Enantioselectivity, with allylsilanes

Other reactions not described here are formal [3 -i- 2] cycloadditions of a,p-unsaturated acyl-fluorides with allylsilanes [116], or the desymmetrization of meso epoxides [117]. For many of the reactions shown above, the planar chiral Fe-sandwich complexes are the first catalysts allowing for broad substrate scope in combination with high enantioselectivities and yields. Clearly, these milestones in asymmetric Lewis-base catalysis are stimulating the still ongoing design of improved catalysts. 

The allylation of a-iV-tosylimino esters, highly activated imines, proceeds by catalytic use of a Lewis acid. Tol-BINAP-Cu(l) complexes are effective catalysts of asymmetric allylation of these imines with allylsilanes (Equation (28)).124 125 The presence of an aryl group at the position j3 to silicon improves the enantioselectivity. 

Condensation of achiral aldehydes with allylsilanes promoted by CAB catalyst (2) (20 mol %) at —78 °C in propionitrile produces homoallylic alcohols with excellent enantioselectivity (eq 7). 

The CAB-catalyzed reactions of aldehydes with allyl- and crotylsilanes are summarized in Table 11-28 [288, 309], These data show that the CAB-catalyzed crotylation reactions with crotylsilanes 123b and 460 are highly diastereo- and en-antioselective, while the corresponding allylation reactions with allylsilanes 131 occur with somewhat lower enantioselectivity. 

Cross-coupling of o-trimethylsilylbenzylmagnesium bromide with alkenyl bromides catalyzed by the PPFA-palladium complex gave optically active allylsilanes 11 (eq. 6). Allylsilane 11a reacted enantioselectively with the prochiral carbonyl compounds 12 in the presence of TiCl4 to produce alcohols 13 of over 90% enantiomeric purity (eq. 7). ... 

Other reactions of oxetanes reported include the asymmetric ring expansion of 2-substituted oxetanes in an enantioselectively catalysed reaction with diazoacetic esters to give 2,3-disubstituted tetrahydrofiirans <01T262I>, and the Lewis acid promoted reactions of 2-methoxy-2-siloxyoxetane with allylsilane <00MI651 >. 

MacMillan and co workers have significantly expanded the scope of this enamine-mediated procedure by the addition of stoichiometric amounts of oxidant that leads to the in situ formation of a radical cation (12.57). This intermediate then undergoes enantioselective radical-based addition with a range of unsaturated substrates (12.58). For example, a-allylation with allylsilanes such as (12.60) can be effected with high ee using CAN as oxidant in the presence of imidazohdinone (12.61) as catalyst, while an a-heteroarylation occurs using N-Boc pyrrole. Furthermore, an asymmetric a-enolation of a range of aldehydes can be achieved by addition of silyl enol ethers such as (12.64). 

Enantioselective organocatalyzed inverse-electron-demand hDA reactions of p,y-unsaturated a-keto esters with enecarbamates (14CEJ16753) or with allylsilanes (14AGE6131) led to 2-substituted 3,4-dihydro-2ff-pyrans and of a,P"Unsaturated aldehydes with acyl phosphonates gave access to... 

Catalytic enantioselective allylation with allylsilanes has also been achieved with chiral Lewis acid catalysts such as Ti/binol " and chiral (acyloxy)borane complexes (Scheme 3-89). 

In pioneering studies, Yamamoto reported catalytic, asymmetric allylation reactions of aldehydes with allylsilanes in the presence of chiral acyloxy-borane complexes (CAB, 188) as shown in Equation 11 [120]. The observed enantioselectivities are impressively high for y-alkyl-substituted allylsilanes (>90% ee), but only modest for the parent allyltrimethylsilane (55% ee). 

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... 

Addition of HSiMeCl2 to cyclopentadiene is catalyzed by (42) at room temperature at low catalyst loading (0.01 mol.%) to give the (S)-allylsilane in moderate ee.125 This enantioselectivity can be enhanced dramatically with incorporation of perfluoroalkyl groups into the amine functionality (Scheme 26).126... 

In the asymmetric hydrosilylation of 1,3-cyclohexadiene 38 (Scheme 10, Table 4), catalyzed by chiral ferrocenylphosphines 5 and 40, the enantioselectivity is higher with phenyldifluorosilane than that with trichlorosilane or methyldichlorosilane (entries 1—4). The reaction of 38 with phenyldifluorosilane in the presence of a palladium catalyst coordinated with ferrocenylphosphine 40b gave allylsilane (A)-39c with 77% ee.58,59 The use of (j3-N-sulfonylaminoalkyl (phosphine 35a for the reaction of 38 with methyldichlorosilane exhibited the same level of asymmetric induction (entries 5-6).53 In this asymmetric hydrosilylation, combination of trichlorosilane and... 

The enantioselective addition of an allylsilane to an aldehyde catalyzed by chiral acyloxyborane (CAB) 13 is an excellent method for obtaining optically active homoallyl alcohols.Itsuno and Kumagai reported that the synthesis of a new optically active polymer with chirality on the mainchain is possible by applying this reaction to the asymmetric polymerization of bis(allylsilane) and dialdehyde (Scheme 12.11). ... 

Lewis acid-catalyzed stereoselective addition of crotylsilanes to chiral 74 has been studied in detail111,112. The presence of the chiral auxiliary at C2 (e.g. p-tolylsulfinyl or menthoxy carbonyl group) induces the diastereofacially selective addition of cyclopentenones with crotylsilanes. Thus, ( )-crotylsilane favors the erythro product, whilst (Z)-isomer favors the threo product. High enantioselectivity is observed in both reactions (equation 48). In a similar manner, conjugated addition of allylsilane to 75 proceeds with high efficiency (equation 49)113. Interestingly, the yield and enantiomeric excess of the product is dependent on the amount of TiCL used and the best selectivity... 

Sharpless asymmetric dihydroxylation of simple allylsilanes yields the corresponding diols 182 with moderate enantioselectivity (equation 154)273. However, when 183 is treated under similar conditions, substituted /-lactones 184a and 184b are obtained in high diastereo- and enantioselectivities (equation 155)274-277. 

It is noteworthy that the ene product 227 is obtained exclusively when allylsilane 226 is treated with butynone in the presence of Znl2 catalyst and molecular sieve (ms) 4A(equation 185)331. Methyl vinyl ketone behaves similarly. High enantioselectivity has been observed in product 228 when triphenylallylsilane is coupled with methyl glyoxylate in the presence of (R)-(BINOL)TiCl2 catalyst (equation 186)332. In addition to [2 + 2] cycloaddition, TCNE undergoes a regiospecific ene reaction with y-alkyl substituted allylsilanes to yield the substituted olefins 229 (equation 187)333. 

The allylation reaction between ketones and allylsilanes was achieved in 2005. Yamamoto and Wadamoto developed the asymmetric allylation reaction in the presence of AgF-Difluorphos (Scheme 9.6).12 The reaction of ketones and allyltrimethoxysilane in the presence of AgF and Difluorophos afforded the corresponding tertiary homoallyhc alcohols with high enantioselectivities. Additionally, a,(3-unsaturated ketones could be used as substrates, and this catalytic system could be applied for the asymmetric crotylation reaction to obtain anti adducts preferentially (Schemes 9.7 and 9.8). When a,p-unsaturated ketones were used as substrates, 1,2-addition products were obtained exclusively. As described before, the anti adducts were obtained predominately, regardless of the geometry of crotyltrimethoxysilane. 

Asymmetric hydrosilylation of 1,3-dienes provides convenient access to optically active a-chiral allylsilanes.107 1073 1071 The combination of 7r-allylpalladium chloride dimer with axially chiral monophosphine ligand 17 realizes high catalytic activity and enantioselectivity in the reaction of cyclic 1,3-dienes with HSiCl3.108,108a The allyltrichlorosilanes obtained react with aldehydes in a syn-Se mode to give homoallyl alcohols with high diastereo-and enantioselectivity (Scheme 9). 

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

Treatment of the (.V)-( A )-[(.svr-allylo y )dimcsitylsilyl]stannanc with -BuLi provides a (R)-(.E)-allylsilane with high enantioselectivity, in which 1,3-chirality transfer occurs during the [2,3]-sila-Wittig rearrangement (Equation (118)).290... 

Somfai et al. demonstrated that it is also possible to use A-sulphonyl groups (Somfai and Ahman 1992 Ahman and Somfai 1992 Weinreb 1997) using the same concept for the cyclization step developed by Speckamp with the allylsilanes (Esch et al. 1987). In this case, the starting product was the L-pyroglutamic acid, inside the succinimide (Esch et al. 1987), allowing the enantioselective synthesis of the (+)-anatoxin-a. The key step consisted of an intramolecular cyclization of an 7V-tosyl iminium ion, catalysed by a Lewis acid, TiCl, to set up the desired bicyclic ring system (Somfai and Ahman 1992). 

Allystannanes are more nucleophilic than allylsilanes. Addition of achiral allylstannanes to achiral aldehydes in the presence of (1) (20 mol %) and Trifluoroacetic Anhydride (40 mol %) also affords homoallylic alcohols with high diastereo- and enantioselectivities (eq 8). 

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