X-ray structures of heterocyclic compounds

Details of bond lengths and bond angles for all the X-ray structures of heterocyclic compounds through 1970 are listed in Physical Methods in Heterocyclic Chemistry , volume 5. This compilation contains many examples for five-membered rings containing two heteroatoms, particularly pyrazoles, imidazoles, Isoxazoles, oxazoles, isothlazoles, thlazoles, 1,2-dlthloles and 1,3-dlthloles. Further examples of more recent measurements on these heterocyclic compounds can be found in the monograph chapters. 

A summary of bond lengths and angles from X-ray structures of heterocyclic compounds studied up to 1970 has been published <1972PMH(5)1>. This compilation contains many examples, particularly of furans, thiophenes, and pyrroles and their benzo derivatives further examples are contained in the appropriate chapters of the three editions of Comprehensive Heterocyclic Chemistry. The Cambridge Structural Database (CSD) maintains a comprehensive collection of heterocyclic structures. 

As mentioned in Section 5.2.2.6, the reaction of [TcCl3(PPh3)2(NCCH3)] (412) with Zn dust with heterocyclic amines produces complexes of Tc in oxidation states +III, -fll, and -fl. When [TcCl3(terpy)] (421) prepared in this manner is reacted with Zn dust in pyridine, the two Tc complexes [Tc(terpy)(py)3] (530) and tra i-[TcCl(terpy)(py)2] (529) are formed. The X-ray structures of both compounds were elucidated. Electrochemical data for the complex (529) shows a... 

Similarly to the above section, no additional specific nuclear magnetic resonance (NMR) data have been published since CHEC(1984) and CHEC-II(1996) <1996CHEC-II(8)249> NMR data for new substituted compounds are routinely reported. Several X-ray structures of bioactive molecules possessing this heterocyclic core have been reported C1999BML1979, 1999JME779>. 

An early compilation of X-ray data for heterocyclic compounds <1972PMH(5)1> contains many examples of six-membered rings. More recent data are contained in the Cambridge Structural Database (CSD) . Determination of structures by X-ray crystallography is now routine and details of structures of specific heterocycles are best found using the CSD or by consulting the appropriate chapters in the three editions of Comprehensive Heterocyclic Chemistry. 

When 8-azaheptafulvene 263 and nitrile oxides such as 264 are mixed in MeOH at room temperature, two interconverting heterocyclic products are formed l-oxa-2,4-diazaspiro[4,6]undeca-2,6,8,10-tetraene 265 and [4,9-a]-dihydrocyclohepta-l-oxa-2,4-diazine 266 (Equation 65) <2002EJ0569>. The X-ray structure of a cycloadduct of 266 (i.e., 35) has been described in Section 9.05.3.1.5 and the thermal decomposition of compounds related to 266 is considered in Section 9.05.6.1.1. 

If, on the other hand, unsymmetrically substituted carbonyl compounds such as monosubstituted benzophenones (X = OCH3, CH3, Cl), tert-butyl methyl ketone, acetophenone, acetaldehyde, or benzaldehyde are used for trapping 39a, diastere-omeric mixtures are formed in each case they could all be resolved except for the products obtained with p-methoxybenzophenone and acetophenone 33>. An X-ray structure analysis has been performed for the E-isomer 57g 36) which, in conjunction with H-NMR studies, permitted structural assignment in cases 56 and 57e, g and h35>. Additional chemical evidence for the structure of the six-membered heterocycles is provided by the thermolysis of 56 a considered in another context (see Sect. 3.1). In general the reaction 39a- 56 or 57 is accompanied by formation of phosphene dimers, presumably via [4 + 4]- and via [4 + 2]-cycloaddition 35). 

Much work has been devoted to [2 + 3] cycloadditions both in the synthesis of and in reactions of this kind of heterocycle. Dimroth rearrangement is another feature which has been explored widely. X-ray structural analysis was extensively used alongside physico-chemical methods to elucidate the structure of novel compounds. 

Limited knowledge exists with respect to detailed molecular structures of simple pyrans and heteropyrans. The semiempirical CNDO/2 calculations, using a full geometry optimization procedure, predict 4 and 5 to exhibit small deviations of the heterocyclic rings from the planar conformations.34 Unfortunately, similar calculated data for thiopyrans 6, 7, and 9 were not presented.45 Accounts of several X-ray diffraction investigations on compounds possessing pyran or thiopyran fragments are available.46 51... 

Se = C(C6H4R-p)H ] (R = H, OMe, CF3) with bis( -butylthio)ethyne. The structure of one 2H-selenete complex 132 (R = H) was confirmed by an X-ray structural analysis.247 Based on spectra of these compounds, an equilibrium between the 2H-selenete complexes (132) and their thioselono-carboxylic ester isomers (133) was proposed (Scheme 36). The fraction of 133 in the eqilibrium was estimated to be about 6-10%. Decomplexation of the heterocyclic ligand in 132 (R = H) with [NEt4]Br afforded the first uncoordinated 2//-selenete. In addition, a 3,4-dihydro-l,2-diselenine was obtained.247... 

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