Lewis enantioselective allylation

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

The first example of a chiral Lewis base promoted allylation was given by Denmark and coworkers in 1994 [45], Stoichiometric amounts of chiral phospho-ramide R,R)-20 facilitated the enantioselective allylation (Scheme 15). There was a complete stereochemical correlation between the geometry (ElZ) of allylsilane and the diastereomeric ratio (synlanti) of the products. 

Amine A-oxides, possessing the property of Lewis basicity, have also been exploited in an enantioselective allylation. Maikov and Kocovsky prepared a series of chiral A-oxide catalysts and found, that ligands 23 and 25 afforded good yield and stereoselectivity (Scheme 17) [49-51]. 

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. 

Chiral (acyloxy)borane (CAB) is known as an effective chiral Lewis acid catalyst for enantioselective allylation of aldehydes. Marshall applied the CAB complex 1 to the addition of crotylstannane to achiral aldehydes and found that the CAB catalyst gives higher syn/anti selectivity than BINOL/Ti catalysts in the reaction [4]. CAB complex 2 was utilized in asymmetric synthesis of chiral polymers using a combination of dialdehyde and bis(allylsilane) [5] or monomers possessing both formyl and allyltrimethylsilyl groups [6]. 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

A spectacular activation of the chiral zirconium-BINOL Lewis acid complex was achieved by the addition of the (achiral ) r-butyl-calix[4]arene. Less than 2% of the catalyst were sufficient in the enantioselective allylation of various aldehydes by allyltributyltin to reach enantiomeric excesses of more than 90%, see Casolari, S. Cozzi, P. G. Orioli, P. Tagliavini, E. Umani-Ronchi, A. Chem. Commun. 1997, 2123-2124. 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

Chiral Lewis bases, 81,82, and 83 modified from HMPA and biquinoline dioxide were developed by several groups for a new type of enantioselective allylation (Sch. 59) [102], Aromatic and conjugate aldehydes react with allyltrichlorosilane with moderate to good ee values under mild eonditions but for aliphatic aldehydes reactivity and selectivity are poor. 

The allylation reactions of carbonyl compounds catalyzed by chiral Lewis acids represent a powerful new direction in allylmetal chemistry. Yamamoto and coworkers reported the first example of the catalytic enantioselective allylation reaction in 1991, using the chiral (acyloxy)borane (CAB) catalyst system (see below) [288]. Since then, several additional reports of the catalytic allylation reaction have appeared. To date, the most effective catalyst systems reported for the enantioselective reaction of aldehydes and Type II allyl- and crotylstannane and silane reagents include the Yamamoto CAB catalyst and catalysts complexes composed of various Lewis acidic metals and either the BINOL or BINAP chiral ligands [289-293]. Marshall and Cozzi have recently reviewed progress in the enantioselective catalytic allylation reaction [294, 295]. 

The l,T-binaphthalene-2,2 -diol complexes of Lewis acids have received considerably more attention. Mikami hrst reported the apphcation of BtNOL/ri(IV) complexes for enantioselective allylations of glyoxylates.ii Keck" and Umani-Ronchi/Tagliavinin independently devised allylation procedures with (/ )-BtN0L/Ti(0-iPr)4 and (5)-BIN0L/Ti(0-iPr)2Cl2, respectively (Scheme 5.2.83). The use of molecular sieves is essential for high reactivity and stereoselectivity. 

Highly activated imines such as u-N-losylimirio esters can be catalytically allylated wifh a Lewis acid. j0rgensen [420] and Lectka [227 b] have reported the Lewis acid-catalyzed enantioselective allylation of an a-imino ester wifh allylsilanes (Scheme 10.148). Tol-BINAP-Cu(I) complex is effective in this asymmetric process as well as the Mannich-type reaction wifh silyl enolates. Aromatic substituents on fhe allyl group dramatically improve fhe enantioselectivity. 

To accelerate aUylation with aUylstaimanes, addition of a Lewis acid is often required, because coordination of the Lewis acid to the carbonyl can enhance the electrophilicity of the substrate and facilitate the couphng reaction. Since Yamamoto showed that a nonracemic Lewis acid, chiral (acyloxy)borane (CAB), catalyzed enantioselective allylation [73], chiral Lewis acid catalysts have been extensively developed. Above all, easily available chiral compounds such as BINOL and BINAP have been most frequently used as chiral auxiliaries [74], and enantioselective allylations of C-N double bonds and of carbonyl groups have been achieved [48a, 75]. 

Asymmetric allyation of carbonyl compounds to prepare optically active secondary homoallyhc alcohols is a useful synthetic method since the products are easily transformed into optically active 3-hydroxy carbonyl compounds and various other chiral compounds (Scheme 1). Numerous successful means of the reaction using a stoichiometric amount of chiral Lewis acids or chiral allylmetal reagents have been developed and applied to organic synthesis however, there are few methods available for a catalytic process. Several reviews of asymmetric allylation have been pubHshed [ 1,2,3,4,5] and the most recent [5] describes the work up to 1995. This chapter is focussed on enantioselective allylation of carbonyl compounds with allylmetals under the influence of a catalytic amount of chiral Lewis acids or chiral Lewis bases. Compounds 1 to 19 [6,7,8,9,10,11,12,... 

Catalytic enantioselective allylations of aldehydes already published can be classified into two methods carried out under the influence of chiral Lewis acid catalysts and chiral Lewis base catalysts. The process by chiral Lewis acid catalysts generally uses allyltrimethylsilane or allyltrialkylstannane as an allylating agent, both of which show low reactivity toward aldehydes without these catalysts. The process by chiral Lewis base catalysts employs allyltrichlorosilane or allylmetals possessing relatively higher reactivity. Both processes can be successfully applied to various substituted allylmetal compounds or allenylmetal compounds. 

Various methods which use a stoichiometric amount of chiral Lewis acids or chiral Lewis bases are available for enantioselective allylation and related reactions... 

S. E. Denmark and J. Fu, Catalytic enantioselective allylation with chiral Lewis bases, Chem. Commun., (2003) 167-170. 

The enantioselective allylation of aldimines 8 with the tetraallylsilane-TBAF-MeOH system with use of the chiral bis-7i-allylpalladium catalyst 20a under catalytic, non-Lewis acidic, essentially neutral, and very mild reaction conditions has been achieved (Eq. 11) [9]. The reaction of imines 8 with 1.2 equivalents of tetraallylsilane 25d in the presence of 5 mol% of the chiral bis-Ti-allylpalladium catalyst 20a, 25 mol% of TBAF, and 1 equivalent of methanol in THF-hexane (1 2) cosolvent furnished the corresponding homoallylamines 9 in high yields and good to excellent enantioselectivities. 

Asymmetric ene reaction of N-sulfinylcarbamatesf The ability of Lewis acids to promote ene reactions (11,413,414 12,389) is useful for asymmetric reactions. Thus the SnCU-promoted reaction of chiral N-sulfinylcarbamates (1) with alkenes results in thermally unstable adducts (2) in 65-91% yield. Use of trans-2-phenylcyclohexanol (13,244) or 8-phenylmenthol as the source of chirality results in high diastereoselective induction in generation of the new carbon to sulfur bond (usually >95 5). This reaction is applicable to both (E)- and (Z)-alkenes, but the former react more readily. These ene adducts can be transformed into optically active allylic alcohols (4) by N-alkylation and conversion to an aryl allylic sulfoxide (3), which undergoes rearrangement in the presence of a thiophile (piperidine) to 4, with retention of configuration at carbon imparted in the ene reaction. The overall process effects enantioselective allylic oxidation of an alkene with retention of the original position of the double bond. 

ABSTRACT. Pentacoordinate organosilicon species behave as Lewis acid as well as nucleophile. This unique character provides several interesting applications in selective reactions useful for organic synthesis. Accordingly, new reduction of carbonyl compounds with pentacoordinate hydridosilicates, new regio- and enantioselective allylation of aldehydes with pentacoordinate allylsilanes, and the new cleavage reaction of C-Si bonds are discussed. In these reactions, it is demonstrated that unique characters of pentacoordinate silicon species as a Lewis acid play an important role. 

Gutmann, V. (1998). The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York. Denmark, S. E., and Wynn, T. "Lewis Base Activation of Lewis Acids Catalytic Enantioselective Allylation and Pro-pargylation of Aldehydes."/. Am. Chem. Soc., 123,6199-6200(2001). 

Stereoselective Allylation Reaction. The enantioselective allylation of aldehydes with allyltrimethylsilane is now possible with good control. One example is the use of a chiral acyloxy borane (CAB) catalyst. Besides that, chiral Ti(OiPr)2X2-BINOL and TiFa-BINOL catalysts are effective for the enantioselective allylation of glyoxylates and aldehydes, respectively (eq 35). Chiral homoallylamines can be prepared from the reaction of imines and allyltrimethylsilane using chiral tt-allylpalladium complexes with TBAF (eq 36) or an (5)-Tol-BINAP-CuPFe system. Free radical allyl transfers from allyltrimethylsilane provides another method for this enantioselective C-C bond formation. Promoted by chiral Lewis acids, both... 

Demnark SE, Fu J (2000) On the mechanism of catalytic, enantioselective allylation of aldehydes with chlorosilanes and chiral lewis bases. J Am Chem Soc 122 12021-12022... 

Denmark SE, Fu J (2003) Catalytic enantioselective allylation with chiral lewis bases. Chem Commnn 167—170... 

Denmark SE, Wynn T (2001) Lewis base activation of lewis acids catalytic enantioselective allylation and propargylation of aldehydes. J Am Chem Soc 123 6199-6200... 

More than a decade ago, Yamamoto and coworkers demonstrated that the chiral acyloxyborane (6) is an excellent catalyst for the Sakurai-Hosomi allylation of aldehydes to furnish the homoallylic alcohols in good yields and enantioselectivities (Equation 60) [59]. This system remains to date, the most effective catalytic enantioselective allylation of aldehydes based on boron Lewis acids. 

---