Reformatsky reaction cerium enolates

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. 

Cerium enolates may also be generated reductively in THF from oi-iodo or a-bromo esters in the presence of an aldehyde or ketone using cerium metal turnings and catalytic HgCk (or cerium amal-gam). This leads to a Reformatsky-type reaction (Volume 2, Chapter 1.7) producing the (3-hydroxy ester (47) as a mixture of diastereomers (equation 16). 

Aldol Additions to Ketones. Traditionally, cerium enolates or the Reformatsky-type reaction have been employed to achieve high-yielding aldol additions to enolizable ketones. In this regard, methyl trichlorosilyl ketene acetal provides a reliable alternative for the synthesis of tertiary -hydroxy esters. In the absence of a Lewis base promoter, the aldol additions of 1 to ketones are too slow to be synthetically useful. On the contrary, with pyridine A-oxide as catalyst, methyl trichlorosilyl ketene acetal reacts smoothly with nearly all classes of ketones (7) (Scheme 1). Good yields of the tertiary alcohol products (8) are obtained (eq 4), table 2 from aromatic (entries 1-2 and 4—6), hetereoaromatic (entry 3), olefinic (entries 7-8), acetylenic (entries 9-10), and aliphatic (entries 11-14) ketones. The only poorly performing substrate is 2-tetralone (7o), which affords a 45% yield of the addition product and returns 45% of unreacted starting material, most likely from competitive enolization. 

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