01. Amauri F. Patroc Nio.P65

J. Braz. Chem. Soc., Vol. 12, No. 1, 7-31, 2001. c 2001 Soc. Bras. Química Printed in Brazil 0103 - 5053 $6.00+0.00 Review Acylsilanes and Their Applications in Organic Chemistry Amauri F. Patrocínio and Paulo J. S. Moran* Instituto de Química, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas - SP, Brazil Estudos sobre o emprego de acilsilanos em rotas sintéticas e as novas metodologias de preparação desenvolvidas nos últimos anos, tornaram estes organo-silanos importantes reagentes para síntese de compostos orgânicos. Esta revisão apresenta algumas aplicações recentes e vários métodos desenvolvidos para síntese de acilsilanos. Current studies concerning the use of acylsilanes in a variety of organic synthetic routes and the improved methodologies of their preparation have turned organosilanes into important reagents for organic chemistry. This review discusses the recent employment of acylsilanes in organic synthesis and also effective methods for their preparation. Keywords: organosilicon compounds, Brook rearrangement, carbonyl compounds, stereocontrol Introduction are compounds that present particular physical and chemical properties. The spectral data of acylsilanes are well described ´ 2 3 Acylsilanes (RCOSiR 3) are compounds that have the in the reviews by Brook and Page and co-workers . The silicon directly attached to the carbonyl group, exhibiting inductive effect of the silicon favors the polarization of the unique chemical properties. The use of acylsilanes in organic carbonyl group, which absorbs at a lower frequency than synthesis has increased significantly over the last few years due simple ketones in the infrared and ultraviolet spectra. In 13C to the discovery of valuable new reactions and the improvement NMR spectroscopy, the signals for the carbonyl carbon are of acylsilane synthesis methods. The substantial effect of the quite different from the corresponding ketones, appearing at electronic properties and the bulk of trimethylsilyl group may higher δ values. The anisotropy effect and electronegativity be used to control the stereochemistry of reactions1. One of the differences also lead to higher δ values in the 1H NMR spectra well-established uses of acylsilanes in organic synthesis is as an for hydrogens attached to the α-carbon of acylsilanes (except aldehyde equivalent, in which a stereoselective nucleophilic for α,β-unsaturated). Table 1 shows some examples of IR and attack on the carbonyl group, α to a chiral center, is followed by NMR spectral data for acylsilanes. Another interesting stereospecific replacement of the silyl group by hydrogen. characteristic of acylsilanes is the abnormally long Si-CO Moreover, acylsilanes can be used as ester equivalents for bond (1.926 Å), first observed by Trotter8 based on X-ray chirality induction, since acylsilanes can be smoothly oxidized analysis, which can be compared to the analogous bond to esters. The enantioselective reduction and the cyclization length in C-CO (1.51 Å)2 compounds. The same authors reaction of acylsilanes appear to have substantial synthetic determined that the carbonyl bond length is essentially potential. In the first part of this review we present some physical normal, despite the low vibrational frequency. properties of acylsilanes as well as some classical reactions involving organosilicon compounds. In the second part, we Table 1. Infrared C=O absorption and NMR data of some acylsilanes2,3. 13 1 present some new work in this area as a complement to the Acylsilane IR C NMR H NMR ν (cm-)a δ C=Oa δ CHCOa previous reviews2-7 and finally, in the third part, useful methods C=O MeCOSiMe 1645 (1710) 247.6 (215) 2.20 (2.08) of acylsilane synthesis. 3 PhCOSiMe3 1618 (1675) 233.6 (207) — MeCOSiPh3 1645 240.1 2.30 (2.01) Physical properties and classical reactions of PhCOSiPh3 1618 (1692) — — the acylsilanes t-BuCOSiMe3 1636 249.0 (215) — Me3SiCOSiMe3 1570 318.2 — PhCH COSiMe 1635 — 3.77 (3.55) In the organosilicon compounds class, the acylsilanes 2 3 CH2=CHCOSiMe3 1641 237.0 6.28 (6.88) aValues in parenthesis are for analogues in which the silicon atom has e-mail: [email protected] been replaced by carbon. 8 Patrocínio & Moran J. Braz. Chem. Soc. Studies have shown that acylsilanes in general behave towards nucleophilic addition9,10. His work is historically as ordinary ketones. However, in some cases, these important because it led to a better knowledge of the reaction compounds have abnormal chemical behavior. Due to the pathway. A nucleophilic attack of alkoxide ion on the silicon intrinsic properties already mentioned, these compounds atom of acylsilane 7 was initially proposed for the formation should undergo many unusual reactions. For example, in of aldehyde 8 (Scheme 3, pathway a). Later, Brook proposed reactions of aroylsilanes with nucleophiles, the Brook another competitive pathway, which involves a nucleophilic rearrangement9 is very common. An example is the known attack by the alkoxide ion at the carbonyl group of 7 hydrolysis of aroylsilanes 1 to the corresponding aldehydes (Scheme 3, pathway b). Using the optically active acylsilane 5, promoted by traces of OH- (Scheme 1). (-)-9 in reactions with different alkoxide ions, 10 was reduced The Brook rearrangement, after carbonyl addition of a by LiAlH4 to give (-)-11 (Scheme 4). Although the optical nucleophilic reagent, is commonly observed in aroylsilanes purities of (-)-11 were observed to be dependent on the bulk due to the relative stabilization of the carbanion of the alkoxide ion (EtO- 22% vs. t-BuO- 65%), in all intermediate 3 by the aromatic ring. Two classical reactions with chiral acylsilane (-)-9 the enantiomer (-)-11 examples in this context are presented in Scheme 2, where was predominant, showing the retention of configuration at the silyloxyalkene 6 is formed only when an anion- silicon. Therefore the bulkier alkoxides find it more difficult stabilizing group is attached to the carbonyl group2. to attack at silicon and, consequently, the attack at the Brook investigated the great reactivity of acylsilanes carbonyl group becomes relatively easier11. Brook rearrangment Ar Ar O O R3SiOH OH- H O R Si Ar R3SiO C 2 R3SiO C H R3Si Ar 3 OH OH OH ArCHO 1 2 3 45 Scheme 1. O O R3SiO CH2N2 -N R3Si C Ar 2 C R3Si Ar H2C Ar CH2 N2 1 6 O CH PPh 2 3 -PPh3 R3Si C Ar CH2 PPh3 Scheme 2. O Ph O Ph O Ph pathway a RO cleavage - Ph Si Ph Ph Si OR Ph Si Ph RO ROH H Ph Ph Ph Ph 8 7 - pathway b RO Brook O OSiPh3 O rearrangement - RO ROSiPh Ph3Si C Ph HC Ph H 3 ROH Ph OR OR 8 Scheme 3. Vol. 12 No. 1, 2001 Acylsilanes and Their Applications in Organic Chemistry 9 Schemes 5 and 6 show recent examples of acylsilane optically active Grignard reagent to reduce benzoyl- reactions involving the Brook rearrangement. The triphenylsilane and benzoyltrimethylsilane in low acylsilane 12 reacts with amines to form the imine 13 or enantiomeric excess. Due to the relative facility that the silyl aminals 1412. The acylsilane 15 reacts with cyanide anion moiety can be removed and replaced by hydrogen, generally under liquid-liquid (CH2Cl2-H2O) phase-transfer catalytic by the action of fluoride ion, acylsilanes can be considered as conditions to form O-silylated cyanohydrin 1713. aldehyde equivalents in nucleophilic additions. Addition of nucleophiles can occur with a high Cram selectivity when Novel work involving acylsilane chemistry the chiral center in the acylsilane is on the α-carbon, as in 18 (Scheme 7), and even in some cases where the chiral center is β 1 Herein some of the most important synthetic on the -carbon , as in 19 (Scheme 8). applications of acylsilanes will be presented, focusing In general, the protiodesilylation (replacement of the especially on reports of the last ten years. R3Si moiety by H) occurs with a high level of stereoselectivity through the Brook rearrangement15. Two a. Stereocontrolled nucleophilic additions recent examples are shown in the Schemes 8 and 9 in which high stereocontrol was obtained in asymmetric induction The first study involving enantioselective addition to in the syntheses of calcitriol lactone derivatives 2016 and acylsilanes was reported by Mosher14 in which he used an β-aminoalcohols 21 and 2217. Scheme 10 illustrates a key Me O O Me RO /ROH Me O Si Np LiAlH Me Me Si 4 H Si Me Si Np Me H Ph Np Np OR Ph Ph Ph OR (-)-9 10 (-)-11 Np: Naphthyl Scheme 4. O O H H2NCH2CH(CH3)2 Ph3Si C Ph Ph3SiO C Ph Ph3Si Ph "dark" 12 NH2CH2CH(CH3)2 NHCH2CH(CH3)2 H N "dark" PhCH NCH2CH(CH3)2 Ph3SiOH 13 H H O N H Ph Si C Ph Ph3SiO C Ph 3 NCPh H N Ph3SiOH N N 14 Scheme 5. Brook O rearrangem ent t-BuMe2SiO KCN t-BuMe2Si O PTC t-BuMe Si R CN R CN 2 R CH2Cl2-H2O 15 (1:1) 16 17 (74-96% R = Me, CH2CH2OMe, Ph, R' PTC: n-Bu4PBr (20 mol%) Scheme 6. 10 Patrocínio & Moran J. Braz. Chem. Soc. step of Corey’s total synthesis of pentacyclic triterpene 24 rearrangement and coupling with allylic bromide, resulted of the β-Amyrin family18. The addition of 2-propenyl- in the Z-olefin in an overall yield of 82% with excellent lithium to acylsilane 23, followed by a Brook stereoselectivity (> 95%). Me Me Me BuLi Bu TBAF Bu SiMe3 Ph Ph Ph SiMe3 H HO O HO 18 (99:1) Scheme 7. SiMe3 Nu Nu SiMe3 H O Nu-M OH TBAF OH CH2 Cl2 iv Ti OBn OBn OBn 19 (93-95% d.e., 90% yield) syn-20 (93-95% d.e., 52-96% yield)d Nu-M = Allyl-SiMe3 , Methallyl-SiMe3 Scheme 8. O HO SiMe2Ph HO H Allyl brom ide/In TB AF SiMe2Ph THF/1 5 h/rt NHBoc NHBoc NHBoc (98% d.e., 96% yield) 21 (9 2 %) OSiMe2t-Bu HO SiMe2Ph HO O OEt O H TB AF OEt OEt BF3 Et2 O, CH2 Cl2 -7 8 o C, 48 h NHBoc NHBoc (98% d.e., 67% yield) 22 (98% d.e., 67% yield) Scheme 9.

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