Protonation imines and

However, when these (H)(p-H)Os3(CO),o(imine) derivatives are dissolved in polar solvents such as methanol or acetone, both syn- and anti-isomers are present. [45] Thus it is clear that the overall stereochemistry of the resulting complexes is dependent upon the competition between intra- and intermolecular hydrogen-bonding interactions. It is worth to note that H relaxation measurements show that in the syn-isomers the distance between the imine proton and the terminal hydride is significantly longer when the compounds are dissolved in the polar solvents rather than when they are dissolved in non-polar solvents (i.e. ca 4.0 A against 2.1 A). This finding has been accounted for on the basis of differences in the orientation of the imine... 

The linear terdentate hybrid ligands (74) with the donor set N—N—As, also yield compounds of the types [MnXjCN—N—As)] (X = Cl, Br or I), [Mn(N—N—As)2](C104)2 and [Mn(NCS)2-(N—N—As)], in which all three donor atoms appear to be bonded. Similarly, all four donors of the quadridentate N—N—N—As ligand (75) appear to be coordinated in the compound [Mn-(N—N—N—As)], as in which both an imine proton and an arsenic methyl group are lost upon reaction of the ligand with Mn2(CO)io. ... 

Three-bond couplings, /HCd = 44.4Hz, have been observed by Sale-hzadeh et al between the imine proton and 111/113-cadmium nuclei in the spectrum of the Cd(II) complex of a new hexadendate base ligand derived from an asymmetric tripodal tetraamine and 2-pyridinecarboxaldehyde. 

Vicinal VnAg coupling of 8.57 Hz between the imine proton and the silver atom has been reported by Kay et for the four-coordinate Ag complex of the ligand tris[4-(2-thienyl)-3-aza-3-butenyl]amine (TTME), [Ag(TTME)](PF6) H2O. It has been determined from the selectively decoupled INEPT spectrum of this compound. 

The low field shift of the imine proton and strong metal carbonyl bands at 1907 and 1792 cm-i are consistent with the proposed structure. Dissolution of the solid 11 in THF results in quantitative spectroscopic conversion to 4. Dynamic behavior is observed in the NMR spectra of 11 with the most noticeable change being the inequivalence of the two rt/i -fluorines in the slow exchange limit spectrum recorded at -83 °C. Coalescence was observed at -30 °C and the expected three resonance pattern was obtained at 0 °C. 

Under acidic conditions, imine 12 is protonated to give the iminium ion 13 which undergoes an electrophilic aromatic substitution reaction to form the new carbon-carbon bond. Rapid loss of a proton and concomitant re-aromatization gives the tetrahydroisoquinoline 14. 

Formation of C—Nu The second mode of nucleophilic addition, which often occurs with amine nucleophiles, involves elimination of oxygen and formation of a C=Nu bond. For example, aldehydes and ketones react with primary amines, RNH2, to form imines, R2C=NR. These reactions proceed through exactly the same kind of tetrahedral intermediate as that formed during hydride reduction and Grignard reaction, but the initially formed alkoxide ion is not isolated. Instead, it is protonated and then loses water to form an imine, as shown in Figure 3. 

If the formation and breakdown steps of a mechanism involving a tetrahedral intermediate respond differently to changes in pH or catalyst concentration, then one can find evidence from plots of rate versus pH or rate versus catalyst concentration for a change in rate determining step and thus for a multistep mechanism. An example would be the maximum seen in the pH rate profile for the formation of an imine from a weakly basic amine (such as hydroxylamine). On the alkaline side of the maximum, the rate determining step is the acid-catalyzed dehydration of the preformed carbinolamine on the acid side of the maximum, the rate determining step is the uncatalyzed addition of the amine to form the carbinolamine. The rate decreases on the acid side of the maximum because more and more of the amine is protonated and unable to react. 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

Because an FI catalyst has a pair of non-symmetric phenoxy-imine ligands, it potentially possesses five isomers stemming from the coordination modes of ligands. Zr-, Ti-, and Hf-FI catalysts 1-3 display three sets of signals in XH NMR, attributed to the imine proton, suggesting that these FI catalysts exist as isomeric mixtures in solution, which is probably an intrinsic feature of FI catalysts. On the basis of the symmetry of the possible isomers A-E (Fig. 10), as well as the relative formation... 

Chiral amines can also be produced using aminotransferases, either by kinetic resolution of the racemic amine or by asymmetric synthesis from the corresponding prochiral ketone. The reaction involves the transfer of an amino group, a proton and two electrons from a primary amine to a ketone, and proceeds via an intermediate imine adduct. A variety of chiral amines can be obtained with high to very high ee-values. Several transformations have been developed and can be carried out on a 100-kg scale [94]. 

A number of pendant arm ligand derivatives based on hexamine macrocyclic backbones have been reported." One such species was formed by the [2 -f 2] Schiff base condensation between 3,3-iminobis(propylamine) and diformyl-p-cresol followed by sodium borohydride reduction of the four imine functions so generated. Potentiometric studies indicate that a range of both mononuclear and dinuclear species are formed in solution with manganese(II) incorporating various both protonated and nonprotonated forms of the ligand. 

Mechanistically the reaction is proposed to proceed via a nine-membered transition state with the chiral phosphoric acid simultaneously activating the imine by protonation and the phosphite by coordinating to the hydroxyl group (Fig. 9). 

Until 2006, a severe limitation in the field of chiral Brpnsted acid catalysis was the restriction to reactive substrates. The acidity of BINOL-derived chiral phosphoric acids is appropriate to activate various imine compounds through protonation and a broad range of efficient and highly enantioselective, phosphoric acid-catalyzed transformations involving imines have been developed. However, the activation of simple carbonyl compounds by means of Brpnsted acid catalysis proved to be rather challenging since the acid ity of the known BINOL-derived phosphoric acids is mostly insufficient. Carbonyl compounds and other less reactive substrates often require a stronger Brpnsted acid catalyst. 

The A-oxoammonium ion is theoretically expected to react with an amine, eliminating a proton and forming the hydroxylamine while the amine is converted to imines and/or nitriles (Scheme 4). These were indeed observed by Semmelhack and Schmid (equation 28)". ... 

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