Allylsilanes reactions with carbonyl compounds

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

Fluoride ion promoted cleavage of propynylsilanes and subsequent reaction of the carbanion with carbonyl compounds produces allenic compounds. The reaction with formaldehyde and pivaldehyde fails, but both the allenic and acetylenic products are obtained from the reaction with acrolein and benzaldehyde [49]. Allylsilanes react with carbonyl compounds to produce but-3-en-l-ols [50],... 

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. 

Allylsilane and Propargylsilane Synthesis. (lodomethyl)-trimethylsilane has been used to prepare aflylsUanes, which are t)q)icafly utilized in Lewis acid or fluoride ion-promoted reactions with carbonyl compounds to provide homoallylic alcohols. Treatment of Mc3SiCH2l with methyltriphenylphosphonium bromide and base produces an intermediate phosphonium salt which can be further utilized to prepare allylsilanes from carbonyl compounds. This procedure has been modified so that this series of transformations can be done as a one-pot procedure (eq 1). ... 

T.3.3.3.5.2. Reactions of Allylsilanes with Carbonyl Compounds Introduction... 

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... 

Allyl transfer does not take place with 76 under nucleophilic conditions39. By contrast, the reactions of allylsilanes with carbonyl compounds are activated by fluoride ion353. 

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. 

In summary, the SMS reaction is a truly efficient process, possessing a broad scope and applicable to a number of carbonyls, allylsilanes, alcohols (silyl ethers) or amines. Its usefulness has been validated in several total syntheses and demonstrated by the preparation of chiral homoallylic alcohols. During the development of the SMS reaction, Melkafia and Marko [48] realized that the homoallylic alcohol (ether), if connected to an allylsilane, would form novel annelating agents that would lead to tetrahydropyran derivatives via condensation with carbonyl compounds. This reaction was called IMSC for intramolecular Sakurai cydization and will be discussed in the next section. 

Alkylation of ketones, As expected 1 reacts readily with carbonyl compounds to form allylsilanes. The reaction can be conducted as a one pot procedure with yields of 60-85%. In the presence of Lewis acids, allylsilanes react with various electrophiles (alkyl halides, acid chlorides, ethylene oxide). A typical sequence is shown in equation (I). 

Allenic alcohols.3 The reaction of allylsilanes with carbonyl compounds catalyzed by this salt to give homoallylic alcohols (9, 455-446) has been extended to a synthesis of a-allenic alcohols. Use of TiCl4 as catalyst results in a 2-chloro-l,3-diene. 

The addition of an allylsilane to an electrophile was first documented in 1948 by Sommer et al. [6]. These workers predicted that the allylsilane would react with an electrophile to generate a silicon-stabilized cationic intermediate. In 1956, Galas and co-workers demonstrated that allylsilanes undergo an allylic shift in the protiodesilylation of a cyclohexenylsilane to afford a methylidenecyclohexane [7], The first report of the reaction of allylsilanes with carbonyl compounds (1974) is also due to Galas [8]. These authors used activated substrates such as perfluoro-acetone and chloroacetone and AIGI3, GaGl3 or InGl3 as Lewis acids to promote... 

The reactions of ailylboranes, -silanes and -stannanes with carbonyl compounds and imines always take place with double-bond migration, and the structural stability of the reagent is a common problem that is encountered ( 2.7). The reactions of ailylboranes and -boronates occur without catalysis, while those of allylsilanes and -stannanes usually require the presence of a Lewis add [253], The mechanism of the reactions of allylboron derivatives is concerted, and the addition occurs via a six-membered cyclic transition state. A slightly distorted chair transition state model in which the oxygen of the carbonyl group is coordinated to the boron atom is usually invoked (Figure 6.42). Various steric and polar interactions dictate whether the Si or Re face of the prochiral aldehyde is attacked (models C j... 

Allylation. The reaction of allylsilanes with carbonyl compounds, acetals, and ortho esters is catalyzed by Cp2Ti(OTf)2. Rapid reaction rates and good conversion... 

Other Lewis acid mediated C-C bond forming reactions of silylated C-nucleophiles include the Mukaiyama aldol and Michael additions of enolsilanes ), the related reaction of allylsilanes described by Sakurai ) as well as similar reactions of Me3SiCN ). Other "inert C-nucleophiles such as dialkylzinc compounds also react with carbonyl compounds in the presence of Lewis acids ). We ourselves have utilized many of these processes in performing stereoselective reactions, particularly in achieving diastereofacial selectivity in the reactions... 

Essentially all allylsilanes (M = SiR3, Section D.l.3.3.3.5.) with the exception of fluorosil-iconates11 and most of the trialkyl(allyl)stannancs (Section D.l. 3.3.3.6.), which have only very weak Lewis acidic properties, require a strong Lewis acid to trigger the reaction with a carbonyl compound by the preceding formation of an x-oxycarbenium ion, which attacks the allylic compound in an ionic open-chain pathway. These Lewis acid catalyzed carbonyl additions offer new possibilities for the control of the simple and induced diastereoselectivity12. 

Allyltrialkoxy- or -tris(dialkylamino)titanium reagents are not capable of chelation-controlled addition reactions with oxy- or amino-substituted carbonyl compounds due to their low Lewis acidity87. To attain chelation control, the application of allylsilanes (Section 1.3.3.3.5.2.2.) and allylstannanes (Section I.3.3.3.6.I.3.2.) in the presence of bidentate Lewis acids like titanium(IV) chloride, tin(lV) chloride or magnesium bromide are the better options. 

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... 

Silyl enol ethers are powerful intermediates in organic synthesis. Reactions of silyl enol ethers with various electrophiles provide effective methods for the synthesis of various carbonyl compounds. In this section we will briefly touch on the electrochemical reactions of silyl enol ethers and related compounds. The electrochemical behaviour of silyl enol ethers is expected to be closely related to that of allylsilanes and benzylsilanes because silyl enol ethers also have a silyl group ft to the re-system. 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

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... 

Indium-mediated allylation of trialkyl(difluoroacetyl)silane 70 in aqueous media gives homoallylic alcohol 71 exclusively (Scheme 60). Both water and THF are essential for the allylation reaction. It is worth noting that homoallylic alcohol 71 is formed exclusively under these reaction conditions. On the contrary, enol silyl ether 72 is a major product of the fluorinated acylsilanes reaction with other organometallic compounds than indium via a Brook rearrangement and defluorination. Indium-mediated allylsilylation of carbonyl compounds provides a facile route to 2-(hydroxyethyl)allylsilanes. The allene homologs are similarly prepared (Scheme 61).244,244a... 

As we mentioned before, a classical Grignard reaction is formally described by the coupling of a covalent (albeit polarized) electrophile with an anionic nucleophile. Reactions shown in Scheme 2.41 (opposite) exemplify the alternative approach involving an interaction between cationic intermediates generated from carbonyl compounds (or their derivatives) under the action of Lewis acids and a purely covalent nucleophile, an allylsilane such as I09a or 109b. Similar electrophiles used in reactions with covalent silyl enolates such as 110 result in the formation of the aldol-Iike products (the Mukaiyama reaction ). 

Coverage in this chapter is restricted to the use of alkenes or alkynes as enophiles (equation 1 X = Y = C) and to the use of ene components in which a hydrogen is transferred. Coverage in Sections 1.2 and 1.3 is restricted to ene components in which all three heavy atoms are carbon (equation 1 Z = C). Thermal intramolecular ene reactions of enols (equation 1 Z = O) with unactivated alkenes are presented in Section 1.4. Metallo-ene reactions are covered in the following chapter. Use of carbonyl compounds as enophiles, which can be considered as a subset of the Prins reaction, is covered in depth in Volume 2, Chtqiter 2.1. Addition of enophiles to vinylsilanes and allylsilanes is covered in Volume 2, Chapter 2.2, while addition of enophiles to enol ethers is covered in Volume 2, Chapters 2.3-2.S. Addition of imines and iminium compounds to alkenes is presented in Volume 2, Part 4. Use of alkenes, aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.9. Coverage of singlet oxygen, azo, nitroso, S=N, S=0, Se=N or Se=0 enophiles are excluded since these reactions do not result in the formation of a carbon-carbon bond. 

The Lewis acid-mediated addition of allyl silanes and allyl stannanes to carbonyl compounds has been a topic of interest for many years [103,104]. Stereochemical studies of reactions of allylsilanes and allylstannanes with aldehydes have included FeCl3-mediated intramolecular reactions (Sch. 20) [105],... 

Allylsilanes work as allylating reagents if the transient )8-silyl cation smoothly collapses with desilylation to form a carbon-carbon double bond. In certain circumstances, however, desilylation is retarded and becomes a slower process than a second path, which might be an unusual yet synthetically interesting reaction. Reaction of allyltrimethylsilane with a,/3-unsaturated carbonyl compounds in the presence of... 

Usefiil levels of stereoselectivity were obtained in intermolecular addition reactions of C(3)-sub-stituted allylsilanes to chiral aldehydes. Lewis acids that are citable of chelating to heteroatoms have been used to direct the stereochemical course of allylsilane additions to a-alkoxy and a,p-dialkoxy carbonyl compounds. The allylation of a-benzyloxy iddehyde (94) in the presence of TiG4 and SnOt furnished products with high levels of syn stereoselection (syn-9. In contrast, under nonchelation-controlled reaction conditions (BF3-OEt2) allyltrimethylsilane reacted to form predonunantly the anti-1,2-diol product (anti-95), as shown in Scheme 45. 

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