Sulfur d-orbitals

Clearly, there exists a good agreement between theoretical predictions (and calculations) based on the participation of sulfur 3d-orbitals and available experimental results. Thus, the important role of the sulfur d-orbitals in determining the structure and, consequently, the chemistry of sulfones and sulfoxides in general, and of strained smallring sulfones and sulfoxides in particular, has been established. 

And finally, we may mention that a statistical analysis of the sulfur d orbital problem has been carried out with dimethyl sulfoxide as a model compound36. The results provide a clear answer to the sulfur d orbital problem, since no simultaneous reproduction of experimental geometry and an adequate approximation to the variationally optimum total energy have been possible without including d polarization functions on sulfur . 

Very recently there has been an experimental and theoretical study of electronic substituent effects in 4-aminoaryl (4-substituted aryl) sulfones146. PMR, 13C NMR and infrared measurements were involved and semi-empirical all-valence CNDO/2 calculations, with and without sulfur d orbitals, were carried out. Various correlations between spectral results and substituent constants are presented. There is good agreement between experimental and theoretical data, which does not depend on the inclusion or exclusion of the sulfur d orbitals from the calculations. 

In view of the limited capacity of the sulfur atom in the sulfoxide and sulfone functional groups to transmit conjugative effects due to the insulating effect of the LUMO sulfur d-orbitals , the application of the UV technique even in the case of the cyclic vinyl sulfones (e.g. thiete dioxides 6b) cannot be expected to find extensive use. UV spectra of substituted thiete dioxides in which an extended conjugated system (e.g. 194) exists in the molecule, did provide useful information for structure elucidation . However, the extent... 

Strict selectivity of the reaction is explained by electroacceptor properties of sulfur d orbitals. These orbitals are not as high in energy as oxygen d orbitals are (Testaferi et al. 1982). 

The half-wave reduction potentials (HWP) of dibenzothiophene and some of its derivatives have been measured for comparison with those of dibenzofuran and dibenzoselenophene. A shift to more negative HWP was observed for all of the methyl derivatives studied, the magnitude of which depended on the position of substitution. These shifts are in accord with LCAO molecular orbital theory predictions if the sulfur d orbitals are excluded from the calculations (Section III, A). ... 

Cyclic sulfamides (14) exhibit enhanced acidity versus their acyclic counterparts by about 4 pA units. The sulfur d-orbitals and/or the added ring strain may be responsible <84JCS(P2)l85l>. 

The unoccupied TTF MOs are schematically shown in Fig. 6.15. The energy values associated with the orbitals are those calculated for a single molecule with the same geometry as for the neutral solid. The symmetry labels are those appropriate for idealized D2h symmetry. For simplicity, the sulfur d orbital contributions have not been shown in these schematic drawings. 

The application of ESR spectral data to the study of conjugated sulfur compounds seems attractive because it could help to resolve the problem of sulfur d-orbital participation in conjugation.79... 

Potts and McKeough81 have recently obtained several interesting mesoionic azapentalenes 83, 84 by reaction of the appropriate 1,4-dicarbonyl compound with phosphorus pentasulfide. Compounds 83 and 84 can be written as dipolar structures, but participation of sulfur d-orbitals allows nonpolar forms to be envisaged. MO calculations neglecting rf-orbital participation predict the thieno[3,4-c]pyrrole system (84) to be very unstable.430 A related system 86 has been prepared by Japanese workers63 from the diamino-r-triazole (85) and sulfur dichloride. 

A few of the corresponding sulfur systems are given in Scheme 4 because of the availability of sulfur d-orbitals, the neutral species lH-thiin is now a possibility and some substituted derivatives exist. These are the thiabenzenes , and with just the single heteroatom in the ring (C5S rings) they are very unstable, non-planar structures which are largely ylidic in character (CHEC 2.25). Aza substitution results in stabilization. 

Many MO calculations have been carried out on thiophene and the nonclassical thienofuran, thienopyrrole and thienothiophene. Triplet ground states were predicted for these heterocycles. The theoretical controversies over sulfur d- orbital participation in these systems provided an added impetus to the experimental chemists and these nonclassical heterocycles became the objects of chemical investigation since 1967 (75ACR139). 

The compound has the R configuration. Note that the unshared electron pair on the sulfur atom in the formula shown below is oriented towards the observer. Contributions of the sulfur d-orbitals to the S-0 double bond are not considered in the determination of the priority order of the groups attached to the chirality centre. Thus the decyloxy residue has precedence over the doubly bonded oxygen atom. This compound has been used in the synthesis of liquid crystalline substances [6]. 

The ratio fa /p(Me)//0 /P(r-Bu) is 15,700 for the alkylbenzenes but only 69 for the 2-alkylthiophenes. Likewise the fa /m(Me)//0 /m(t-Bu) ratios are 44,700 (alkylbenzenes), 76.4 (2-alkylthiophenes), and 17.6 (3-alkylthi-ophenes). These results are free from any bond-order effects (which would equally affect the activation by both methyl and r-butyl substituents). They indicate that the 2-t-butyl group produces more hindrance than the 3-/-butyl group, possibly through buttressing between it and the sulfur d orbitals. This view is reinforced by results for electrophilic substitution of benzo[b]thiophene (Chapter 8, Section 2.A.b.i). 

The second possibility requires the assumption that the lone pair on nitrogen (but not the sulfur d orbitals) has a smaller space requirement than a C—H bond. Consequently, when the probe is adjacent to nitrogen, the empty p orbitals in the carbocation in the transition state are better able to overlap with the p orbitals in the azole ring, and the reactivity will be enhanced. One must also assume that this is relatively unimportant in pyrolysis, which is reasonable since less charge is developed in the transition state. A parallel discrepancy in reactivity adjacent to nitrogen is evident in pyridine chemistry, in which the results tend to support the former interpretation this is discussed further in Chapter 9. 

Lastly, the 2- versus 3-positional reactivity order for benzo[6]thio-phene appears to be affected by steric hindrance from the sulfur d orbitals [82JCS(P2)1489], and this is discussed in detail in Section 2.A.C. For indoles it has been proposed that 2-substitution may take place by initial attachment of the electrophile to the 3-position, followed by migration (69T227). 

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