Lewis-base asymmetric allylation reactions

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 chemistry of asymmetric allylation of carbonyl compounds has further progressed since the review in Comprehensive Asymmetric Catalysis [1] and plenty of papers including reviews [2,3] on chiral catalysts for the reaction have since appeared. This chapter describes new examples of catalytic enantioselec-tive allylation of carbonyl compounds with allylmetals in the presence of a catalytic amount of chiral Lewis acid or chiral Lewis base (Scheme 1). Compounds 1-36 [4-49] shown in Fig. 1 are the chiral catalysts reported since 1998, which have been used in the asymmetric allylation or propargylation of carbonyl compounds. Chiral compounds 37-40 [50-53], which have been utilized in the stoichiometric version, are also candidates for the chiral catalyst (Fig. 2). 

Catalytic asymmetric allylations of aldehydes or ketones are roughly classified into two methods, namely, those using chiral Lewis acid catalysts and those using chiral Lewis base catalysts. The former method uses less reactive allylsilanes or allylstannanes as the allyl source. The latter method requires allyltrichlorosi-lane or more reactive allylmetals. Both processes are applicable to the reactions with substituted allylmetal compounds or propargylation. 

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. 

The observed activation of allyltrihalosilanes with fluoride ion and DMF and the proposition that these agents are bound to the silicon in the stereochemistry-determining transition structures clearly suggested the use of chiral Lewis bases for asymmetric catalysis. The use of chiral Lewis bases as promoters for the asymmetric allylation and 2-butenylation of aldehydes was first demonstrated by Denmark in 1994 (Scheme 10-31) [55]. In these reactions, the use of a chiral phos-phoramide promoter 74 provides the homoallylic alcohols in high yield, albeit modest enantioselectivity. For example, the ( )-71 and benzaldehyde affords the anti homoallylic alcohol 75 (98/2 antUsyn) in 66% ee. The sense of relative stereoinduction clearly supports the intermediacy of a hexacoordinate silicon species. The stereochemical outcome at the hydroxy center is also consistent with a cyclic transition structure. 

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,... 

The catalytic asymmetric allylation of aldehydes is another reaction that has received a great deal of attention. Both allylstannes and the less reactive allylsilanes undergo addition to aldehydes with high ee in the presence of enantiomerically pure Lewis acids and Lewis bases and asymmetric versions of the chromium-catalysed Kishi-Nozaki-Hiyama reaction utilising allyl halides have recently been developed. 

Allylation of simple ketone is not possible under usual conditions, but the reaction can be carried out under selected conditions. Asymmetric allylation of the chiral racemic o -methylcyclohexanone 161 with allyl carbonate proceeded in the presence of LDA as a base with or without MesSnCl as a Lewis acid at room temperature to provide the allylated ketone 162 in very high yield with 82 % ee when (5,5)-Trost L-1 was used. The choice of base is crucial, and it was claimed that no reaction took place when Na or K bases were used in this reaction [57]. Asymmetric allylation of a-aryl and heteroaryl ketones has been carried out. Asymmetric allylation of 2-indolylcyclohexanone 163 took place at 0 C to give the the allyl ketone in 82 % yield with 84 % ee. In this reaction, NaHMDS was used as a base and Trost L-2 as chiral ligand [58]. Asymmetric allylation of the tetralone 164 with allyl acetate was carried out using Trost L-6 in the presence of CS2CO3 to provide the allylated ketone with 91 % ee in 90% yield [59]. 

However, Lewis bases of this type are less practical than the previously described phoshoramides (Figure 21.1) and ALoxides (Figures 21.2 and 21.3) as they are commonly required in more than stoichiometric amounts. Furthermore, these chiral promoters are rarely recovered due to reduction of the sulfoxide functionality or decomposition during the reaction. A positive nonlinear effect observed in the asymmetric allylation of aldehydes using chiral sulfoxide 21.30 as a promoter suggests a transition state with two molecules of the catalyst coordinated to silicon in the carbon-carbon bond-forming event. - ... 

To be accurate, the definition should be restricted to asymmetric reactions catalyzed by a combination of l,r-binaphthalene-2,2 -diol (BINOL, 4) and Ti(0 -Pr)4. Nonetheless, this chapter will give some background on non-chiral Lewis acid promoters, and include other asymmetric catalytic systems. We will not discuss the allylations that are promoted by Lewis bases, which are reviewed elsewhere, nor cover the reactions with other electrophiles. Excellent reviews already exist on "Selective Reactions Using Allylic MetaM and Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , as well as in the comprehensive monograph "Modern Carbonyl Chemistry. The use of BINOL-based catalysts in other fields of organic synthesis has also been reviewed. ... 

Originally, enantiosdective allylboration was developed using chiral allylbo-ranes and allyl boronates. These reactions require multistep preparahons of chiral reagents that are used in stoichiometric amoimts, and are therefore impractical. Recently, catalytic asymmetric allylborations were developed. These reactions can apply either chiral Lewis bases or BBonsted acids as the catalysts, hi particular, chiral BlNOL-phosphoric acids were demonstrated to provide high optical yields in the enantioselective allylboration reaction between allylboronate 1 and aldehydes. For example, the catalytic asymmetric allylboration of benzaldehyde 2 proceeded quantitatively yielding the corresponding homoallyl alcohol 3 with 98% ee ( heme 3.1). 

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). 

According to Mayr s nucleophilicity scale (N), silyl enol ethers derived from aldehydes (N > 3.5) and ketones (N > 5) and, in particular, silyl ketene acetals (N > 8) [70] represent powerful nucleophihc reagents. Indeed, the aldol-type addition of trichlorosilyl enol ethers 76a-d to aldehydes 1 proceeds readily at room temperature without a catalyst (Scheme 15.14), which is in contrast with the lack of reactivity of allyl silanes in the absence of a catalyst. As a result, the reaction exhibits simple first-order kinetics in each component [71, 72]. Nevertheless, the reaction is substantially accelerated by Lewis bases, which provides a sohd ground for the development of an asymmetric variant The required trichlorosilyl enol ethers 76 can be generated in various ways, for example (i) from the corresponding trimethylsilyl enol ethers on reaction with SiCLt, catalyzed by (AcO)2Hg,... 

Leighton et al. have applied this concept of strained silacydes [50-54] for the asymmetric allylation in a series of publications [48, 55-57]. Leighton s aUyUc silacyclopentane 21 [55] (Scheme 16.14) allows the aUylation of aromatic and aliphatic aldehydes in the absence of additional Lewis bases (promoter activator) or Lewis acids with high yield and enantioselectivity. The reaction proceeds probably over a cyclic transition state with a trigonal bipyramidal geometry at a penta-coordinated silicon [47, 58]. 

Asymmetric allylations of ArCHO with allyltrichlorosilane in CH2CI2 to form homoallylic alcohols has been Lewis base catalysed by chiral bisformamide-type catalysts (with ee < 83%) and by (f )-methyl p-tolyl sulfoxide. Mechanistic study of the latter reaction supports a dissociative pathway via an octahedral cationic complex with two sulfoxides. The greater stereoselectivity of A-oxide-catalysed allylations, compared to propargylations, has been explained by a simple electrostatic model that should enable design of suitable catalysts for both reactions. ... 

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