Phosphate-mediated catalysis

Mechanistic Studies of the Maillard Reaction with Emphasis on Phosphate-Mediated Catalysis... 

Potman, R.P., T.A. van Wijk, Mechanistic studies of the Maillard reaction with emphasis on phosphate-mediated catalysis, in Thermal Generation of Aromas, T.H. Parliment, R.J. McGorrin, C.T. Ho, Eds., Amer. Chem. Soc., Washington, D.C., 1989,... 

Various salts such as hypervalent iodines [69] aryldiazonium salts [70] and sulfo-nium salts [71] undergo facile StUle coupling. Acetates [72], carbonates [73] and phosphates [74] are also quite reactive. Finally, heteroaromatic tioethers [75] and sulfonyl chlorides [76] were recently reported to couple under palladium-catalyzed copper-mediated catalysis. 

Catalysis by adenylyl cyclases involves cation-mediated attack of the 3 -OH on the a-phosphate of 5 -ATP, with PPj as leaving group. It is a reversible Adenylyl Cyclases. Figure 3 Membrane localization, bireactant sequential mechanism with free cation and topology, and regulation of mammalian adenylyl cyclases. cation 5 -ATP as substrates and cAMP, cation-PP , and... 

Two years later, Terada and coworkers described an asymmetric organocatalytic aza-ene-type reaction (Scheme 28) [50], BINOL phosphate (7 )-3m (0.1 mol%, R = 9-anthryl) bearing 9-anthryl substituents mediated the reaction of A-benzoylated aldimines 32 with enecarbamate 76 derived from acetophenone. Subsequent hydrolysis led to the formation of P-amino ketones 77 in good yields (53-97%) and excellent enantioselectivities (92-98% ee). A substrate/catalyst ratio of 1,000 1 has rarely been achieved in asymmetric Brpnsted acid catalysis before. 

Chiral phosphoric acids mediate the enantioselective formation of C-C, C-H, C-0, C-N, and C-P bonds. A variety of 1,2-additions and cycloadditions to imines have been reported. Furthermore, the concept of the electrophilic activation of imines by means of phosphates has been extended to other compounds, though only a few examples are known. The scope of phosphoric acid catalysis is broad, but limited to reactive substrates. In contrast, chiral A-triflyl phosphoramides are more acidic and were designed to activate less reactive substrates. Asymmetric formations of C-C, C-H, C-0, as well as C-N bonds have been established. a,P-Unsaturated carbonyl compounds undergo 1,4-additions or cycloadditions in the presence of A-triflyl phosphoramides. Moreover, isolated examples of other substrates can be electrophil-ically activated for a nucleophilic attack. Chiral dicarboxylic acids have also found utility as specific acid catalysts of selected asymmetric transformations. 

The conversion of the single starting compounds was studied under the reaction conditions, revealing that at pH-7 and higher the monosaccharide was partially consumed by alkali-mediated decomposition,also shown to be enhanced by phosphate catalysis. Therefore all further experiments were performed at pH-5.6, which is the pH obtained by mixing the starting monosaccharides and glycine. 

The degree of phosphate catalysis (DPC = ratio of reaction rate in the presence and absence of phosphate) varies with the nature of the dipeptide. The plots suggest that, within each series, DPC is negatively correlated with the relative rate in the absence of phosphate. Phosphate is postulated to act as a proton transfer mediator between the dipeptide carboxy group and the reactive imine centre (see Scheme 3.4). If direct transfer from the dipeptide carboxy group is already sufficiently efficient, the enhancement is decreased. 

The reaction between diethyl phosphite and a-chloroketones can also be accomplished by fluoride ion-mediated deprotonation reaction in DMF (dry KF or KFTHjO as fluoride ion source), which constitutes anonbasic route to diethyl 1,2-epoxyalky Iphosphonates. Unfortunately, the application of this technique provides a mixture of 1,2-epoxyalkyIphosphonate (about 50%) and vinyl phosphate (about 30%) whatever the a-chloroketones. The reaction between 3-(co-bro-moacetyOcoumarin and diethyl or di- -butyl phosphite has been performed with 50% NaOH in CgHg under phase-transfer catalysis conditions with TEBA as catalyst. The only products of the reaction are the epoxyphosphonates isolated in 70% and 34% yields, respectively. In the presence of EtjN, diethyl phosphite reacts with 2-chloroacetoacetate to give diethyl 2-(ethoxycarbonyl)-l-methyl-l,2-epoxyethylphosphonate in 72% yield as a mixture of cis and trans isomers. ... 

Elegant isotope labelling studies by Rose and coworkers (Rieder and Rose, 1959 Rose, 1962) established the general mechanistic features of the TIM reaction. Proton transfer is mediated by a single enzymatic base, and the reaction proceeds via an intermediate that is either a cis enediol phosphate (Figure 2) or one of the tvo possible symmetrical enediolates. The evidence for enzyme-mediated proton transfer is the catalysis by the enzyme of loss of radioactive label from substrate to solvent water, or of exchange-in of label from deuterated or tritiated water to... 

---