Ketones allylsilanes

Table 9. TiCVassisted 1,4-addition of allylsilanes. Unsaturated ketone Allylsilane...

CONJUGATE ALLYLATION OF a. -UNSATURATED KETONES WITH ALLYLSILANES 4-PHENYL-6-HEPTEH-2-0NE (6-Hepten-2-one, 4-pheny1-)... 

Titanium(IV) is a powerful but selective Lewis acid which can promote the coupling of allylsilanes with carbonyl compounds and derivatives In the presence of titanium tetrachlonde, benzalacetone reacts with allyltnmethylsilane by 1,4-addition to give 4-PHENYL-6-HEPTEN-2-ONE. Similarly, the enol silyl ether of cyclopentanone is coupled with f-pentyl chloride using titanium tetrachlonde to give 2-(tert-PENTYL)CYCLOPENTANONE, an example of a-tert-alkylation of ketones. 

Ring formation by intramolecular reaction between an allylsilane and an aldehyde or ketone is well precedented54, 55. 

Of particular interest are reactions between a-hydroxy ketones and allyl(trifluoro)silanes which proceed with participation of the hydroxy group via a pentacoordinated allylsilane, with intramolecular delivery of the allyl group to the ketone. Excellent stereoselectivity is obtained at up to three contiguous stereogenic centers66. 

Chelation control of the intramolecular reaction between an allylsilane and an aldehyde or ketone has been carefully investigated. Excellent stereoselectivity was found for cyclization of B-oxo esters using titanium(IV) chloride as the Lewis acid, less good selectivity for cyclization of /l-diketones70. 

Almost 15 years ago Sakurai and Hosomi, in pioneering work, showed that intermolecular addition of an allylsilane to a,j6-unsaturated ketones in the presence of titanium(IV) chloride as the Lewis acid gave the desired 1,4-addition products1 4. In the case of 4,4a,5,6,7,8-hexahy-dro-2(3//)-naphthalenone, reaction was shown to proceed by 1,4-addition with exclusive production of the ris-fused product in high chemical yield. 

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... 

Similar conditions are used to effect reactions of allylsilanes with acyl halides, resulting in (3,y-unsaturated ketones.128... 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

A one-pot reaction between a tryptophan ester, benzotriazole, and 2,5-dimethoxytetrahydrofuran in acetic acid gives the diastereomeric benzotriazolyl tetracycles, 349, in good yield. Substitution of the benzotriazole by reaction with silyl enol ethers and boron trifluoride etherate gives the corresponding ketones 350 and 351, and reaction with allylsilanes gives the corresponding alkenes 352 and 353. If the boron trifluoride etherate is added to the mixture before the silane, elimination of benzotriazole from 349 is also observed (Scheme 83) <1999T3489>. 

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... 

Treatment of a-(benzotriazol-l-yl)alkyl thioethers 831 with ZnBr2 weakens the bond with benzotriazole, and the obtained complex 832 may partially dissociate to thionium cation 835 that can be trapped by even mild nucleophiles. Thus, trimethylsilyl cyanide added to the reaction mixture causes substitution of the benzotriazole moiety by the CN group to give a-(phenylthio)carbonitrile 834. In a similar manner, treatment with allylsilane leads to y,S-unsaturated thioether 833. Addition of species 835 to the double bond of a trimethylsilyl ot-arylvinyl ether followed by hydrolysis of the silyloxy group furnishes (i-(phenylthio)alkyl aryl ketones 836 (Scheme 132) <1996TL6631>. 

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... 

The most direct route to the 1,4-dicarbonyl equivalent required for the aldol condensation would be to couple the enol ether of an aldehyde with the enol ether of a ketone. However, this sequence proved impractical due to the hydrolytic instability of the ketone enol ether. Even after an extensive effort, the substrate for the electrolysis reaction could not be reproducibly prepared in high yield. These problems were readily avoided with the use of an allylsilane based... 

Intramolecular cyclization of tethered phenyl ketones (58 X = Br, SiMe3) show contrasting stereochemical outcomes for indium catalysis of the alkyl bromides and fluoride ion-induced reaction of the allylsilanes. The reactions thus allow complementarity in product diastereoselectivity, and the difference appears to be related to an... 

Homoallylk ethers. lodotrimethylsilane catalyzes the allylation of acetals and ketals by allylsilanes, with transposition of the allylic group (equation I). It does not catalyze allylation of aldehydes and ketones. Note that TiCI can catalyze both of these reactions (7, 370-371). In this respect, ISi(CH3)3 resembles (CH3)3SiOTf (this volume). 

The size of silyl groups influences the carbon-carbon bond formation between sily-lalkenes and ketones. Treatment of allylsilanes 12 or vinylsilanes 14 with ketones in the presence of MnC>2 and acetic acid at elevated temperature gives a-alkylation products 13 and 15, respectively (equations 9 and 10)18. The steric effect resulting from the silyl groups plays an essential role on the exclusive C—C bond formation at the terminal sp2 carbon of silylalkenes 12 and 14. The yield of the alkylation is inverse to the size of silyl groups and follows the order listed in entry 7 of Table 1. 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

Parallel to an earlier work on the highly diastereoselective reactions of aliphatic aldehydes with allylsilane in the presence of 3976, treatment of methyl ketones under the same conditions yields the corresponding tertiary homoallylic ether with a diastereomeric excess of up to 90% (equation 25)77. 

The TiCU-induced three-component coupling reaction of an a-haloacylsilane, allylsilane and another carbonyl compound gives 48 in good yield. A silyl enol ether intermediate is suggested (equation 31)82. The reaction of a cyclopropyl ketone with allylsilane yields a mixture of skeletal rearranged products83. 

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. 

A highly selective synthesis of homoallylic alcohols has been reported by Tietze et al.,917 who reacted methyl ketones, the chiral norpseudoephedrine derivative 285, and an allylsilane in the presence of a catalytic amount (0.2 mol%) of triflic acid [Eq. (5.340)]. The transformation was interpreted as an SN2 attack of the allylsilane to the protonated mixed acetal 286. The obtained ethers were then cleaved to the final product, homoallylic alcohols. 

Allylsilanes.1 A general route to allylsilanes involves addition of the anion of 1 lo aldehydes and ketones to form an adduct (2). Mesylation in situ, of the adducts followed by sodium amalgam reduction, results in allylsilanes (4) in 85-95% overall yield, based on the carbonyl compound. 

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