Substrate chelation

Whatever the route to a rhodium dihydride alkene complex, the hydrogen must be transferred sequentially to the double bond. It had always been assumed that the first C-H bond is formed / to the amido-group, so that the more stable Rh-substrate chelate is formed. This is the alkylhydride isomer observed in stoichiometric NMR studies at low temperatures, and is supported by studies under catalytic turnover conditions, assuming a normal isotope effect... 

As expected, the trend in the R values for PhCHO, PhOH, and PhC02H followed that for their aqueous acidities. Benzoic acid and phenol proved to be feebly acidic towards 1 while absolutely no interaction between benzaldehyde and X was observed even with lO -fold excess of this substrate. Chelation has a large impact on the reactivity of these functional groups towards 1. Thus, the RpCQ2H was 8reater than that for the parent benzoic acid. 

C2-symmetric bis(oxazolinyl)pyridine (pybox)-Cu (II) complex 27 has been shown to catalyze highly enantioselective Mukaiyaraa aldol reactions between (benzyloxy)acetaldehyde and silyl ketene acetals by Evans and co-workers as exemplified in Scheme 1-9 [38]. Here, the requirement for a chelating substituent on the aldehyde partner is critical to catalyst selectivity, as a-(terl-butyldimethylsil-oxy)acetaldehyde gave lower enantioselectivity (56% ee). In addition, P-(benzyl-oxy)propionaldehyde provided the racemic product, indicating a strict requirement for a five-membered catalyst-substrate chelate. 

Keywords Rhodium, Ruthenium, Diphosphines, Enamides, Substrate chelation, Pressm e and temperature dependence. Dihydrides, Alkyl hydrides. Transient NMR... 

If one wishes to extract stereochemical information about substrates in solution from the data obtained in the NMR experiments, two physical-mathematical models are available. For shifts induced by a dipolar (pseudocontact) mechanism, the McConnell-Robertson equation (Equation 1) (28, 29)—where r, i, and are the spherical polar coordinates of the ith resonating nucleus in the coordinate system of the principal magnetic axes—relates the direction and magnitude of the shift to the geometry of the substrate-chelate complex. If the substrate-chelate... 

Catalytic enantioselective Friedel-Crafts type addition reactions of electron-rich arenes (indoles, pyrroles etc.) with Michael acceptors have been well developed in the past years. These reactions generally are enabled by chiral Lewis acids or organocatalysts (Scheme 6.24). Chiral Lewis acid-catalyzed processes usually require bidentate substrates (chelating a,p-unsatu-rated compounds) for strong chelation activation. Monodentate compounds (simple a,p-unsaturated compounds) prove to be less selective. With chiral... 

Synonyms Hexamethylenediaminetetra (methylene phosphonic acid) HMDTMP Empiricai C10H28N2O12P4 Properties Wh. powd. m.w. 492.23 Uses Scale/corrosion inhibitor for industrial water systems aq. process additive for paper, textiles, and metals industrial and commercial formulations for cleaning various substrates chelating agent... 

As will be seen below, when coordinating atoms are present in the substrate chelation is an important factor in regioselectivity control. Linear... 

The first catalytic asymmetric version of [2 + 2] cycloaddition reaction was realized by Narasaka in 1989 using chiral titanium catalyst derived from TADDOL. It was found that the reaction of ketene dithioacetal with acryloyloxazolidinone derivatives proceeded smoothly in the presence of 10 mol% of TADDOL-TiCl2 to give the cyclobutane derivatives in high yields (64-96%) and good to excellent enantioselectivities (80-98%) [183]. The reaction is presumed to proceed via a carbonyl substrate chelated TADDOL-TiCb intermediate although the exact reaction mechanism is unclear. Moreover, alkynyl, alkenyl, and 1,2-propadienyl... 

The fact that no substrate-chelate compounds are formed with Mn++... 

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