Carbonyl compounds orbital treatment

The orbital interaction treatment of C=C n bonds and other types of p-p n bonds is given here. Carbonyl compounds are treated separately in Chapter 8, and organo-metallic 7i-type bonding is briefly described in Chapter 13. 

The sulfur analogues of enolates have recently received attention in the context of synthetic applications. Thiocarbonyl compounds bear a-protons which are rather acidic. Kresge et al. [120] has shown that their pKas are 10 units less than those of carbonyl compounds. Thus enethiolates are easily formed with a variety of bases, and they exhibit thermal stability [1]. They are ambident nucleophiles and the sulfur vs carbon regiochemistry has been rationalised by Anh [119] using frontier orbital treatment. 

The high reactivity towards nucleophiles, as compared to carbonyl compounds, is easily explained by frontier orbital treatment [119]. However the regioselectivity of addition, whether on sulfur or on carbon atoms, has not yet been fully rationalised. However this has not prevented numerous successful applications [1]. 

The introduction of the photochemically excited triplet mechanism leading to CIDEP of the resulting radicals has added a new dimension to the potentials of the CIDEP techniques in photochemistry. In liquid photochemical systems, very little is known experimentally about the exact nature of the intersystem crossing process, but the rate or efficiency of such ISC process can sometimes be estimated by chemical (86) and optical methods (51,105). The treatment of the phototriplet mechanism in CIDEP of radicals in liquid solution is consistent with the following conclusions (1) ISC occurs mainly by the spin-orbit coupling mechanism in carbonyl compounds, (2) spin polarization of the triplet sub-levels is obtained via the selective ISC processes, and (3) the chemical reaction rate of the triplet is at least comparable to its depolarization rate via spin-lattice relaxation. 

It is well known that the tricarbonylchromium-complexed benzylic anions and cations are stabilized due to overlapping between d-orbital of the chromium and p-orbital of the benzylic carbon [1]. Tricarbonylchromium complexes of a-te-tralone and a-indanone having a carbonyl group at the side chain underwent a deprotonation of the exo-benzylic protons by treatment with base to give the stereo-controlled tricyclic compounds (Eqs. 1 and 2) [2]. In these cases, Robinson annulation products were formed in less than 10% yield. Also, base treatment of benzyl ether chromium complex having a chlorine at the side chain 3 gave cyclization product as a diastereomeric mixture (Eq. 3) [3]. 

---