Enamines oxyallyl cations

Fe2(CO)g and CeClj—SnClj the latter present some advantages like lower toxicity, lower temperature and homogeneous reaction conditions. The first step in the reaction ofa,a -dibromoketone 56 with CeClj—SnClj (equation 10) involves a single debromina-tion in analogy with a Reformatsky reaction to afford a cerium a-bromo enolate 57, This intermediate suffers from further elimination of bromide via ionization to oxyallyl cation 58 which undergoes the [3 + 2] cycloaddition to the enamine. The aminocyclo-pentanonc 59 obtained was easily deaminated to cyclopentenone 60. 

If iron carbonyl is used to generate the oxyallyl cation, the iron supplies two extra electrons in the complex 90 which now adds23 to mono-enes to give cyclopentanones 91. We shall see more examples of transition metal complexes altering selectivity later in the book. To get a cyclopentenone 92 we need an extra degree of unsaturation, best provided by using the enamine of a ketone instead of a simple alkene. 

This cycloaddition works best with substituted oxyallyl cations, like the one from the dibromo-ketone 93 which reacts with the morphol ine enamine of cyclohexanone 95 to give the cyclopentenone 96 in excellent yield.24 You should mark the difference in structure between the Nazarov products, e.g. 70, formed in an electrocyclic reaction, and these cycloadducts, e.g. 96. Each strategy has its part to play and must be chosen according to the structure of the target molecule. 

Analogously, fulvene n-systems can act as a triene unit for enamines (Scheme 6.46a) [103], 2-oxyallyl cations (Scheme 6.46b) [104], chromium alkenyl carbene complexes (Scheme 6.46c) [105], 3-oxidopyrylium (Scheme 6.46d) [106], 3-oxidipyridinium betaines (Scheme 6.46e) [107], dienolates (Scheme 6.46f) [108], azirines (Scheme 6.46g) [109], and azomethine ylides (Scheme 6.46h) [110]. 

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