Group 16 systems heteroatomic

The use of group 6 heteroatom stabilized carbene complexes (Fischer carbene complexes) in organic synthesis is relatively recent and, in spite of that, it has already produced impressive synthetic results [1]. These versatile organometallic reagents have an extensive chemistry, and they are probably one of the few systems that undergo cycloadditions of almost any kind. For instance, [1+2], [2+2], [3+2], [3+3], [4+1], [4+2], [4+3], [6+3]... 

The syntheses from [4+1] atom fragments, in which the Group 16 heteroatom is introduced between two nitrogen atoms, are the most widely applicable and versatile methods available for construction of the 1,2,5-thiadiazole ring system. These methods have been applied to the synthesis of monocyclic and polycyclic aromatic forms of these ring systems in addition to the direct formation of 1-oxides and 1,1-dioxides, 2-oxides, quaternary salts, and reduced forms. The earliest use of the [4+ 1] synthesis dates back to 1889 when Hinsburg prepared 2,1,3-benzothiadiazole (I) from o-phenylenediamine and sodium bisulfite. 

The 13C NMR chemical shifts and one-bond coupling constants 7(13C-H) have been obtained for (1), (2) and (5) in carbon disulfide (Table 8) (74OMR(6)430) (see also Section 4.01.3.4). The chemical shifts for the 3a-position in the title compounds are the reverse of what would be expected on the basis of a purely inductive effect of the Group VIA heteroatoms. If the chemical shifts are controlled by through-bond effects, then mesomeric interaction of (2) is of considerable importance in these systems. Thus, the 13C NMR... 

This stereoelectronic effect may explain other cases of different mass spectra of stereoisomers. The more pronounced loss of axial versus equatorial methyl groups in heteroatom-containing decalin systems, which has been attributed... 

The utility of routes requiring electrophilic attack on an aromatic nucleus, used so extensively in the synthesis of benzheterocycles, has been little explored as a method for generating dibenzometalloles. The only systems for which this route has been reported are those that contain a Group V heteroatom ... 

Experimental data as well as calculations 284, 330) reveal two important differences between silicon-carbon bonds and carbon-carbon bonds, i.e., torsional barriers are small and gauche interactions unimportant for the Si—C bond when compared to the C—C bond. Low torsional barriers may imply a more flexible system (lower barriers to ring inversion), and small gauche interactions may imply lack of substituent preference or low substituent preference when compared to the parent carbocycle. Although calculations for heavier Group IV heteroatoms have not been published, it is probable that results will be similar to those obtained for Si. 

Not unexpectedly the systems that have received the most attention are derivatives of cyclohexane containing a silicon or phosphorus heteroatom. Although cyclohexane is a saturated system, the results of these studies should provide insight for future work on unsaturated systems. Until future studies provide data to the contrary, it will be assumed that Si provides the model for Group IV heteroatom behavior and P a model for Group V behavior. The first aspects to be considered are associated with ground-state geometry and substituent preference. 

The systematic IR studies of the functional group and heteroatom effect on the position of the tautomeric equilibrium indicated that the stability of thiols (with respect to the corresponding thione forms) is considerably higher than the stability of the hydroxy forms (with respect to the oxo forms) in the same heterocyclic systems, so the mercapto tautomers of mercaptopyridines should be more favored in the equilibrium than their oxygen analogs (92JPC6250). 

To determine the stability and reactivity of heterocyclic compounds, it is useful to compare them with their carbocyclic analogues. In principle, it is possible to derive every heterocycle from a carbo-cyclic compound by replacing appropriate CH2 or CH groups by heteroatoms. If one limits oneself to monocyclic systems, one can distinguish four types of heterocycles as follows ... 

Not only alkyl hahdes but also group 16 heteroatom compounds are reduced with Sml2 upon photoirradiation. This Smh-hi system is extremely effective for the reduction of organic selenides. On the other hand, the photoinduced reduction of sulfides with Smij does not proceed at all, while dodecane is obtained in 38 % yield by the photoinduced reduction of dodecyl phenyl telluride with Sml2 (Scheme 2.35) [95]. 

Theoretically this kj -value is again given by an electronic matrix element and a Franck-Condon factor. Systems with the same free enthalpy change should have the similar Franck-Condon factors, since the latter is a function of the former. However, the yields are scattered, which should be reduced to the difference of the electronic matrix elements. Small yields were observed for the systems with a component molecule having carbonyl group or heteroatom. In these molecules, an n-Tr triplet level is placed between or near the initial electron transfer and the final triplet states, which may accelerate the intersystem crossing. The kj -values increase and the formation yield of ion radicals is reduced. This interpretation is consistent with the result that the yield is determined by the chemical properties of the component molecules. 

Both and C n.m.r. spectra have been obtained for a number of 1-hetero-cyclohexan-4-ones. The data indicated that all systems (heteroatoms = S, SO, SO2, O, or NMe) exist predominantly in the chair conformation, and that no significant transannular interactions between the heteroatom and the carbonyl group occur. [Eu(fod)3]-induced shifts are smaller in thiacycloalkanones than in the corresponding carbocycles, due to the decrease in the basicity of the carbonyl oxygen. ... 

Fig. 2 The basics of structure-activity reiationship modifications. From a hit compound containing a terminai phenyi moiety (center), SAR involves addition of eiectron-withdrawing groups (EWG) (path 1) and electron-donating groups (EDG) (path 2) at varying positions around the ring system. Heteroatom substitution can alter the binding and physicai properties of the molecule (path 3), and bioiso-steric replacements (path 4) can facilitate significant structural changes without loss of on-target potency...

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