Sp2 Nitrogen Atom Research Articles

Intramolecular "hydroiminiumation and -amidiniumation" of alkenes: a convenient, flexible, and scalable route to cyclic iminium and imidazolinium salts.

Addition of a stoichiometric amount of HCl to alkenylaldimines, -formamidines, and -amidines results in the protonation of the sp2-nitrogen atom. The resulting alkenylaldiminium, -formamidinium, and -amidinium salts can be isolated and fully characterized, including single-crystal X-ray diffraction studies. Heating solutions of these salts induces ring closure cleanly and regioselectively via formal "exo" addition of the nitrogen-hydrogen bond to the pendent carbon-carbon double bond, affording the corresponding cyclic aldiminium, dihydroisoquinolinium, and imidazolinium salts. Of special interest, novel 4,4-disubstituted imidazolinium salts are accessible via this synthetic route. Similarly, addition of phosgene to alkenyl ureas and alkenyl amides, followed by gentle heating, cleanly affords C-chloro imidazolinium, and cyclic C-chloro iminium salts, respectively. Treatment of the latter with tetrakis(triphenylphosphine)palladium allows for the preparation of the first transition-metal complex bearing a cyclic arylaminocarbene as ligand. Deuterium labeling experiments suggest that the mechanism of the hydroiminiumation and -amidiniumation reactions involves an intramolecular proton transfer to the double bond in the rate-determining step. This novel synthetic methodology gives access to a variety of N-heterocyclic carbene (NHC) and cyclic alkyl- and arylaminocarbene (CAAC) precursors.

First example of antimoniated Schiff bases: Hypervalent Sb ← N(sp2) bonds

The first example of antimoniated Schiff bases viz. 1-[2-(bis-{2-[(1-R-p-tolylethylimino)methyl]phenyl}stibanyl)benz-E-ilidene]-(1-R-p-tolylethyl)amine (1) and tris[(R)-2-benzyliden-2-yl-amino)butan-1-ol]stibine (2) containing imino groups at the ortho position of aryl ring have been synthesized and characterized. The compounds were obtained by the reaction of tris(o-formylphenyl)stibine with R-4-dimethyl benzyl amine or (R)-2-aminobutan-1-ol, respectively. These chiral antimony based Schiff bases were characterized by IR, mass, 1H, 13C, NMR spectroscopy. The X-ray crystal structures of these two Schiff bases show hypervalent interactions between antimony and sp2 nitrogen atoms. Additionally compound 2 shows intramolecular Sb–O interaction also giving eight-coordination of antimony, both of these observations are uncommon in literature. These two compounds 1 and 2 show helicoidal chirality, which is a very new concept in antimony chemistry.

The replacement of five-membered N-donor heterocycles from aminotrichlorogold(III) complexes. A comparison with pyridines

The kinetics of the process AuCl3(nu) + Cl−→ AuCl 4 − + nu (nu = one of a number of five-membered N-donor heterocycles covering a wide range of basicity, namely: oxazole, 2,4,5-trimethyloxazole, thiazole, 5-methylthiazole, 4-methylthiazole, 4,5-dimethylthiazole, 2,4-dimethylthiazole, 2,4,5-trimethylthiazole, imidazole and 2-methylimidazole) have been studied in methanol at 25 °C. The reactions follow the usual two-term rate law, rate = (k 1 + k 2[Cl−])[complex], observed in a square-planar substitution associative-mechanism. The second-order rate constants, k 2, indicate that the discriminating ability of Au(III) in these complexes is good and markedly influenced by the nature of the leaving group. A linear-free-energy relationship, logk 2 = 0.53pK a + constant, is observed between the rate constant and the basicity of the leaving group for its replacement by chloride. The results are compared with data from the literature regarding a series of complexes of the type AuCl3(py) (py = one of a number of pyridines) reacting with the Cl− anion under the same experimental conditions. The reactivity depends not only upon ligand basicity but also upon the nature of the ligand in the order: pyridines> five-membered heterocycles. Steric factor due to the presence of a single methyl group ortho to the sp2 nitrogen atom in the ring has no influence on the rate of substitution while, surprisingly, when there are two ortho methyl groups a remarkable steric retardation effect is observable. The results are discussed in terms of reaction-profile in the associative-substitution reaction and bonding interactions in the ground and transition states.

On the structural regularity in nucleobases and amino acids and relationship to the origin and evolution of the genetic code.

To explore how chemical structures of both nucleobases and amino acids may have played a role in shaping the genetic code, numbers of sp2 hybrid nitrogen atoms in nucleobases were taken as a determinative measure for empirical stereo-electronic property to analyze the genetic code. Results revealed that amino acid hydropathy correlates strongly with the sp2 nitrogen atom numbers in nucleobases rather than with the overall electronic property such as redox potentials of the bases, reflecting that stereo-electronic property of bases may play a role. In the rearranged code, five simple but stereo-structurally distinctive amino acids (Gly, Pro, Val, Thr and Ala) and their codon quartets form a crossed intersection "core". Secondly, a re-categorization of the amino acids according to their beta-carbon stereochemistry, verified by charge density (at beta-carbon) calculation, results in five groups of stereo-structurally distinctive amino acids, the group leaders of which are Gly, Pro, Val, Thr and Ala, remarkably overlapping the above "core". These two lines of independent observations provide empirical arguments for a contention that a seemingly "frozen" "core" could have formed at a certain evolutionary stage. The possible existence of this codon "core" is in conformity with a previous evolutionary model whereby stereochemical interactions may have shaped the code. Moreover, the genetic code listed in UCGA succession together with this codon "core" has recently facilitated an identification of the unprecedented icosikaioctagon symmetry and bi-pyramidal nature of the genetic code.

Ru3(CO)12‐Catalyzed Silylation of Benzylic C—H Bonds in Arylpyridines and Arylpyrazoles with Hydrosilanes via C—H Bond Cleavage.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ru3(CO)12-catalyzed silylation of benzylic C-H bonds in arylpyridines and arylpyrazoles with hydrosilanes via C-H bond cleavage.

Ruthenium-catalyzed silylation of sp3 C-H bonds at a benzylic position with hydrosilanes gave benzylsilanes. For this silylation reaction, Ru3(CO)12 complex showed high catalytic activity. This silylation proceeded at the methyl C-H bond selectively. For this silylation reaction, pyridyl and pyrazolyl groups, and the imino group in hydrazones, can function as a directing group. Several hydrosilanes involving triethyl-, dimethylphenyl-, tert-butyldimethyl-, and triphenylsilanes can be used as a silylating reagent. Coordination of an sp2 nitrogen atom to the ruthenium complex is important for achieving this silylation reaction.

Position-Addressable Nano-Scaffolds. I. The Preparation of N,O-, N,C- and N,N-Bridged Sesquinorbornadiene Succinimides as Compact, Highly Functionalized Addressable Building Blocks

Succinimide-functionalized N,O-, N,C-, and N,N-bridged benzo[2]polynorbornanes (benzosesquinorbornadienes) are reported for the first time and are prepared by the cycloaddition of either five-membered cyclic 1,3-dienes onto N-substituted benzo-7-azanorbornadienomaleimides or N-substituted isoindoles onto the appropriate norbornadienomaleimides. These products contain an end-fused norbornene (or a 7-substituted relative) and have been designed to act as alkene BLOCKs for the production of nano-scaffolds with up to six addressable sites supplied by each norbornene unit. The N-bridges in these BLOCKs (and their coupled products) allow N-substituent mobility by invertomerization of the sp3 nitrogen whereas the N-substituents on the succinimides are attached to an sp2 nitrogen atom and remain static. Preferred invertomer geometry can be determined using molecular modelling.

Syntheses of optically active 2‐(2‐benzothiazolylimino)‐heterazolidines

AbstractThe syntheses of compounds 4R, 5R‐2‐[2‐benzothiazolylimino]‐3‐hydro‐4‐methyl‐5‐phenylthiazolidine (5a), 4S,5S‐2‐[2‐benzothiazolylimino]‐3,4‐dimethyl‐5‐phenylthiazolidine (5b), 4R,5S‐[2‐benzothiazolylimino]‐3‐hydro‐4‐methyl‐5‐phenyloxazolidine (5c), 4R,5S‐2‐[2‐benzothiazolyl‐imino]‐3,4‐dimethyl‐5‐phenyloxazolidine (5d), and 4R,5S‐2‐[2‐benzothiazolylimino]‐1,3,4‐trimethyl‐5‐phenylimidazolidine (5e) are reported. The stereochemistry of the reaction products and the X‐ray diffraction analyses of compounds 5a–d are discussed. Compounds 5a–d present planar structures. We have found short distances between the thiazolidine S1 atom and the benzothiazole N9 atom for 5a and 5b (2.782 and 2.824 Å, respectively) and the distance between the oxazolidine O1 and the benzothiazole S7 for 5d (2.721 Å). These distances are shorter than the sum of the van der Waals radii. These distances, together with the coplanarity of the heterocycles, seem to indicate the existence of a weak coordination bonding from sp2 oxygen or nitrogen atoms towards the sulfur atom. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:586–593, 2001

Regioselective Synthesis of 8-Hydroxytetrahydroquinolines by the Cyclization of m-Hydroxyphenethyl Ketone O-2,4-Dinitrophenyloximes

Abstract 8-Hydroxy-1,2,3,4-tetrahydroquinolines are synthesized from m-hydroxyphenethyl ketone O-2,4-dinitrophenyloximes by the cyclization on the sp2 nitrogen atom of the oxime derivatives using sodium hydride and sodium cyanoborohydride.

SN2 Substitution on sp2 Nitrogen of Protonated Oxime

Abstract Theoretical studies on intramolecular nucleophilic displacement of the protonated oxime oxygen with an aryl ring revealed that the substitution reaction on the sp2 nitrogen atom is a low energy process. This seemingly anomalous substitution is due to the low-lying σ*-orbital of the O-protonated oxime. The theoretical conclusion received experimental support.

15N NMR studies of guanidines. I—hydrogen bonding and electronic effects in conformationally flexible guanidines

AbstractHydrogen bonding and electron distribution in some heterocyclic guanidines were studied by 15N NMR spectroscopy, using other substituted guanidines as points of reference. Solvent effects were used to distinguish the 15N NMR signals for their two sp2 nitrogen atoms, since ring nitrogen forms an intramolecular hydrogen bond and so is much less sensitive to solvent than is the guanidine unit. The effect of substituent is discussed in terms of competition between through‐resonance and the more localized resonance of the π‐inductive effect. These are favoured, respectively, by oxygen (nitro, acyl) and nitrogen (cyano) acceptor groups, whereas heterocycles, also nitrogen acceptors, occupy an intermediate position. Among the latter class, there are indications that the balance between these forms of resonance may be influenced by competitive conjugation with ring π‐donor heteroatoms.

Amino/Aromatic Interactions in Proteins: Is the Evidence Stacked Against Hydrogen Bonding?

We investigate the suggestion that aromatic rings can act as hydrogen-bond acceptors in proteins, by an analysis of 55 non-homologous high-resolution protein chain structures. Approximately 10% of interactions between sp2 hybridized nitrogen atoms, from either side-chains or main-chains, and phenylalanine or tyrosine rings have the nitrogen atom positioned above the ring. In these instances, however, the sp2 nitrogen atoms tend to form stacked interactions with the aromatic rings, these geometries outnumbering amino/aromatic hydrogen bonds by around 2·5:1. The statistically expected distribution, in contrast, would have only a few stacked structures and many more with larger interplanar angles, corresponding to amino/aromatic hydrogen bonds. Thus, although we do find some of these unconventional hydrogen bonds, they are clearly disfavoured relative to stacked geometries.In stacked geometries, the nitrogen-bearing groups are observed to fulfil their hydrogen-bonding potential by forming conventional, energetically stronger, hydrogen bonds with other groups in protein or solvent. This may explain the favourability of stacking. Thus, although ab initio calculations of the gas phase interaction energies for three model systems generally favour the amino/aromatic hydrogen-bonded over the stacked geometries, the differences are small enough to be outweighed easily by the additional conventional hydrogen bonding in stacked structures.

Synthesis and x-ray structure of N[(CH2)2O(2,6-C6H3N)O(CH2)2]3N: a D3 macrobicyclic ligand capped by two sp2 nitrogen atoms

Synthesis and x-ray structure of N[(CH2)2O(2,6-C6H3N)O(CH2)2]3N: a D3 macrobicyclic ligand capped by two sp2 nitrogen atoms

ChemInform Abstract: 2,5‐DIHYDRO‐1,3,2‐OXAZAPHOSPH(V)OLES. PREPARATION AND PROPERTIES

AbstractDie Azidooxirane (I) liefern mit den Phosphinen (II) in Pentan oder Ether bei 0 bis 25°C die Oxazaphosphole (III).

2,5-Dihydro-1,3,2-oxazaphosph(V)oles. Preparation and properties

2,5-Dihydro-1,3,2-oxazaphosph(V)oles, prepared by reaction of 2-azido-oxirans with trico-ordinated phosphorus compounds, exhibit a trigonal bipyramidal structure with the sp2 nitrogen atom apical and the oxygen atom equatorial.

Sulphone structures. Part II. Crystal and molecular structure of N-p-tolylsulphonylbenzylamine, 1-methyl-3-p-tolylsulphonylaminoindole, 1-methyl-3-p-tolylsulphonyliminoindoline-2-spirocyclopentane, and 1,2,3,4-tetrahydro-1,2,4-trimethyl-4-p-tolylsulphonylamino-3-p-tolylsulphonyliminoquinoline

The crystal and molecular structures of the four title compounds have been determined by heavy-atom methods. (I): N-p-Tolylsulphonylbenzylamine, triclinic, a= 9·686, b= 9·156, c= 9·749 A(all ±0·003 A), α= 68·1, β= 64·5, γ= 60·5°(all ±0·1°), space group P, Z= 2. Four-circle diffractometer data, R 0·066 for 2515 independent reflections. (II): 1-Methyl-3-p-tolylsulphonylaminoindole, monoclinic, a= 20·69, b= 17·39, c= 9·53 A(all ±0·01 A), β= 118·25°± 0·03°, space group C2/c, Z= 8, linear diffractometer data, R 0·065 for 1948 independent reflections. (III): 1-Methyl-3-p-tolylsulphonyliminoindoline-2-spirocyclopentane, monclinic, a= 16·08, b= 11·42, c= 20·51 A(all ±0·01A), β= 105·0°± 0·1°, space group C2/c, Z= 8. Linear diffractometer data, R 0·055 for 1448 independent reflections. (IV): 1,2,3,4-Tetrahydro-1,2,4-trimethyl-4-p-tolylsulphonylamino-3-p-tolylsulphonylaminoquinoline, triclinic, a= 7·67 ± 0·01, b= 13·71 ± 0·02, c= 13·57 ± 0·02 A, α= 110·4 ± 0·1, β= 109·2 ± 0·1, γ= 92·8 ± 0·2°, space group P, Z= 2. Visually estimated data, R 0·103 for 1481 independent reflections. In the four compounds the bond lengths and interbond angles at any sulphur atom are unaffected by the substituents at the nitrogen atom bonded to the sulphur atom. The bond lengths, but not the interbond angles, are the same for an sp2 and an sp3 nitrogen atom and the angle between the planes through the O–S–O sulphonyl atoms and through the S–N–C atoms is restricted to the range 53–65 for an sp3 and 83–88° for an sp2 nitrogen atom.