2- - 1 -halo-1 -alkene 2132 Transformation

In 1913, Kishner observed in one instance that under standard Wolff-Kishner reduction conditions, 2-hydroxy-2,6-dimethyloctan-3-one underwent eliminative reduction upon treatment with hydrazine hydrate and base at elevated temperatures to afford 2,6-dimethyloctan-2-ene (Scheme 7). This same reaction was later found to occur in the case of a-methoxy ketones and has since been referred to as the Kishner eliminative reduction. The reaction entails initial formation of the hydrazone and elimination of the a-substituent to afford the intermediate alkenyldiazene, which subsequently collapses to the desired alkene. Given the facile transformation of ketones into a-halo ketones, these conditions have been used to introduce alkenes regioselectively in the 2a-halocholestan-3-one series as shown in Scheme 8. Yields of 2-cholestene parallel the resistance of the a-halogen to undergo competitive elimination reactions. 

The transformation of an a-halo sulfone into an alkene takes place in three steps (Scheme 7). Step 1 involves reversible formation of the a -anion. Under the basic reaction conditions, there is a rapid equilibration of the a-halo sulfone with its a- and a -anions. Only the a -anion leads to step 2 which is intramolecular displacement of a halide ion to give a thiirane 1,1 -dioxide, generally as a mixture of cis and trans isomers. This is the rate-limiting step. Finally, in step 3, the thiirane dioxides lose SO2 to give the stereoisomeric alkenes. This transformation can take place via two distinct pathways one is thermal, while the other involves base catalysis. Both pathways are stereospecific trans- or c/5-thiirane 1,1-dioxides give only ( )- or (Z)-alkenes, respectively. Consequently, the stereochemical outcome of the overall reaction is determined at the ring-forming step 2. 

Palladium-catalyzed reactions of conjugated dienes in the presence of a halide anion can be controlled to selectively give l-acyloxy-4-halo-2-alkene under appropriate reaction conditions. The catalyst for this system is a palladium(II) salt, usually Pd(OAc)2 or Li2 PdCl4. The reaction may be either intermolecular or intramolecular. In most cases, this transformation is stereoselective and provides a 1,4-ds-adduct of the diene. The products obtained from these reactions are useful synthetic intermediates since they have two allyhc leaving groups with a large difference in reactivity (Section 11.3.1.2.3). 

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. Salts of palladium, in particular PdCl2 in the presence of a base, catalyze the CCI4 oxidation of alcohols to aldehydes and ketones. Allylic alcohols carrying a terminal double bond are transformed to 4,4,4-trichloro ketones at 110 °C, but yield halo-hydrins at 40 °C. These can be transformed to the corresponding trichloro ketones under catalysis of palladium acetate (eq 56). The latter transformation could be useful for the formation of ketones from internal alkenes provided the halohydrin formation is regioselective. 

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