Tetrazole
Tetrazole contains three pyridine- and one pyrrole-like N-atom. It is aromatic because it con- tains six delocalized -electrons. Tetrazole has the highest ionization potential of all azoles, i.e. 11.3 eV. The tetrazole ring resists even strong oxidizing agents because of its low HOMO. The dipole moment in the gas phase is 2.19 D and in dioxane 5.15 D. From this great difference, it can be concluded that the 2 -form predominates in the gas phase and the l -form in solution. Its associated...
Xt Xt
2,6-Lutidine does not undergo nuclear substitution with S03, but yields 20 by N-addition. However, 2,6-di-tert-butylpyridine is converted into the 3-sulfonic acid 21 under mild conditions S03, liq. S02, -10 C with oleum at higher temperature the sulfone 22 is obtained as well as 21. The ready formation of 21 indicates steric hindrance towards attack of the N-atom by the bulky tert-butyl groups accordingly, only nuclear substitution to the alkyl-activated base occurs. Halogenation of pyridine...
D Hxb
Oxazole is a colourless liquid with a smell similar to pyridine, of bp 69-70 C, and is soluble in water. l,3-Oxazolium-5-olates 16 are known by the trivial name of Miinchnones. Their reactions have been extensively studied by HuiSGEN in Munich. They are prepared by cyclodehydration of -substituted N-acyl a-amino acids with acetic anhydride Mesomeric zwitterions like Miinchnones are known as mesoionic compounds, which also occur in other heterocyclic systems 70 . Miinchnones are crystalline...
Ad
3 -Diazirines are structural isomers of diazoalkanes. They are gases or colourless liquids, e.g. 3,3-dimethyldiazirine, bp 21 C. Liquid 3 -diazirines can decompose explosively. Their basicity is very low. Unlike diazoalkanes, they react with acids only slowly, with the liberation of nitrogen. The dediazoniation of 3 -diazirines can be effected thermally or photochemically 22 . In the absence of carbene acceptors, the initially formed carbenes isomerize to give olefins, e.g. 3 -Diazirines are...
Quinoline
The topology of pyridine allows three benzene-annulated products, namely quinolizinium ion 1 benzo a pyridinium ion , quinoline 2 benzo b pyridine and isoquinoline 3 benzo c -pyridine The neutral systems 2 and 3 will be discussed first. A Quinoline is derived from naphthalene by replacement of one of its a-CH groups by nitrogen. 2-and 3-Methylquinoline quinaldine and lepidine , 2-quinolone carbostyril , 4-quinolone and the quinolinium ion are important derivatives of quinoline. Quinoline has...
Oxadiazole 1
The trivial name furazan is still in wide usage for 1,2,5-oxadiazole. The 1,2,5-oxadiazole mole- cule is planar and exists as a regular pentagon. The ionization energy is 11.79 eV and the dipole moment 3.38 D. Both values are greater than those of isoxazole. The chemical shift S in the -NMR spectrum is 8.19, and in the 13C-NMR spectrum 139.4 both in CDC13 . 1,2,5-Oxadiazole is aromatic. The following 7r-bond orders were calculated by the HMO method A considerable derealization of the -electrons...
Dihydroisoxazole
4,5-Dihydroisoxazole has previously been known as A2-isoxazoline or 2-isoxazoline. 4,5-Dihydroisoxazoles are accessible mainly by two synthetic pathways 1 Cyclocondensation of a, -unsaturated ketones with hydroxylamine. This reaction is analogous to isoxazole formation via the corresponding ketoxime 2 1,3-Dipolar cycloaddition of nitrile oxides to alkenes. This reaction is the standard synthesis for 4,5-dihydroisoxazoles 87 . For instance, acid 1 is formed regioselectively from benzonitrile...
Pyrazole
Pyrazole, like its structural isomer imidazole, contains a pyrrole-like and a pyridine-like N- atom, but in the 1- and 2-positions 1,2-diazole . The pyrazole molecule is planar. Bond lengths and bond angles have been calculated from microwave spectra see Fig. 5.16 . Consistent with the structural formula, the bond between atoms 3 and 4 is the longest. Bond lengths in pm, bond angles in degrees The ionization energy of pyrazole is 9.15 eV. It follows from a comparison with pyrrole 8.23 eV that...
Benzimidazole
The UV and 13C-NMR spectra of benzimidazole show the following characteristics UV ethanol 13C-NMR methanol-d4 244 3.74 , 248 3.73 , 266 3.69 , C-2 141.5, C-4 115.4, C-5 122.9, C-6 122.9 272 3.71 , 279 3.73 C-7 115.4, C-3a 137.9, C-7a 137.9 The short wavelength bands at 244 and 248 nm are due to electronic excitation of the imidazole ring, the others to electronic transitions in the benzene ring. There is no detailed analysis of the A2B2 system of the two benzene-type protons in the iH-NMR...
Thiirane
Thiiranes are also known as episulfides. As a result of the greater atomic radius of the S-atom, the three atoms form an acute-angled triangle see Fig. 3.2 . bond lengths in pm, bond angles in degrees The thermochemically determined strain enthalpy of thiirane of 83 kJ mol-1 is less than that of oxirane. The ionization potential amounts to 9.05 eV, the dipole moment to 1.66 D. Both values are below those of oxirane. The chemical shifts in the NMR spectra are S 2.27, Sc 18.1. g The properties of...
Azirine
Unlike l -azirine, 2 -azirines can be used preparatively, although the ring strain is substantially greater than that of the saturated three-membered heterocycles. The ring strain enthalpy amounts to approximately 170 kJ mol 2 f-Azirine is thermally unstable and has to be stored at very low temperatures. Substituted 2H-azirines are more stable. They are liquids or low melting solids. Their basicity is substantially lower than that of comparable aliphatic compounds. For instance,...
Thiazole
Thiazole 1,3-thiazole possesses a pyridine-like N-atom and an S-atom as present in thiophene. - The univalent radical is known as thiazolyl. The thiazole molecule is planar and the C-S bond length is 171.3 pm, similar to that in thiophene see Fig. 5.13 Bond lengths in pm, bond angles in degrees A comparison with the bond lengths of oxazole see Fig. 5.9, p 123 leads to the conclusion that the derealization of the --electrons in thiazole is greater. Thus, the aromaticity of thiazole is greater...
Azetidine
Azetidine was previously called trimethyleneimine. The activation energy of the ring inversion is 5.5 kJ moF and is therefore only slightly below the value for cyclobutane 6.2 kJ mol-1 - The conformer with an equatorial N-H bond is lower in energy g Azetidines are thermally stable and less reactive than aziridines. They behave in their reactions - almost like secondary alkylamines. The pKa value of azetidine is 11.29 and so it is more basic than aziridine pKa 7.98 and even dimethylamine pKa...
Isoxazole
Isoxazole 1,2-oxazole contains a pyridine-like N-atom, but differs from oxazole by the pres- ence of an N-0 bond. The bond energy of such a cr-bond amounts only to 200 kJ moF, much lower than that of N-C or O-C bonds. The univalent radical is known as isoxazolyl. The isoxazole molecule is planar see Fig. 5.11 . Again, it is evident that the heteroatoms impair the derealization of the -electrons. This is more pronounced than in oxazole, as can be deduced from a comparison of the bond lengths...
Indazole
Indazole benzo d pyrazole has the following UV and NMR spectral data UV H20 pH 4 1H-NMR DMSO-d6 13C-NMR DMSO-d6 250 3.65 H-3 8.08 H-6 7.35 C-3 133.4 C-6 125.8 284 3.63 H-4 7.77 H-7 7.55 C-4 120.4 C-7 110.0 296 3.52 H-5 7.11 C-5 120.1 g Indazole, pKa 1.25, is less basic than pyrazole but a stronger N-H acid, pKa 13.86 121 . - The tautomerism of indazole is a special case inasmuch as the 2 -indazole possesses an o- For this reason, the equilibrium lies to the left. However, the energy difference...
Systematic Nomenclature of Heterocyclic Compounds
Many organic compounds, including heterocyclic compounds, have a trivial name. This usually originates from the compounds occurrence, its first preparation or its special properties. Structure Trivial name Systematic name ethylene oxide oxirane fj pyromucic acid furan - 2 - carboxylic acid nicotinic acid pyridine - 3 - carboxylic acid The derivation of the systematic name of a heterocyclic compound is based on its structure. Nomenclature rules have been drawn up by the IUPAC Commission and...
B Def
For the reactions of quinoline, addition and substitution processes are to be expected in view of its similarity to naphthalene and pyridine. It is of interest to note the extent to which the fused benzene ring influences the positional and relative reactivities. Electrophilic substitution reactions As in pyridine, it is the nitrogen in quinoline which undergoes protonation, alkylation, acylation and, with peroxyacids, oxidation to the TV-oxide. SEAr reactions occur on the ring C-atoms,...
Pyrylium ion
The pyrylium ion possesses the structure of a planar, slightly distorted hexagon with C-C and - C-0 bonds of more or less equal length. This follows from the X-ray analysis of substituted pyrylium salts, e.g. the 3-acetyl-2,4,6-trimethyl system see Fig. 6.1 . Fig. 6.1 Bond parameters of the pyrylium system in the 3-acetyl-2f4,6-trimethyl pyrylium ion bond lengths in pm, bond angles in degrees Fig. 6.1 Bond parameters of the pyrylium system in the 3-acetyl-2f4,6-trimethyl pyrylium ion bond...
Isothiazole
In isothiazole 1,2-thiazole , the pyridine-like N-atom is bonded to the S-atom. This cr-bond is also the weakest link in the molecule and is cleaved by ring-opening reactions. The isothiazole molecule is planar its ionization energy amounts to 9.42 eV and its dipole moment to 2.4 D. Isothiazole absorbs at longer wavelengths than isoxazole and thiazole, due to a tt- gt n transition g Isothiazole is aromatic. The NMR spectra confirm a largely undisturbed derealization of the n- electrons. In...