Nerol, prochiral allylic alcohol

The chiraUy modified cluster HRu3(CO)10(i -OCNCH2CH2CH2CHCH2 OCH3) catalyzes the conversion of the prochiral allylic alcohol nerol (78) into the chiral aldehyde citronellal (79)... 

The mechanisms of the reactions of the cluster Ru3(CO)i2 with halide ions, alkoxide ions and amines, all of which involve initial rapid nucleophilic addition at a carbonyl hgand, have been reviewed.In a related study, addition of 5-proline methylester or 5-methoxymethyl pyrrolidine to a carbonyl ligand of Ru3(CO)j2 has yielded chiral carbamoyl clusters of the type (84) R = C02Me or CH20Me, Eq. (16). Such chiral clusters may have potential as new enantioselective catalysts, as shown by the observation that cluster (84), R = CH20Me) catalyzes the isomerization of the prochiral allylic alcohol nerol to give the chiral aldehyde citronellal with an enantiomeric excess of 12%. 

Prochiral allylic and homoallylic alcohols are hydrogenated in a highly enantiose-lective manner by use of BINAP-Ru complexes (Scheme 2.2) [60]. Geraniol or nerol is converted quantitatively to citronellol in 96-99% e.e. in methanol at an initial hydrogen pressure higher than 30 atm. The S/C approaches 50 000 in the reaction using the Ru bis(trifluoroacetate) catalyst. Only allylic alcohol double bond is hydro-... 

Allylic alcohols also provide a suitably activated substrate for hydrogenation. Ryoji Noyori s asymmetric reduction of the prochiral geraniol (E-double bond geometry) and nerol (Z-double bond geometry) to enantiomerically pure citronel-lol in the presence of Ru(OAc)2 BINAP is a well-known example. The nonallylic olefin is not reduced appreciably, which indicates the importance that the allylic alcohol functionality plays in this reduction. (Homoallylic alcohols are also reduced by this system, but when the olefin in question is three or more bonds distant from the alcohol moiety, the compound is inert.) Either enantiomer of cit-ronellol is accessible regardless of which substrate is used depending on the chirality of the Ru-BINAP catalyst used (8). This type of relationship implies that the reaction s mechanism possesses high facial selectivity. 

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