Homopropargylic alcohols, asymmetric

An asymmetric version of the Pd-catalyzed hydroboration of the enynes was reported in 1993(118]. The monodentate phosphine (S)-MeO-MOP was used as a chiral ligand for the palladium catalyst. Enantioselectivity of the asymmetric hydroboration was estimated from the enantiopurity of homopropargyl alcohols, which were obtained from the axially chiral allenylboranes and benzaldehyde via an SE pathway (Scheme 3.78). 

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). 

Scheme 1.3.17 Asymmetric synthesis of homopropargyl alcohols via a-elimination of alkylidene aminosulfoxonium ylides.

The asymmetric approaches include the preparation of the configurationally stable chiral aUenyltin starting from enantio-enriched propargyUc precursors. When submitted to transmetaUation with Sn, Bi or In Lewis acids before addition to the aldehyde, the homopropargyl alcohol is obtained in a 95 5 anti/syn ratio and in a 90% ee . On the other hand, the use of a chiral aUenyltin reagent, without prior transmetaUation, gives the syn adduct selectively (95 5) (equation 12) . However, the use of chiral aUenyltin and chiral aldehydes may lead to the same match/mismatch effect that was observed with allyltins . Both approaches were applied to the synthesis of macrolides subunits ° i . 

Optically active allenyltrichlorosilanes can be prepared by the Pd-catalyzed asymmetric hydrosilylation of 1,3-enynes with I ISiCb, (Scheme 10.138) [389]. In this reaction use of bisPPFOMe (125) as the chiral ligand achieves high enantioselectivity. The allenylsilanes smoothly react with PhCHO in DMF to give optically active homopropargyl alcohols, and the axial chirality of allenylsilanes is completely transferred to the central chirality of the products by a syn-SE process. 

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