Hydrogen bonds in “push-pull” enamines

Abstract

A comparison of hydrogen bond strengths in various enamines was made by monitoring the differential shifts Dd(X) as the difference of NMR chemical shifts between E and Z forms of the nuclei directly involved in hydrogen bonding, i.e., X = 15 N, 1 H, 17 O atoms. The interpretation of these differential values was aided by ab initio calculations and X-ray derived geometric parameters for selected compounds. It is shown that Dd(N 1 H) and Dd( 15 N) give conclusive results and their changes are rationally interpreted by invoking established contributing effects that influence the values of chemical shifts. The Dd( 17 O) parameter is sensitive to intramolecular geometry, mainly bond angles around oxygen and co-planarity of the atoms forming the hydrogen bonds. The latter factor is important in determining the strength of hydrogen bonds. Even a weak acceptor such as the lactone function gives a relatively strong hydrogen bond as compared with sp 3 -hybridized sulfonyl or sulfinyl acceptors. The hybridization of the penultimate atom of an acceptor plays a crucial role in determining the strength of hydrogen bonding. P A P E R NJC www.rsc.org/njc 1562 N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 5 6 2 -1 5 6 7

Effects of methyl substitution in 4-silathiane S-oxides on the stereochemistry and1JCHcoupling constants: Buttressing effect of axial sulfinyl group as the origin of the reverse Perlin effect

“…We believe that this is a manifestation of the aforementioned buttressing effect of the S=O group on the C-3-H-3ax and C-5-H-5ax bonds and, circuitously, on the Si-C bonds. The buttressing effect, by analogy with that of the sulfinyl and sulfonyl groups in enamines [25] should result in shielding of the C-3 and C-5 carbons, i.e., in the γ -effect, and this is what is observed in the spectra (table 1). …”

A 19F NMR study of C–I⋯π halogen bonding

“…To broaden the range of methods used in systematic studies, we tested if some of the techniques previously used to study hydrogen bonding can also be applied to examine halogen bonding. One of these techniques is NMR spectroscopy since it allows the detection of small changes in chemical shifts upon hydrogen bond formation [30][31][32]. 19 F NMR spectroscopy is of particular interest since halogen bonding requires a positively charged region on the halogen atom, the so-called r-hole [21], which is easily achieved by perfluorinated side chains on the halogen atom.…”

One-Pot Synthesis of Highly Substituted Nicotinic Acid Derivatives Based on a Formylation Strategy of Enamino Keto Esters

“…The trifluoromethyl derivative 1c was prepared by potassium tert -butoxide-assisted addition of ethyl acetoacetate to trifluoroacetonitrile, generated in situ from 2,2,2-trifluoroacetamide . It may be noted that the enamino keto esters 1a – k are capable of existing as two geometrical isomers . Careful examination of proton NMR spectra of these enamino keto esters revealed that they exist as E -isomers, as the two amino protons showed separate signals approximately at 5.5 and 11 ppm.…”

“…15 It may be noted that the enamino keto esters 1a−k are capable of existing as two geometrical isomers. 16 Careful examination of proton NMR spectra of these enamino keto esters revealed that they exist as E-isomers, as the two amino protons showed separate signals approximately at 5.5 and 11 ppm. In the E-geometry, one of the amino protons forms an intramolecular hydrogen bond (N− H•••O bond) with the acyl oxygen and thus appears downfield at around 11 ppm.…”

“…The desired enamino keto esters 1l – r bearing homologous keto functionality on the α-carbon were prepared from benzonitrile and appropriate β-keto esters as described in the case of 1a . Enamino keto esters 1l – r also existed as the E -isomers . The treatment of 3-oxopentanoate ester 1l with Vilsmeier reagent under optimized reaction conditions resulted in the formation of two distinct products (TLC).…”