Alkenyl Alcohols to Aldehydes and Ketones

Although thus far a number of reports have been made on mthenium complex-catalyzed isomerization of alkenyl alcohols to saturated aldehydes or ketones, the mechanistic details of these reactions have not yet been fully elucidated. It has been generally accepted that isomerization of the alkenyl alcohol forming a carbonyl compound proceeds via an intermediate enol. However, developments in mechanistic studies have advanced recently, and a new mechanism which involves an intermediary mthenium alkenylalkoxide is proposed by Trost et al. for isomerization of allylic alcohols catalyzed by CpRuCl(PPh3)2 (Cp = [1]. Gmbbs et al. also proposed 

Chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium, CpRuCl(PPh3)2, effectively catalyzes isomerization of allylic alcohols 1 to saturated carbonyl compounds 2, aldehydes or ketones, in the presence of NH4PF6 (Eq. 12.2). 

The use of an indenyl complex as catalyst instead of the cyclopentadienyl analogue enhances the reactivity due to the opening of a coordination site by valence tautomerization. The reaction is highly chemoselective, and nonallylic alcohols and allyl ethers are not isomerized (Eq. 12.3). 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

The fact that allylic ethers are not isomerized to the corresponding enol ethers by this catalytic system is clearly consistent with the above mechanism involving the metal alkoxide intermediate. 

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