Bronsted acid phosphine

Ionic liquids with Bronsted acidity have also been prepared from neutral liquids, such as [BMIM]BF4 and [BMIMJPFg, by dissolving well-known Bronsted acids, -toluenesulfonic acid (TsOH), pyridinium -toluenesulfonate (PPTS), and triphenyl phosphine hydrobromide (TPP.HBr) (101). The strong acidities of the catalysts are maintained in the liquids. 

Some complexes of nickel(0) with phosphines have been found to react with several Bronsted acids in non-aqueous solvents and under an inert atmosphere to give hydrido complexes of nickel(I) and nickel(II) (equations 85-87).264,268,269... 

Fig. 45. Effect of calcination on the 31P MAS NMR spectrum at 80.96 MHz of P(CH3)3 adsorbed on zeolite H-Y (202). Samples were degassed at 80°C for 1 hr prior to measurements. Samples calcined at (a) 400°C (b) 500°C (c) 600°C (d) 700°C. The resonance at ca. —3 ppm is assigned to [(CH3)3PH] + complexes formed on the Bronsted acid sites resonances in the region of ca. - 32 to - 58 ppm in samples calcined at 500°C correspond to the phosphine on the Lewis acid sites and the signal at —58 ppm in samples calcined at high temperatures are due to the phosphine on A1203 clusters in the zeolitic cavities. Chemical shifts are in ppm from 85% aqueous H3P04.

Because PH3 is the very weak parent acid of the strong Bronsted base P3, we can form phosphine by protonating phosphide ions with a Bronsted acid. Even water is a sufficiently strong proton donor ... 

In contrast to chiral amines, phosphorus-based chiral catalysts were less developed for asymmetric MBH transformations. As with amine-based reactions, the selectivity of the addition in phosphine-mediated reactions depends clearly on the nature of the complementary Bronsted acid co-catalysts used. 

Scheme 5.19 The achiral phosphine and chiral binaphthol-derived Bronsted acid-catalyzed MBH reaction of cyclohexenone and 3-phenyl propionaldehyde.

As discussed previously for the MBH reaction, the aza-MBH reaction involves rate-limiting proton transfer in the absence of added protic species (Scheme 5.22) [93]. In contrast to the MBH reaction, however, the aza-MBH exhibits no autocatalysis. Bronsted acidic additives lead to substantial rate enhancements through acceleration of the elimination step. It has been shown that phosphine catalysts - either alone or in combination with protic additives - may trigger epimerization of the aza-MBH product by proton exchange at the stereogenic center. This fact indicates that the spatial arrangement of a bifunctional chiral catalyst in this reaction is crucial not only for the stereodifferentiation within the catalytic cycle but also to prevent subsequent epimerization. 

Asymmetric organocatalytic Morita-Baylis-Hillman reactions offer synthetically viable alternatives to metal-complex-mediated reactions. The reaction is best mediated with a combination of nucleophilic tertiary amine/phosphine catalysts, and mild Bronsted acid co-catalysts usually, bifunctional chiral catalysts having both nucleophilic Lewis base and Bronsted acid site were seen to be the most efficient. Although many important factors governing the reactions were identified, our present understanding of the basic factors, and the control of reactivity and selectivity remains incomplete. Whilst substrate dependency is still considered to be an important issue, an increasing number of transformations are reaching the standards of current asymmetric reactions. 

Typical Procedure for the Triethyl Phosphine and Chiral Binaphthol-Derived Bronsted Acid Co-Catalyzed Asymmetric MBH Reaction [43] (pp. 173 and 232)... 

The catalysts were cationic palladium-phosphine systems prepared from palladium acetate, an excess of triphenylphosphine (PPhs) and a Bronsted acid of a weakly or noncoordinating anion (e. g., p-tosylate (OTs ) methanol was used as both the solvent and a reactant. An unexpected change in selectivity was observed upon replacement of the excess of PPhs by a stoichiometric amount of the bidentate l,3-bis(diphenylphosphino)propane (dppp). Under the same conditions. 

Other suitable molecules include trialkyl phosphines and trialkyl phosphine oxides 134-36]. Phosphines bound to Bronsted acid and Lewis acid sites and physisorbed molecules yield distinguishable resonances, although the latter two are poorly resolved from each other. There is no indication of exchange broadening in these cases. 

Other useful classes of basic probe molecules used to examine silica, alumina, and silica-alumina surfaces (as well as zeolite systems) include small organic phosphines and phosphine oxides, which rely on the highly convenient P nuclide (/ = 1/2, 100% natural abundance). As Lunsford and coworkers demonstrated for zeolites [89], the P NMR signal of trimethylphosphine is a useful probe for Bronsted acid sites on surfaces. The basis for this approach is the formation of R3P -H B( ) sites at surface Bronsted acid sites, H-B(. ... 

At first sight, this may seem like a very simple question with a simple answer, yes. The problem is, of course, that many organic and inorganic ions are in chemical equilibrium with neutral species. Depending on the position of this equilibrium -and the latter is a function of temperature and pressure - even a pure ionic liquid may contain significant amounts of neutral molecules. Of course, this will greatly infiuence all properties of the substance. Volatility, viscosity, chemical reactivity etc. will greatly differ from the hypothetical mixture of the individual ions if free molecular species such as amines, phosphines, Bronsted-acids or acid esters form as neutral molecules in an equilibrium reaction under the conditions of the ionic liquid application. 

In addition to tertiary amines, triphenylphosphine is also an effective promoter and the use of enantiomerically pure amines or phosphines to catalyse the reaction is an interesting prospect, since the products would be synthetically useful. In addition there is also the potential for Lewis acid/Bronsted acid-catalysed asymmetric MBH reactions. While early attempts at the development of a catalytic asymmetric variant were only moderately successful, providing products with up to 50% ee, some recent progress has been made in this area and high ees have been obtained in both the MBH and aza-Baylis-Hillman reaction of a,p-carbonyls with imines. 

A number of BINOL-based bifunctional organocatalysts, for example (7.171-7.173), containing both Bronsted acidic and Lewis basic sites have been used to good effect in the asymmetric MBH reaction. The amine-thiourea (7.171) promotes the MBH reaction of aliphatic aldehydes with 2-cyclohexenone with ees ranging from 80 to 94% while both the (pyridinylaminomethyl)BINOL (7.172) and phosphine (7.173) catalyse the aza-Bayhs-Hilhnan reaction of simple a,p-carbonyls such as MVK and phenyl acrylate with N-tosyl arylaldmines with similar levels of enantioselectivity. 

The hydridic character of the rhenium-bonded hydrogen of 34a, b facilitated the hydride abstraction upon treatment with either Bronsted acids or Lewis acids [32]. Depending on the nature of the phosphine substituent, the 16e Re(—I) dinitrosyl cations [Re(NO)2(PR3)2][BAr 4] (35, R = iPr a, Cy b) could be accessed from the reaction of either 34a with [H(OEt2)2][BAr 4] or 34b with [PhaC] [BAi 4]. Further anion modification to access the corresponding [B(C6F5)4] salts [Re(NO)2(PR3)2l[B(C6F5)4] (36, R = Pr a, Cy b) showed enhanced solubilities in nonpolar solvents [105]. 

This concept was applied to the intramolecular alkylation of imidazole and benzimidazole derivatives (Scheme 19.86) [121]. Preliminary results showed that while the Wilkinson catalyst was effective for this transformation, standard ruthenium catalysts were unreactive. Further screening using [RhCl(coe)2]2 as the rhodium source with various phosphine ligands revealed that electron-rich ligands such as PCyj yielded the best results. As the reaction was found to be more efficient in the presence of the Lewis or Bronsted acid, an optimized protocol was devised through the use of [HPCyjKQ] imder microwave irradiation [122]. Mechanistic studies showed that the reaction is zero order in substrate and first order in catalyst. 

It is common to use Bronsted acids as phosphine scavengers in olefin metathesis phosphine bearing catalysts. On the other hand, it is obvious that HCl plays a different role in catalyst 4d, since this complex does not carry a phosphine ligand. To understand more about the activation mechanism, an NMR study was undertaken. 

Ni-Cp interactions of the ligand trans-in [ucnQc exerted by the other auxiliary ligands has been discussed and the following approximate /r< //i -influence order has been suggested alkyl, hydride > phosphine, methoxide > arylamide, aryloxide, arylthiolate, bromide. A study of the Ni-amido derivatives with Bronsted acids HX has led to the observation that the Ni-X bonds have a significant electrostatic component. 

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