Aldehydes asymmetric allylation

Enamines derived from ketones are allylated[79]. The intramolecular asymmetric allylation (chirality transfer) of cyclohexanone via its 5-proline ally ester enamine 120 proceeds to give o-allylcyclohexanone (121) with 98% ee[80,8l]. Low ee was observed in intermolecular allylation. Similarly, the asymmetric allylation of imines and hydrazones of aldehydes and ketones has been carried out[82]. 

The best procedure reported to date for the asymmetric allylation of aldehydes using tributyl(2-propenyl)stannane involves the catalyzed addition with the BINOL-TiCl2 complex as catalyst. Good yields and ee s were obtained for both aromatic and aliphatic aldehydes using 20 mol% of the catalyst127. 

Asymmetric Allylation. One of the recent new developments on this subject is the asymmetric allylation reaction. It was found that native and trimethylated cyclodextrins (CDs) promote enantiose-lective allylation of 2-cyclohexenone and aldehydes using Zn dust and alkyl halides in 5 1 H2O-THF. Moderately optically active products with ee up to 50% were obtained.188 The results can be rationalized in terms of the formation of inclusion complexes between the substrates and the CDs and of their interaction with the surface of the metal. 

In asymmetric reactions, chiral phosphine ligands such as BINAP derivatives are used as effective chiral ligands in silver complexes. In particular, an Agr-BINAP complex activates aldehydes and imines effectively and asymmetric allylations,220-222 aldol reactions 223 and Mannich-type reactions224 proceed in high yield with high selectivity (Scheme 51). 

A fast and efficient molybdenum-catalyzed asymmetric allylic alkylation under noninert conditions has been reported using MW-accelerated reaction [178]. Inter-molecular hydroacylation of 1-alkenes with aldehydes has been presented as a greener alternative to classical approach using a homogeneous catalyst in toluene. 

Systematic studies of the reactions of tartrate allyl-boronates with a series of chiral and achiral alkoxy-substituted aldehydes show that conformationally unrestricted a- and /f-alkoxy aldehyde substrates have a significant negative impact on the stereoselectivity of asymmetric allyl-boration. In contrast, con-... 

As an alternative approach, chiral Lewis base has been tested for catalytic allylation. Compound 139, reported by Iseki et al.,88 was the first example of a chiral Lewis base that effectively serves as a catalyst in asymmetric allylation in combination with HMPA. Allylation of aliphatic aldehydes with allyl- and crotyltrichlorosilanes in the presence of 139 provides up to 98% ee (Scheme 3-49). 

This procedure describes the preparation and use of an effective chiral catalyst for the asymmetric allylation of aldehydes. A previous synthesis of optically pure 1-(phenylmethoxy)-4-penten 2-ol requires seven steps from D-mannitol.4 This procedure has been employed successfully with other aldehydes,5 and also with methallyltributylstannane5 (see Table). Catalysts prepared from (R)- or (S)-BINOL and Ti(0-i-Pr)4 at 2 1 stoichiometry have also proven useful in these reactions.The olefinic products may be regarded as latent aldol products between aldehydes and the enolate of actetaldehyde or acetone. In all cases examined thus far, enantioselectivity... 

CATALYTIC ASYMMETRIC ALLYLATION REACTIONS WITH OTHER ALDEHYDES... 

Leighton has combined this concept of strained silacycles [62-66] with the asymmetric allylation chemistry in a series of pubhcations [60, 67-70], Leighton s aUyhc silacyclopentane 29 [67] (Scheme 20) works equally for allylation of aromatic and ahphatic aldehydes in the absence of additional Lewis bases (promoter activator) or Lewis acids with high yield and enantioselectivity. The mechanism of... 

Furthermore, next to aldehydes, acylhydrazones have been used in the allylation reaction. Kobayashi and coworkers found that achiral phosphineoxides catalyze the allylation of acylhydrazone [71, 72], Next, a method for an asymmetric allylation of iV-acylhydrazone with chiral BINAP dioxide 30 was developed... 

Next to P(0) or N(0) Lewis bases, there are very rare cases where enantiopure sulfoxides are used in combination with silanes. Kobayashi and coworkers reported a highly diastereoselective and enantioselective allylation of hydrazones with chiral sulfoxide 32 (Scheme 23) [76], Massa [77,78] and Bamess [79] reported the asymmetric allylation of aldehydes with enantiopure sulfoxides, respectively, with moderate selectivity. 

Acylals (geminal diacetates) are frequently used as protecting groups for aldehydes because of their stability to neutral and basic conditions [8]. In addition, the acylal functionality can be converted into other useful functional groups [9]. For example a novel synthesis of chiral allylic esters has been developed using palladium-catalyzed asymmetric allylic alkylation of gem-diesters [10]. The allylation of... 

SCHEME 122. Lewis acid-catalyzed asymmetric allylation of aldehydes. 

Asymmetric allylation of aldehydes.3 In the presence of (S)-l-[l-methyl-2-pyrrolidino]methylpiperidine (1, 2 equiv.) and Sn(OTf)2 (2 equiv.) prochiral al-... 

Keck reported an asymmetric allylation with a catalytic amount of chiral titanium catalyst [24]. The enantioselective addition of methallylstannane to aldehydes is promoted by a chiral catalyst 13 prepared from chiral BINOL and Ti(0-i-Pr)4 (Scheme 9.10). An example of asymmetric amplification was reported by using (R)-BINOL of 50% ee, and the degree of asymmetric amplification was dependent on the reaction temperature. Tagliavini also observed an asymmetric amplification in the enantioselective allylation with a BIN0L-Zr(0-i-Pr)2 catalyst [25]. 

Stereo specific generation and reactions of allylic alkali and alkaline earth metals have been reviewed121 and solvent-mediated allylation of carbonyl compounds with allylstannanes has been explored.122 Chiral phosphoramides derived from (5 )-proliiie have been used to catalyse asymmetric allylation of aromatic aldehydes by allylic trichlorosilanes.123... 

CPrSSiMe3 catalyses the asymmetric allylation of aldehydes using (S)-BINOL-Ti(IV) complex (equation 8)15. 

Chiral (salen)chromium(III) complexes catalyse the asymmetric allylation of a range of aldehyde types, using allylstannane reagents.171... 

A related N-oxide organocatalyst of type 178, developed by Maikov and Kocovsky et al., has been used for successful asymmetric allylation of aldehydes [178]. It is worthy of note that the corresponding N,N -dioxide gave less satisfactory results. In the presence of 7 mol% N-monoxide 178, aromatic aldehydes have been con-... 

Asymmetric allylation of RCHO.1 The reagent converts aldehydes to homoal-lylic alcohols in 94-99% ee, which is significantly higher than the enantioselectivi-ties obtained with B-allyl(diisopinocampheyl)borane. The enantiomer of ( + )-2-car-ene is not known, but B-allylbis(4-isocaranyl)borane (2) behaves as an enantiomer of 1 in allylation of aldehydes. 

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