Hydrogen donors unexpected

One frequent observation is that radicals can more frequently be reduced instead of undergoing the usually more fancy but desired transformation. The unexpected hydrogen donor can be a reagent (such as a stannane, silane, thiol, hypophosphorous acid, etc., or solvent). If the hydrogen shift is suspected to be a culprit, it could be easily unmasked through a deuterium experiment. It may then be necessary to block the unwanted... 

Methods that deduce a pharmacophore, an arrangement in 3D space of features that contribute or detract from binding and look for its presence in the database that is searched. This method places emphasis on features like hydrogen bond donors, hydrogen bond acceptors, acidic or basic units and hydrophobic fragments and opens the possibility of identifying unexpected scaffolds with required features (pharmacophore-based VS or PHBVS). 

A large number of reports have concerned transfer hydrogenation using isopropanol as donor, with imines, carbonyls-and occasionally alkenes-as substrate (Scheme 3.17). In some early studies conducted by Nolan and coworkers [36], NHC analogues of Crabtree catalysts, [Ir(cod)(py)(L)]PF,5 (L= Imes, Ipr, Icy) all proved to be active. The series of chelating iridium(III) carbene complexes shown in Scheme 3.5 (upper structure) proved to be accessible via a simple synthesis and catalytically active for hydrogen transfer from alcohols to ketones and imines. Unexpectedly, iridium was more active than the corresponding Rh complexes, but... 

A wide variety of E-H a bonds (E = boron or transition metal) act unexpectedly as efficient hydrogen bond acceptors toward conventional proton donors, such as O-H and N-H groups. The resulting X-H- - H-E systems have close H- H contacts (175-190 pm) and are termed dihydrogen bonds. ... 

The substituent effects on the a protons in butatrienes correspond to 0.71 a"(R) in Table 8 (R = H, Me, Et, MeO, MeS, Cl, CHO (from 4, 229, and 239)) that is, they are reduced compared with those in allenes. Concerning the remote hydrogen atoms (H3, H4) one observes, if ever, only a slight difference between the H chemical shifts of the Z- and -protons (228). The substituent effects on the proton shifts of these last hydrogen atoms show an unexpected behavior The tt donors Me, Et, EtO shift the resonances to higher field, whereas the 7T donors Cl and MeS yield low-field shifts. Therefore, simple resonance theory which stresses a negative v electron density at the 5 carbon atoms of butatrienes with tt donors does not seem to be appropriate to represent the electron density distributions in butatrienes with second-row substituents (Cl, MeS) (Section IIl.C). 

The clear evaluation of the experimental results is also hindered by the difficulties encountered in the perfect purification of non-aqueous solvents. Several of them are hygroscopic, and even an extremely low water content may cause fundamental changes in the chemical properties of numerous solvents. As an electron-pair donor, the water molecule may behave as a ligand, and as a consequence of the ability of its hydrogen atoms to form hydrogen bonds, it may also act as an acceptor. This may lead to the occurrence of unexpected side reactions. In acidic solvents water behaves as a base, and in basic solvents as an acid, thereby disturbing the courses of the reactions to be investigated. The removal of trace amounts of water and the performance of work under anhydrous conditions is a difficult task. 

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