Electron-rich triarylphosphines as nucleophilic catalysts for oxa-Michael reactions

Abstract

Electron-rich triarylphosphines, namely 4-(methoxyphenyl)diphenylphosphine (MMTPP) and tris(4-trimethoxyphenyl)phosphine (TMTPP), outperform commonly used triphenylphosphine (TPP) in catalyzing oxa-Michael additions. A matrix consisting of three differently strong Michael acceptors and four alcohols of varying acidity was used to assess the activity of the three catalysts. All test reactions were performed with 1 mol % catalyst loading, under solvent-free conditions and at room temperature. The results reveal a decisive superiority of TMTPP for converting poor and intermediate Michael acceptors such as acrylamide and acrylonitrile and for converting less acidic alcohols like isopropanol. With stronger Michael acceptors and more acidic alcohols, the impact of the more electron-rich catalysts is less pronounced. The experimental activity trend was rationalized by calculating the Michael acceptor affinities of all phosphine–Michael acceptor combinations. Besides this parameter, the acidity of the alcohol has a strong impact on the reaction speed. The oxidation stability of the phosphines was also evaluated and the most electron-rich TMTPP was found to be only slightly more sensitive to oxidation than TPP. Finally, the catalysts were employed in the oxa-Michael polymerization of 2-hydroxyethyl acrylate. With TMTPP polymers characterized by number average molar masses of about 1200 g/mol at room temperature are accessible. Polymerizations carried out at 80 °C resulted in macromolecules containing a considerable share of Rauhut–Currier-type repeat units and consequently lower molar masses were obtained.

Tris(2,4,6-trimethoxyphenyl)phosphine – a Lewis base able to compete with phosphazene bases in catalysing oxa-Michael reactions

“…2). 15 This correlation between the alcohol's acidity and the obtained conversion can be rationalized by a more efficient trapping of the intermediately formed zwitterion by more acidic alcohols and thus a faster formation of alkoxide. When strong Michael acceptors are reacted, all alkoxides are nucleophilic enough to form the desired Michael adducts.…”

“…1) is more active in converting poor and intermediate Michael acceptors than triphenylphosphine (TPP). 15 Regrettably, the improvement in activity was not that pronounced that TMPP could rival the performance of phosphazene bases.…”

Tris(2,4,6-trimethoxyphenyl)phosphine - a Lewis base able to compete with phosphazene bases in catalysing oxa-Michael reactions

“…The double bond conversions reached high values of 95 % already after 1 h, which further increased to 97-98 % after 24 h. Using TTMPP or P2-tBu catalysis resulted in the same double bond conversions, which were higher than that obtained with TPP (1 h: 48 %; 24 h: 75 %) or TMPP (1 h: 80 %; 24 h: 90 %). 15 Number average molecular masses (Mn) and dispersities (D) were obtained by subjecting the reaction mixture to the SEC machine after 24 h reaction time and Mn and D values of 1280 ± 100 g/mol and 2.1 ± 0.1 were determined for the TTMPP catalysed reactions (ESI, Fig. S5).…”

Exploiting retro oxa-Michael chemistry in polymers

“…allyl alcohol was used to evaluate different catalysts for the reverse oxa-Michel reaction (Figure 1a). As for the oxa-Michael addition of alcohols Lewis or Brønsted base catalysis is required, 27,28 we expected that also in the retro-oxa-Michael reaction a catalyst would be necessary. Various catalysts (10 mol% in respect to 1) ranging from pure Brønsted bases to pure Lewis bases were screened at a temperature of 140 °C and the conversion towards 2 was monitored by 1 H-NMR spectroscopy after 1 h and 24 h. In presence of strong Brønsted bases such as KOH, potassium tert-butoxide (KO t Bu) and the phosphazene base 1tert-butyl-2,2,4,4,4-pentakis-(dimethylamino)-2λ 5 ,4λ 5 -catenadi-(phosphazen) (P2-t Bu) about 60% of the newly formed adduct 2 could be detected after 1 h (Figure 1b).…”