Intramolecular hydrogen bridg

Besides direct solid phase fixation, the functional OH-group of 49 may be used for further transformations. Esterification with acetyl chloride at the OH-linker (Scheme 27) leads to 50, providing a heteroscorpionate ligand with the protected OH-linker. An intramolecular hydrogen bridge is found in the X-ray structure [d(01-N21) = 2.478 (3) A] between the carboxylic acid and the pyrazole nitrogen (Fig. 32b). 

It is possible that the hi er-melting Z diastereomers also have an intramolecular hydrogen bridge. 

Figure 17. Intramolecular hydrogen bridges in the [p-N2H2 MS2(thiolate) 2] cores of [p-N2H2 M(S ) 2] complexes (N—H S distances in pm were retrieved from [p-N2H2 Fe(NHS4) 2]).

In many cases such as in 0-chlorophenol where relatively weak intramolecular hydrogen bridges are formed, the two bands are present together the position of the equilibrium between the free and the internally banded form can be calculated from the intensities. 

In the case of primary (and secondary60) jff-keto enamines, the enamino keto form 30a or 31a is stabilized by its push-pull conjugated mesomeric form 30b or 31b and by an intramolecular hydrogen bridge which dominates over the tautomeric imino enol form 30c or 31c (equation 3). Similar results are reported for jft-thioketone enamines61. 

Related to tautomeric problems of 1,2,4-triazoles are those concerned with hydrogen bonding. AT-Unsubstituted triazoles (1) or (2) with R or R = H may be linked by inter-molecular bridges between pyrrole NH and pyridine —N= if then R = 2-pyridyl a variety of intermolecular and intramolecular hydrogen bridges are possible. 

In this structure, two likely features, supported by experimental data, should be outlined. Firstly, the ligand C should be perpendicular to the plane Sn3-O40-Sn2. The identical intensities of the 2D HMQC crosspeaks H27-Sn2 and H27-Sn3 arising from V( Sn, H) coupling pathways are in agreement with this view. Secondly, as the hydroxylic proton resonance of ligand C at 10.90 ppm is rather sharp, in the same way as those around 14 ppm in both M and mi are, the proposed structure is likely to be stabilized by an intramolecular hydrogen bridge 029-H. . . 040. 

Substitution at C3 shifts the absorption of this carbon atom towards lower fields. The extreme value 105.9 corresponds to the CHO substituent (82CJC133). Also, absorptions at C4 are affected. Thus, depending on the absence or the presence of an intramolecular hydrogen bridge between the OH group at C4 and the group R two completely different ranges are... 

Scheme 4.15 Intramolecular hydrogen bridges with fluorine as the acceptor stabilize two different conformations of 2-fluoro- and 6-fluoronore-pinephrine (2-F-NE and 6-F-NE, respectively), inducing different kinds of biological activity [21].

More than one hydrogen bond can be formed at once. For example, the adduct between 4,6-dimethyl-pyrimidin-2-one and urea derives its stability from two, intermolecular, non-equivalent N-H- -0(=C) bridges [26, 27], and the physical properties of usnic acid are strongly affected by its three intramolecular hydrogen bridges located in three different molecular areas and having different strengths [28, 29]. 

The -NMR spectra indicate that these potentially tautomeric compounds exist exclusively in the enamine form 67a (Scheme 15). The amine protons are found as broad resonance signals in the region 5 5.25-6.85 and are similar to conjugated primary amines. From these values, an intramolecular hydrogen bridge (cf. 67c) to the neighboring ester carbonyl can be excluded all data are given in references 12, 13, and 58. 

As has been shown by Ernst (71, 72) for naphthalene derivatives, para SCS can amount up to +10 ppm, and can be correlated linearly with total charge density changes calculated by INDO MO methods. This also seems to hold for 6- and 7-substituted coumarins, because fairly good correlations exist between respective para effects in both molecular systems. Exceptions only occur for 3-carboxycoumarin and 3-hydroxycoumarin, which are probably due to intramolecular hydrogen bridging. Analogous observations are noted in connection with ortho effects. 

Thence it can be concluded that such decarboxylations can occur in either of two ways neither of these two routes involves the enol form of the acid or its salt since dimethylacetoacetic acid behaves in the same way. The two terms in the rate equation thus correspond simply to the monomolecular decomposition of the oxo acid or its anion, respectively. Here too decomposition of the oxo acid27 must be assumed to proceed by way of a cyclic intermediate containing an intramolecular hydrogen bridge, as formulated in the introduction (page 1004) for a type 2 reaction. 

Compounds with an intramolecular hydrogen bridge (IMHB), such as 2-(2-hydroxyaryl)-l,3,5-triazines, are used widely for the photostabilization of polymers. They absorb harmful UV radiation and transform it, via a very efficient ESIPT within the IMHB, into vibrational energy. 

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