A new synthetic approach for the production of carbon nanomaterials (CNM) decorated with organophosphorus moieties is presented. Three different triphenylphosphine oxide (TPPO) derivatives were used to decorate oxidized multiwalled carbon nanotubes (ox-MWCNTs) and graphene platelets (GPs). The TPPOs chosen bear functional groups able to react with the CNMs by Tour reaction (an amino group), nitrene cycloaddition (an azido group) or CuAAC reaction (one terminal C–C triple bond). All the adducts were characterized by FTIR, Raman spectroscopy, TEM, XPS, elemental analysis and ICP-AES. The cycloaddition of nitrene provided the higher loading on ox-MWCNTs and GPs as well, while the Tour approach gave best results with nanotubes (CNTs). Finally, we investigated the possibility to reduce the TPPO functionalized CNMs to the corresponding phosphine derivatives and applied one of the materials produced as heterogeneous organocatalyst in a Staudinger ligation reaction.
Tertiary sulfanylphosphine and aminoalkylphosphine oxides can be easily converted into the corresponding tertiary sulfanylphosphine- and aminoalkylphosphine-boranes, respectively, through the facile P=O bond reduction by borane complexes. The easy reduction of the strong P=O bond by BH3, a mild reducing agent, has been achieved through an intramolecular P=O -- B complexation directed by proximal SH or NH activating groups located at the α- or β-position to the P=O bond. A generalized reduction mechanism has been proposed.
Oxidized multiwalled carbon nanotubes were modified anchoring phosphine oxides and used as heterogeneous catalysts. A proper substitution of the phosphine oxides allowed the use of the Tour reaction and the nitrene cycloaddition to obtain functionalized carbon nanotubes (CNT) with a loading up to 0.73 mmol/g of material. The catalysts proved efficient in Wittig reactions, Mitsunobu reactions, and Staudinger ligations. Furthermore, the phosphorus decorated CNT were used to produce nanocomposite with Pd nanoparticles able to catalyze Heck reactions.
α-Oxy-o-xylylene, a highly reactive diene
readily accessible from benzocyclobutenol, undergoes Diels–Alder
reaction with vinylphosphine oxides, yielding the corresponding 2-phosphorylated
1-hydroxy-1,2,3,4-tetrahydronaphthalenes in excellent yields. Use
of unsubstituted and trans-2-aryl-substituted vinylphosphine
oxides leads to cycloadducts with complete regioselectivity and with
cis/trans selectivity up to 19:1 in the most favorable case. In the
case of P-stereogenic trans-2-aryl-substituted vinylphosphine
oxides, a virtually complete chirality transfer from P to C can be
achieved. Dehydration and aromatization of the obtained cycloadducts
bearing the resolved P-stereogenic phosphinoyl groups can be carried
out to afford the valuable P-stereogenic and axially chiral phosphorylated
1,2′-binaphthyl ring system. Cases of restricted rotation around
Csp3–Csp2 single bond in some tetrahydronaphthalene
cycloadducts have also been revealed.
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