1,1 -Disubstituted allene, enantioselective

Enantioselective chemistry see Enantioselectivity) has been also developed with imido complexes. The stereospecific reaction of rac-(ebthi)(thf)Zr=NAr with racemic symmetrically or unsymmetrically 1,3-disubstituted allenes (1,3-diphenylallene, 3,4-heptadiene, 4,5-nonadiene or 1,2-cyclononadiene) provides at room temperature a single diastereoisomeric azazirconacycle (equation 8). In certain cases, this system allows conversion of an allene racemate into a mixtme emiched in one enantiomer. 

In 2008, Willis and co-workers developed a highly enantioselective inter-molecular hydroacylation reaction employing ortfto-S-substituted benzal-dehydes 51 and 1,3-disubstituted allenes 52 (Scheme 8.25). When using a (7 ,/ )-Me-DuPhos-derived cationie rhodium catalyst, excellent yields and ee were obtained. Extension of the substrate scope to a trisubstituted allene led to the product with high ee, but in a disappointingly low yield. It was revealed that the reaction was not a simple kinetic resolution of the racemic allene but rather proceeded by dynamie kinetie asymmetric transformation. 

Toste has described the intramolecular enantioselective hydroamination of y- and 6-aUenyl sulfonamides catalyzed by enantiomerically pure bis(gold) phosphine complexes [42]. For example, treatment of the terminally-disubstituted y-allenyl sulfonamide 59 with a catalytic amount of [(R)-3,5-xylyl-binap](AuOPNB)2 (OPNB =p-nitrobenzoate) formed protected pyrrolidine 60 in 88% yield with 98% ee (Eq. (11.34)). Likewise, treatment of 6-allenyl sulfonamide 61 with a catalytic amount of [(JJ)-Cl-MeObiphep](AuOPNB)2 in nitromethane at 50 °C for 24h formed 2-alkenyl piperidine 62 in 70% isolated yield with 98% ee (Eq. (11.35)). Realization of high enantioselectivity in this protocol required employment of both a terminally disubstituted allene and a sulfonamide nucleophile. 

A 1 2 mixture of [ (5)-(235) (AuCl)2] and AgBp4 has been reported to catalyse the enantioselective hydroamination of chiral, racemic 1,3-disubstituted allenes ArCH=C=CHMe with A-unsubstituted carbamates to form A-allylic carbamates ArCH=CHCH(NHCbz)Me in <92 % ee ... 

As with i -substituted allyl alcohols, 2,i -substituted allyl alcohols are epoxidized in excellent enantioselectivity. Examples of AE reactions of this class of substrate are shown below. Epoxide 23 was utilized to prepare chiral allene oxides, which were ring opened with TBAF to provide chiral a-fluoroketones. Epoxide 24 was used to prepare 5,8-disubstituted indolizidines and epoxide 25 was utilized in the formal synthesis of macrosphelide A. Epoxide 26 represents an AE reaction on the very electron deficient 2-cyanoallylic alcohols and epoxide 27 was an intermediate in the total synthesis of (+)-varantmycin. 

Palladium-catalyzed diboration can be achieved with bis(pinacolato)diboron for strained 1,2-disubstituted alkynes. ° Analogously, palladium-catalyzed addition reactions of diboron compounds provide 1,2-bisborylalkanes from l-alkenes ° or cis-l,4-bisboryl-2-alkenes from 1,3-dienes.2,3-Bisboryl-l-propenes are accessible from allenes (Scheme 5-174). The asymmetric allene diboration was examined by Burks et ai, who showed that monodentate phosphoramidite ligands gave excellent enantioselectivities (up to 97% ee) (Scheme 5-174). The (asymmetric) metal-catalyzed reactions of diboron, silylboron and disilane compounds have been reviewed.Recently, the addition across two triple bonds to give dienes has been explored by Singidi et... 

Catalytic, enantioselective halolactonization of (Z)-l,3-enynes (39) and 1,1-disubstituted alkenes with NBS has been reported to occur in the presence of chiral urea derivatives, such as (41), to produce allene derivatives (40) in <91% ee ... 

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