Synthesis and Properties of Sterically Crowded Triarylphosphines Bearing Naphthoquinone Moieties

Abstract

Sterically crowded triarylphosphines bearing naphthoquinone moieties were synthesized by the Suzuki-Miyaura coupling of the corresponding (phosphinoaryl)boronic acid derivatives with 2,3dichloro-1,4-naphthoquinone. The triarylphosphine-naphthoquinone unit can be extended by employing a triarylphosphine bearing a chloronaphthoquinone moiety as a substrate. As an example, an oligomer bearing three triarylphosphine and two naphthoquinone moieties was synthesized. The triarylphosphines bearing naphthoquinone moieties exhibited purple to blue colors arising from intramolecular charge transfer. A systematic study of various derivatives showed that the corresponding absorption shifted toward longer wavelengths as the difference between the oxidation potential of the triarylphosphine moieties and the reduction potential of the naphthoquinone moieties becomes smaller. On the other hand, the intensity of the charge transfer absorption depends on the number of interacting triarylphosphine-naphthoquinone units. The purple color of the charge transfer turned to pale yellow after protonation of the phosphorus atom with trifluoroacetic acid, while deprotonation by addition of triethylamine regenerated the color as well as the phosphine. The intramolecular charge transfer is sensitive to structural changes, as insertion of a 1,4-phenylene spacer between the triarylphosphine and naphthoquinone moieties leads to a loss of the purple color.

Synthesis and Structure of Sterically Crowded Triarylphosphine Bearing 4-Bromo-2,6-Bis(4-tert-Butylphenyl)Phenyl Group

“…developed by Hart et al 11 We have previously synthesized the 2,6-diarylhalobenzenes including the 4-bromo derivatives in this manner. 9 5-Bromo-1,3-bis(4-tert-butylphenyl)-2-iodobenzene (6) was similarly synthesized by addition of 4-tert-butylphenylmagnesium bromide to 1,3,5-tribromo-2-iodobenzene followed by quenching with iodine (Scheme 1). The lithiation of 6 with n-butyllithium and the reaction with phosphorus trichloride afforded diarylchlorophosphine 7, however, the attempted conversion to the sterically crowded triarylphosphine by the coupling with 2,4,6-triisopropylphenylcopper 12 was unsuccessful.…”

“…In addition, they are distinct from previously most crowded 2 as well as conventional phosphines in that they prefer the one electron oxidation to the formation of metal complexes. We have synthesized various sterically crowded triarylphosphines bearing functional sites such as donors, 5 acceptors, 6 and radicals, 7 and have revealed their structures and properties. Sterically crowded triarylphosphines bearing bromo groups 4a, 7 4b, 6a and 4c 6a played key roles as common synthetic intermediates for various compounds.…”

“…Sterically crowded triarylphosphines bearing bromo groups 4a, 7 4b, 6a and 4c 6a played key roles as common synthetic intermediates for various compounds. 6,7 On the other hand, 2,6-diarylphenyl groups or m-terphenyl ligands have been successfully applied as sterically protecting groups of the low coordinate compounds. 8 We have also applied the 2,6-diarylphenyl groups to the synthesis of the diphosphenes bearing the redox sites, 9 and have also synthesized sterically crowded triarylphosphines 5a, 5b, and 5c bearing the 2,6-diarylphenyl groups.…”

1,2-Dihaloalkenes in Metal-Catalyzed Reactions

“…In 2010, Sasaki and coworkers showed that 2,3-dichloro-1,4-naphthoquinone (84) was diarylated with boronate ester 85 to produce 86 (Scheme 24). 32 Synthesis of symmetrically disubstituted maleimides has been extensively studied. Selected examples are presented in Table 1.…”

Synthesis and redox properties of sterically crowded triarylphosphine and tetraaryldiphosphane bearing phenothiazinyl groups

“…However, even with the help of the stability of phenothiazinylbenzene redox cycle 7, triarylphosphines such as 3a were reversibly oxidized in two steps only at low temperature (Chart ). Tris(2,4,6‐triisopropylphenyl)phosphine ( 1b ) and the related compounds are reported to give more stable redox systems as well as radical cations 4, and we have synthesized crowded triarylphosphines structurally similar to 1b , bearing various functional sites such as donors 8, acceptors 9, and radicals 10. The electrochemical measurement of these functionalized crowded triarylphosphines showed that the phosphine moieties worked as reversible redox sites and the molecules as a whole became multistep reversible redox systems.…”