Ring contraction and expansion reactions

Ring contraction and expansion reactions. Treatment of the cation [7t-C7H7Mo(CO)3]+ with anions such as Y = CN, OMe- or CeHs , affords straightforward addition products [2, 16]. 

However, when Y = the cyclopentadienide or diethylmalonate anion a ring contraction occurs and a neutral arene complex is formed. The postulated mechanism [16] is shown at top of opposite page. 

A reverse reaction, namely ring expansion of an arene ligand to a cycloheptatriene ligand, was discovered during attempts to acylate the mixed sandwich complexes Tr-CsHsCr-Ti-CeHe and yr-CsHsMn-Tr-CsHs [17]. 

The reaction shown below also occurs with aromatic a yl halides, thus benzoyl chloride affords the complex re-C5H5Cr(7r-C7H6-C6H5) [17]. 

Some of the known mixed sandwich complexes are shown in Table 22. These complexes may be prepared by a variety of routes which are exemplified below. 

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In reactions that resemble the pinacol rearrangement, thiopyranoside sulphonate esters undergo ring contraction and expansion reactions via the intermediacy of sulphonium ions, e.g. equations 37 and 38102. Similarly, cyclic sulphonates are able to trap oxime anions at the carbon atom (equation 39)103. Such carbon alkylation of oxime ions is rare, and by suitable choice of substrate both ring-contraction and ring-expansion reactions are possible103. 

The ring-contraction and -expansion reactions of vicinally disubstituted cycloalkanes (397) and cycloalkylmethyl compounds (398) have been reviewed by Conia and Robson. It is worth noting that reactions of this type in small-ring compounds often give a single product in high-yield and that the reactions are synthetically useful. 

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