Catalytic asymmetric allenylation

The catalytic asymmetric allenylation has been explored with a BINOL/Ti system, giving selectively the homoallenyl alcohol with up to 95% ee. Nevertheless, the lower reactivity of aUenyltin compared to allyltin necessitated a nearly stoichiometric amount of 8. Recently, an improvement of the reaction by using i-PrSBEt2 overcame that limitation, making the system truly catalytic, with ee in the range of 81 to 97% . Interestingly, this reaction showed an unexpected equilibration phenomenon, thus producing exclusively... 

Using the same catalyst system, Akiyama has demonstrated the catalytic asymmetric allenylation of a-amino ester 389 with propargylstannane 391 to synthesize allenic amine 392 in good yield and high enantiopurity (Scheme 5.2.87). 22... 

The catalytic asymmetric synthesis of allenes was first achieved by Elsevier and co-workers in 1989 [104]. A palladium-catalyzed cross-coupling reaction of an allenyl-metal compound 250 (M = ZnCl, MgCl or Cu) with iodobenzene in the presence of DIOP 251 gave 252 in 25% ee (Scheme 4.65). The synthesis of 252 by the reaction of 250 (M = Br) with phenylzinc chloride in the presence of a chiral palladium catalyst gave a quantitative conversion but very low enantiomeric excesses (3-9% ee). 

Many noticeable examples of chiral Lewis base catalyzed allylation of carbonyl compounds have also appeared. Iseki and coworkers published a full paper on enantioselective addition of allyl- and crotyltrichlorosilanes to aliphatic aldehydes catalyzed by a chiral formamide 28 in the presence of HMPA as an additive [41]. This method was further applied to asymmetric allenylation of aliphatic aldehydes with propargyltrichlorosilane [40]. Nakajima and Hashi-moto have demonstrated the effectiveness of (S)-3,3 -dimethyl-2,2 -biquinoline N,AT-dioxide (29) as a chiral Lewis base catalyst for the allylation of aldehydes [42]. In the reaction of (fs)-enriched crotyltrichlorosilane (54 , E Z=97 3) with benzaldehyde (48), y-allylated anfi-homoallylic alcohol 55 was obtained exclusively with high ee while the corresponding syn-adduct was formed from its Z isomer 54Z (fs Z= 1 99) (Scheme 6). Catalytic amounts of chiral urea 30 also promote the asymmetric reaction in the presence of a silver(I) salt, although the enantioselectivity is low [43]. 

The catalytic asymmetric propargylation [108] and allenylation [109] of achiral aldehydes has been performed with high levels of enantioselection. The asymmetric propargylation promoted by the chiral Lewis acid derived from bind and Ti(0/-FT)4 are representative. Between 50 and 100 mol% of titanium is required for these reactions to go to completion (Scheme 10-70). The reaction of benzalde-hyde with allenyltributylstannane 170 and the chiral promoter produced the homo-propargylic alcohol 171 in >99% ee and 48% yield (7% of the undesired allenyl alcohol was also obtained). 

Catalytic asymmetric propargylation has been reviewed (131 references). Aldehydes have been propargylated in up to 99% yield and 92% ee by allenyl-zinc reagents, generated in situ from the action of diethylzinc on iodoallene (or 3-iodopropyne), with a bulky diarylprolinol catalyst. The reaction is conveniently carried out in DCM at low temperature and avoids the use of tin reagents. ... 

A catalytic asymmetric Saucy-Marbet Claisen rearrangement of propargyl ethers to -substituted allenyl carbonyls has been reported (Scheme 4). ... 

Asymmetric hydroboration of 1-phenyl-1,3-butadiene (95) catalyzed by Rh-BINAP gave the corresponding optically active 1,3-diol 155 with 72% ee [89,90] (Scheme 2.15). Palladium-MOP complex also exhibited catalytic activity for the asymmetric hydroboration of but-l-en-3-yne (156), giving an optically active allenyl borane 157 [91]. 

Indole derivatives tethered by alkenyl or allenyl groups undergo efficient intramolecular hydroarylation. Treatment of 2-(4-pentenyl)indoles with a catalytic platinum/chiral phosphine complex induced intramolecular hydroarylation to give tetrahydrocarbazole in 93% yield with 90% ee (Scheme 18.14) [13], The similar intramolecular asymmetric hydroarylation was applied to indoles having internal olefins by use of a highly strained planar chiral platinacycle (Scheme 18.15) [14],... 

The first report on the conceptually new asymmetric catalysis described that both a stoichiometric amount of SiCU and a catalytic amount of chiral phosphoramide (107) promote highly enantioselective allylation and propargylation of aromatic aldehydes with allyl- and allenyl-tributylstannane, respectively [41], The allylation does not proceed without (107). In the proposed mechanism, SiCU, a weak achiral Lewis acid, accepts the Lewis base (107) to form a strong chiral Lewis acid by polarization of the Si-Cl bonds. The active Lewis acid promotes the asymmetric reaction to give trichlorosilylated adducts with regeneration of (107) (Scheme 9.74). 

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