Unsaturated cyclic ring systems, carbon

Aromatic compounds are unsaturated cyclic molecules that possess additional stability as a result of the arrangement of 7i-electrons associated with the unsaturation of the ring system. This book will concentrate on the chemistry of benzene (4) and its derivatives and related polynuclear hydrocarbons. Aromatic compounds are also known as arenes they can becarbocydic, indicating that the ring skeleton contains only carbon atoms, or heterocyclic, with at least one atom other than carbon in the ring. These heteroatoms are typically N, O or S. Heterocyclic compounds, which can be aromatic or alicyclic, are covered in another book in this series. 

Many ring systems have been prepared by cycloaddition of acetylenic compounds, following concerted or multi-step intra- or intermolecular reaction mechanisms. In-particular, the placing in close proximity of triple bonds or a triple bond and another unsaturated system, such that intramolecular cycloaddition might lead to four-seven-membered rings, would seem of interest. This section deals with transannular carbon-carbon bond formation of triple bonds in acyclic and cyclic systems. 

Phosphinine is representative of unsaturated cyclic systems built from carbon and phosphorus atoms, while phosphinane is typical of saturated ring systems. 

As was pointed out in Chapter 5, with reference to the conjugated ligand cyclooctatetraene, and as the structures of the cyClobutenyl nickel complex, 7.IS, and of the rhodium complex 7.18 show, it is not necessary for all the annular carbon atoms of a cyclic unsaturated hydrocarbon ligand to be involved in bonding to a metal. Further examples of this are shown in Figure 41, where in complexes derived from S-, 6- and 7-membered ring systems respectively, only 5 electrons are formally involved in the bonding in each case. 

Several saturated parent systems shown in Scheme 1, especially (2), (4) and (7), have been incorporated into spiro compounds or have been fused with a variety of non-aromatic carbocyclic or heterocyclic rings. Unsaturated compounds with an endocyclic carbon-carbon double bond, formally possible in all systems with three adjacent heteroatoms, are only known in very few cases (e.g. cyclic sulfites and sulfates of enediols) or in the form of benzo or other condensed systems (mainly cyclic sulfites and sulfates of aromatic 1,2-dihydroxy compounds). For details concerning the various types of derivatives, see Section 4.33.4.2. 

Aromatic systems that obey Hiickel s 4 + 2 rule where = 0 and so possess two 7i-electrons do exist and are indeed stable. The smallest possible ring is three membered and the derived unsaturated structure is cyclopropene. The theoretical loss of a hydride ion from this molecule leads to the cyclopropenyl cation, which contains two 7t-electrons distributed over the three carbon atoms of the planar cyclic system (Figure 1.8). 

Analogous to the hydrogenation of sp carbons within a cyclic system, unsaturated carbons, located in the exocyclic position in the silacyclobutane subunits are transformed into their saturated derivatives. This is exemplified by the silacyclobutanes 7, 8, and 11 (Scheme 2) as model compounds. From those the products 9, 10, and 12 can be easily obtained in good yield, leaving the four membered ring moiety unchanged. 

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