Allylation using allylsilanes

Allylation of aldehydes or ketones using allylsilanes, known as the Hosomi-Sakurai reaction, is a useful method for obtaining homoallylic alcohols. TiIV compounds have been successfully applied to this reaction (Scheme 21) 80 Besides aldehydes and ketones, acylsilanes, 0,0-acetals, and A-,(7-acetals can be employed.81-83 1,4-Addition of an allyl group to an a,/ -unsaturated ketone has been also reported.84... 

Scheme 7.8 Propagation steps for allylation reaction using allylsilanes...

Scheme 6 Bi(OTf)34H20-catalyzed allylation using a functionalized allylsilane...

The enantioselective allylation of aldehydes can also be carried out by using allylsilanes as the nucleophile. For example, Yamamoto and co-workers313,313a employed allyltrimethoxysilane for the allylation of various aldehydes in... 

Analogous to the allylation with allylsilanes and -stannanes, the transformations, vinylallylation, propargylation, allenylation, alkenylation, alkynylation, and arylation, are viable by the use of an appropriate reagent in the presence of a titanium Lewis acid these are surveyed in the review articles cited both in the Introduction and in this section. The stereochemistry of the reaction of a (vinylallyl)silane in the presence of TiCU has been reported [234]. Equation (113) shows that the major reaction of this silane and isobutyraldehyde occurred mainly in the anti sense with a ratio of anti to syn attack of 90 10 at the terminus remote from the silyl group. Essentially the same stereochemical outcome was observed for the same reaction with the corresponding trimethylsilyl derivative. The intramolecular reaction with an acetal, however, proceeded less selectively the anti syn ratio was 60 40 (Eq. 114) [234]. 

Allylsilanes and allylstannanes are known to undergo the allylation reactions of aldehydes and ketones in the presence of Lewis acids (the most commonly used are TiCU and SnCU). Zirconium Lewis acids have often been compared with the titanium analogs. For most allylation reactions using allylsilanes the best results were obtained when TiCU or SnCU was used. For allylation reactions using allylstannanes, as illustrated in Eq. (15), however, ZrCU was used as the Lewis acid to obtain the bicyclic compound 36 via the intramolecular allylation of stannylaldehyde (35) [16]. 

Allylations of -alkoxy esters were demonstrated under atom and group transfer conditions using allylsilanes and chelating Lewis acid conditions [28]. Equation (13.18) illustrates the basic mechanism for this reaction. Once again, selectivities greater than 100 1 favoring the anti product and yields of 87% are reported. Results from this study suggest that inclusion of Lewis acids may help facilitate the atom transfer step. 

Elimination of silyl groups with p-oxy groups, i.e., Peterson-type elimination, is a useful method for preparing stereodefined alkenes [101,102]. The synthetically useful allylsilanes are effectively synthesized in geometrically and enantio-merically pure forms through the Peterson-type elimination of organosilanes prepared by palladium-catalyzed bis-silylation (Eq. 57) [72, 73]. The intramolecular bis-silylation of optically active allylic alcohols in refluxing toluene af-... 

To achieve asymmetric allylation with allylsilanes, the control of absolute configuration by use of chiral auxiliaries was extensively studied in the nineteen-eighties. Recently, much effort has been directed toward the use of optically active a-chiral allylsilanes and catalytic asymmetric allylation using chiral Lewis adds and bases. 

Acid-induced reactions of silicon-containing unsaturated compounds with A -acyliminium intermediates have proven particularly useful. Allylsilanes lead to the corresponding allyl-substituted products. This reaction proceeds well even for -lactams (equation 49). 5.86 j, g penicillin-derived 4-acetoxyazet-idinone (91) reacts with excess allylsilane in refluxing dichloroethane, catalyzed by TMSOTf, to produce as the sole product trans compound (92). A -Acyliminium ions generated from cyclic ureas and carbamates (93) likewise yield allylated products with virtually complete trans stereoselectivity (equation SO). Allylstannanes react under somewhat milder conditions than allylsilanes, requiring only a catalytic amount of BFsZ EtjO at room temperature for a 3-lactam system (equation 51). This allylstannane procedure was applied to cyclic carbamate systems for the stereoselective synthesis of several isomers of statine. Thus, ethoxycarbamate (94) reacts with methallyltributylstannane in excellent yield and dia-stereoselectivity (equation 52). ... 

Guindon investigated diastereoselective allylation reactions of alkyl halides and phenyl selenides using allylsilanes [68], For example, substrate 116 is allylated by allyltrimethylsilane in the presence of magnesium bromide diethyl etherate using triethylboranc as an initiator (Scheme 25). No tin is required in these reactions. 

Allylations. In reactions using allylsilane as nucleophiles TiCU is one of the most popular catalysts. It is now employed in allylating squaric acid derivatives, 8-phenylmenthyl pyruvate, and (V-acyliminium ions. A remarkable observation is that the four stereocenters of the products from l,8-bis(trimethylsilyl)-2,6-octadiene and two aldehyde molecules are generated with good control. The stereoselective intramolecular reaction is most suitable for the synthesis of bicyclic a-methylene-y-lactones. ... 

DeShong (Scheme 20). Thus, reaction of nitrones with vinyl silanes followed by reduction provides Peterson - type intermediates which can then be eliminated to either Z- or E-products. Homoallylic amines were also prepared using allylsilane in the initial cycloaddition. Substitution reactions of allylic nitro compounds have received considerable attention and some examples are outlined in Scheme 21. In each case, examination of the regiochemistry of the reaction was of paramount concern, the results using palladium being superior to the SnCl mediated process. Allylic sulphides constitute yet another group of... 

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). 

Notes and discussion. Methyl a-D-gluco- and mannopyranosides and a-D-glycopyranosyl chlorides can react with allylsilanes via activation with trimethylsi-lyl trifluoromethanesulfonate or iodotrimethylsilane to stereoselectively afford the corresponding a-C-allylated glycopyranosides in excellent yields. The yields of the reactions depend on the solvent and the amount of catalyst employed. Acetonitrile is the most suitable solvent whereas dichloromethane, the most commonly used solvent for allylation reactions using allylsilanes, does not afford satisfactory results. The reactions proceed very slowly when promoted by less than 5 mol% of the catalyst, however, the use of 50 mol% of catalyst is sufficient to force the reactions to completion. 

Allylsilanes are intrinsically nonnucleophilic. We believe that addition of fluoride ion to an allylsilane forms a silicate anion, which can react at either end of the tc-system. Thus from a mechanistic viewpoint, substitution of an allylsilane under nucleophilic conditions is more appropriately considered an addition-elimination reaction with the terms Se2 and Se2 aptly representing the elimination process. For recent evidence of pentacoordinate silicate intermediates in fluoride ion catalyzed allylation using trimethylallylsilane, see a) M. Kira, M. Kobayashi and H. Sakurai, Tetrahedron Lett.. 28. 4081 (1987) b) M. Kira, K. Sato and H. Sakurai. J. Am. Chem. Soc.. 110, 4599 (1988) c) T. Hayashi, Y. Matsumoto, T. Kiyoi, Y. Ito, S. Kohra, Y. Tominaga and A. Hosomi, Tetrahedron Lett., in press d) H. Sakurai, Lewis Acid Character and Selective Reactions of Pentacoordinate Silicon Compounds, this volume. 

A different approach is to add substituents to or manipulate substituents on an existing allyl- or propar-gyl-silane system. Thus propargyltrimethylsilane itself can be alkylated at the terminus and the triple bond reduced with Brown s nickel catalyst or by hydroalumination-protonation, both of which give the (Z)-allylsilane. Alternatively, zirconium-catalyzed carboalumination of propargylsilanes introduces more substituents onto the allyl framework.Allylsilane anions show some selectivity for alkylation a to the silyl group, and the geometry of the double bcmd can be controlled by using either the kinetic or the thermodynamic allyllithium intermediate (Scheme 78). ... 

Baba et al. found that the Sakurai-Hosomi reaction, a variant of the carbonyl allylation reaction using allylsilanes, was catalyzed by 5mol% of a InCl3-TMSCl catalytic pair [38]. Good to excellent yields of the products from the y-allylation... 

In the pioneering works by Hosomi and Sakurai, a stoichiometric or substoichio-metric amount of TiCU, a conventional Lewis acid, was used for the carbonyl allylation with allylsilanes [106]. Davis and coworkers found that TMS borates such as Me3Si[BX(OTf)3] (X = OTf, Cl) enables an efficient, catalytic Hosomi-Sakurai allylation of aldehydes although (la) and Me3SiI show low catalytic activities (Scheme 9.41) [18, 107]. Ishihara and Yamamoto have demonstrated the utility of (lb) as Lewis acid catalyst of the carbonyl allylation as well as the Mukaiyama aldol reaction [15]. The silicon Lewis acid (lb), generated in situ by the reaction of allyltrimethylsilane with HNTf2, efficiently promotes the allylation of aldehydes and ketones with a loading of 0.5 mol% (Scheme 9.41). 

Scheme 3-113. Chiral Lewis-base catalyzed enantioselective allylation of aldehydes using allylsilanes.

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

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