Cycloaddition of allylsilanes

Lewis acids play a dominant role on the chemoselectivity of the cycloaddition of allylsilanes and 3-butyn-2-ones. AICI3 and EtAlCl2 promote [2+2] cycloaddition predominantly, whereas TiCLt-mediated reaction gives significant amount of the [3+2] adduct288,289. The reaction of 2,2-bis(trifluoromethyl)ethylene-l,l-dicarbonitrile with allylsilane without any Lewis acid catalysts yields the cyclobutane derivative via a nonconcerted fashion290. 

As for oxygen heterocycles, several reports involving the diastereoselective synthesis of highly substituted tetrahydrofuran derivatives have appeared. SnCLj-mediated [3 + 2] cycloaddition of allylsilane with optically active a-ketoesters affords 211 with excellent diastereoselectivity (equation 173)314-317. 

Formation of A2-isoxazolines 214 can be achieved by the cycloaddition of allylsilanes to NOBF4 in excellent diastereoselectivity (equation 175)318. 

The reaction of homochiral allylsilanes with a-enones and a-enals is very valuable for the asymmetric synthesis of multisubstituted cyclopentanes (Equation (40)).161,162 The [3 + 2]-cycloaddition of allylsilanes is applicable to />-quinones163 and />-quinoneimines.164 3-Butyn-2-one undergoes a double cycloaddition with an excess amount of allyltriisopropylsilane to give a bicyclo[3.3.0]octane in good yield (Equation (41)).165... 

Imines and iminium salts also undergo [3 + 2]-cycloaddition of allylsilanes to afford 3-silylpyrrolidines with high diastereoselectivity.121,177,178... 

Benzyl cations generated from benzyl alcohols or quinone methides by the action of S11CI4 undergo [3-(-31-cycloaddition of allylsilanes leading to tetrahydronaphthalenes.1 With secondary and tertiary benzyl cations, a competing [3 + 2]-pathway forms dihydro(177)indenes. 

The first example of Lewis acid-promoted [2 + 2]-cycloaddition of allylsilanes was introduced by Snider et al. in 1979.185 In the last decade, similar cycloadditions to various electron-deficient unsaturated bonds have been developed for efficient syntheses of cyclobutanes, cyclobutenes, oxetanes, and azetidines as described below. 

The [2 + 2]-cycloaddition of allylsilanes is applicable to the synthesis of substituted oxetanes from aldehydes and ketoesters (Equation (49)).176,193 The BF3-OEt2-promoted reaction of iV-tosylaldimines with allylsilanes gives [3 + 2]-cycloadducts, 3-silylpyrrolidines (vide supra).178 In contrast, iV-acylaldimines are converted into [2 + 21-cycloadducts under similar conditions.194... 

Three modes of reaction of aldehydes with allylsilane bearing sterically demanding silyl substituents are mediated by the proper choice of Lewis acid (Eq. 39) [66a]. Thus, influenced by SnC, allyl-t-butyldimethylsilane reacts with aldehyde in 2 1 stoichiometry to afford a ketone derivative. In contrast, use of BF3 OEt2 leads to the formation of a 1,3-dioxane derivative, which is a 1 2 adduct. Furthermore, ZrCU-pro-moted [2 -r 2] cycloaddition of allylsilane and aldehyde fnmishes oxetanes in good yields [66b]. 

The 1,2-silyl migrative [3+2] cycloadditions of allylsilanes are applicable to a variety of electron-deficient multiple bonds. High diastereoselectivity is usually observed. These silicon-directed reactions are, therefore, valuable for stereocontrolled syntheses of highly functionalized five-membered carbocycles and heterocycles. 

The [3+2] cycloaddition of allylsilanes is applicable to imines and iminium salts [419, 484, 485], Highly substituted pyrrolidines can be synthesized with high diastereo and enantio control by reaction of homochiral crotylsilanes with carbon-nitrogen double bond generated in situ from acetals and methyl carbamate (Scheme 10.174) [419]. The cycloaddition to N-tosylaldimines of aromatic aldehydes proceeds with excellent 2,4-ds selectivity whereas the stereoselectivity with aliphatic aldimines is rather low [484]. With N-tosylaldimines, fhe formation of [2+2] adducts is not observed (vide infra). 

Although it was stated that allyltrialkylsilanes were unsuccessful in the Lewis acid mediated addition to acryloylFp complexes, a stereoselective procedure has been developed for [3 + 2] cycloadditions of allylsilanes to enones10. The reaction occurs in the presence of titanium(IV) chloride (TiCl4) as Lewis acid and the single cycloaddition product formed is the m-fused eyclopentane with the trimethylsilyl group in the endo orientation. The reaction appears to follow a similar pathway to the previous process although, as yet, yields and experimental procedures have not been documented. 

A thermal, Lewis acid-catalysed [3+2] cycloaddition of allylsilanes giving the dihydrobenzofurans 57 has also been described (equation (3)) (02TL5349). The mechanism has been interpreted in terms of formation of a p-silyl cation by a zinc-benzoquinone complex, followed by cyclisation, and elimination of isobutene when R = Bu. 

Cycloaddition of allylsilanes with a,P-unsaturated ketones has been known to be catalyzed by TiCU. The same reaction was possible with ZrCU as shown in Equation 59 [64]. 

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. 

Knolker and coworkers also used a domino [3+2] cycloaddition for the clever formation of a bridged tetracyclic compound 4-172, starting from a cyclopentanone 4-168 and containing two exocydic double bonds in the a-positions (Scheme 4.36) [57]. The reaction of 4-168 with an excess of allylsilane 4-169 in the presence of the Lewis acid TiCLj led to the spiro compound 4-170 in a syn fashion. It follows a Wag-ner-Meerwein rearrangement to give a tertiary carbocation 4-171, which acts as an electrophile in an electrophilic aromatic substitution process. The final step is the... 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

Lewis acid treatment of 1,2,4-trioxolanes gives metallated carbonyl oxides which may be trapped by cycloaddition to allylsilanes to give 1,2-dioxolanes 78 <99TL6553>. 

In the cycloaddition of triisopropylallylsilane to a, /J-unsaturated lactams 197, cyclobutane adducts 198 have been found to be the kinetic products whereas the formation of cyclopentanes 199 is thermodynamically controlled285. Reactions of allenylmethylsilanes with activated unsaturated esters and nitriles (equation 163)286 and allylsilanes with unsaturated esters287 are other examples of using [2+2] cycloaddition to construct cyclobutane derivatives. 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

The synthesis of polysubstituted tetrahydrofurans was also achieved in a stereoselective manner by a formal [3+2] cycloaddition of an allylsilane with a-triethylsiloxy aldehydes. An example showing the mechanism is illustrated <02JA3608>. In another approach, allylsilane was also allowed to react with a-keto esters in a [3+2] annulation reaction, providing highly substituted tetrahydrofurans in good yields as single diastereomers <02OL2945>. 

The Lewis acid-promoted reaction of aldehydes with a-substituted allylsilanes forms 3-silyltetrahydrofurans in good to high yields.169-172 The use of homochiral allylsilanes is very valuable for highly diastereo- and enantio-selective syntheses of tri- and tetrasubstituted tetrahydrofurans (Equation (43)). Catalytic asymmetric [3-1-21-cycloaddition of a-substituted allenylsilanes to aldehydes can be achieved by a chiral scandium complex.173... 

Certain ketones are also usable for [3 + 2]-cycloaddition with allylsilanes.174,174a-174c,17S Interestingly, the reaction of unsubstituted allylsilanes at — 78 °C gives 2-silylmethyloxetanes ([2 + 2]-cycloadducts) mainly, although only [3 + 2]-cycloadducts are obtained at 0°C (Equation (44)).176... 

Spontaneous reaction of iV-chlorosulfonyl isocyanate with a-substituted allylsilanes is valuable for the synthesis of 4-silyl-2-pyrrolidinones.179-181 The [3 + 2]-cycloaddition of crotylsilanes proceeds stereospecifically as does the... 

The BF3-catalyzed reaction of a-aminoaldehydes with 10 is valuable for highly stereoselective synthesis of 2,3,5-trisubstituted pyrrolidines with all -cis configurations (Equation (50)).197 The stereochemical outcome like chelation-controlled stereochemistry might result from the inherent conformational arrangement of the aldehyde-BF3 complex. />-Quinoneimines, o-quinones, and a-alkoxyhydroperoxides undergo similar types of [3 + 2]-cycloadditions with allylsilanes to afford dihydro indoles, dihydrobenzofurans, and 1,2-dioxolanes, respectively.164,175,198... 

Allylsilanes act as good acceptors of nitrones and oxyallyl cations. The 1,3-dipole species arising from electronically activated cyclopropanes can be trapped by allylsilanes.203 204 2043 Epoxides as well as aziridines act as 1,3-dipole precursors for inter- and intramolecular [3 + 2]-cycloadditions with allylsilanes.205 2053 206 2063... 

Use of allylsilanes as 1,2-dipole equivalents is valuable also for the construction of six-membered rings. f3-Oxyaldehydes undergo Lewis acid-promoted cycloadditions with allylsilanes to provide substituted tetrahydropyrans (Equation (52)). 3 A similar [4 + 2]-cycloaddition of /V-/-butoxycarbonyl-0,Ar-acetals is available for the synthesis... 

Au-Yeung and Fleming have devised a new alternative synthesis of loganin (590) based upon allylsilane chemistry.438 The bicyclic ketone 586, obtained by cycloaddition of dichloroketene to trimethylsilylcyclopentadiene, was monodechlorina-ted, ring expanded with diazoethane, and again dehalogenated prior to equilibration... 

Sugimura, H., Uematsu, M. Unusual [2+2]cycloaddition reaction of allylsilanes with 2,3-O-isopropylidene derivatives of aldehydo-aldose catalyzed by boron thfluohde etherate. Tetrahedron Lett. 1988, 29,4953-4956. 

Treatment of the title compound with an aqueous base under phase-transfer catalysis conditions generates, by a-elimination, chloro(trifluoromethyl)carbene that can be trapped by cycloaddition to allylsilane and allylstannane derivatives, e.g. formation of 1 and 2. This method represents a fast and mild alternative to the Seyferth method that starts from the same alkyl halide and generates the carbene by thermal decomposition of l-bromo-l,2,2,2-tetra-fluoroethyl(phenyl)mercury (see Section 1.2.1.2.4.1.1). 

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