Arylchlorogermanes/TBAF/“Moist” Toluene: A Promising Combination for Pd-Catalyzed Germyl-Stille Cross-Coupling

Abstract

The trichlorophenyl,-dichlorodiphenyl,-and chlorotriphenylgermanes undergo Pd-catalyzed crosscouplings with aryl bromides and iodides in the presence of TBAF in toluene with addition of the measured amount of water. One chloride ligand on the Ge center allows efficient activation by fluoride to promote transfer of one, two or three phenyl groups from the organogermane precursors.The Pd-catalyzed cross-coupling of organogermanes has thus far received much less attention 1 than the couplings involving organostannanes and organosilanes. 2 This is due to the lower reactivity of tetracoordinated organogermanium species, the less developed syntheses of vinyl/aryl germanyl derivatives, and the higher cost of germanium relative to silicon. 3 The carbagermatranes 1, with internal coordination of nitrogen to germanium, were the first examples of reactive tetracoordinated germanes applied to Pd-catalyzed cross-coupling reactions with aryl bromides 4 (Figure 1). The oxagermatranes 2 were found to be more efficient than carbagermatranes and triethoxygermanes. 5,6 Fluoride-promoted couplings with aryltri(2-furyl)germanes 7 and NaOH-activated couplings with arylgermanium trichlorides 8 or their hydrolyzed and stable sesquioxide alternatives 9 were also reported. The bis(2-naphthylmethyl) arylgermanes 3 were developed as photochemically activated arylgermanes for the synthesis of biaryls. 10,11 The vinyl tris(trimethylsilyl)germanes 4 were employed as transmetalation reagents in "ligand-and fluoride-free" coupling reactions with halides under oxidative wnuk@fiu.edu. † On faculty leave from

Chemoselective Transfer of Allyl or Phenyl Group from Allyl(phenyl)germanes in Pd-catalyzed Reactions with Aryl Halides

“…The yield increased when couplings were performed with addition of a measured amount of water (µ1030 equiv; Entries 68), as observed before for couplings with chloro(phenyl)germanes. 16,17 The SbF 5 /TBAF combination in moist toluene allowed transfer of two phenyl groups from germane 2, as expected based on the couplings of Ph 2 GeCl 2 with aryl halides promoted by TBAF. 17 One-pot treatment of germanes 1, 2, or 3 with SbF 5 /C followed by Pd-catalyzed (TBAF/moist toluene) reactions with 1, 2, or 3 equiv of aryl halides, respectively afforded various biaryls 7b7d in good yields and regioselectivity (Table 4).…”

“…One chloride ligand on the Ge center is sufficient to allow efficient activation by fluoride and subsequent transfer of one, two or three phenyl groups from the organogermane precursors. 16,17 We showed that arylchlorogermanes can render a coupling efficiency comparable to that of the more established stannane and silane counterparts and that their coupling efficiency reflected their ability to generate reactive hypervalent intermediates upon fluoride activation. 17 Hiyama and co-workers utilized tri-, di-, and monoallyl(aryl)-silanes in Pd-coupling with aryl halides to access biaryls.…”

Fluoride-Promoted Cross-Coupling of Chloro(mono-, di-, or triphenyl)germanes with Aryl Halides in “Moist” Toluene. Multiple Transfer of the Phenyl Group from Organogermane Substrates and Comparison of the Coupling Efficiencies of Chloro(phenyl)germanes with their Corresponding Stannane and Silane Counterparts

“…During the optimization of the cross-coupling reactions between chlorogermanes and 1-iodonaphthalene (see Table 2) it became obvious that the outcome of the couplings strongly depended on TBAF/organogermanes ratios 22. TBAF most likely facilitates coupling by generating the more reactive hypervalent fluorogermanium species and the reactivity of these species could be superior in toluene solvent due to weak solvation.…”

“…fluoride, hydroxide etc.) thereby also rendering the Ge centre pentavalent 11–13,17,22. Although the effects of the fluoride or base activation and Pd-catalyst/ligand combination on the coupling of organogermanes are still ambiguous, all but one8 of the successful examples of organogermane couplings with aryl/alkenyl halides involved activation with either base or fluoride.…”

Novel Approaches for the Synthesis of C-5 Modified Pyrimidine Nucleosides

“…), which is also rendering the Ge center pentavalent. 75,110,115,134 Even though the mechanism of fluoride/base activated Pd-catalyzed cross- …”