Sakurai-Hosomi allylation reaction

The Sakurai-Hosomi allylation reaction of benzaldehyde with 2-methallyltri-methylsilane proceeds smoothly in the presence of 5 mol % B(C6Fs)3 (not anhydrous grade) to afford 3-methyl-l-phenyl-3-buten-l-ol (93 %) [151c]. 

As a result of the preliminary examples described in Sect. 3.1 and the quest for automation techniques in solution-phase synthesis, various examples of continuous flow processes appeared in the literature lately which utilized solid-phase-bound chemical catalysts. In a simple example, Yamamoto and coworkers studied the use of super Bronsted acids loaded on polystyrene beads 5 for use in a single-pass column system (Fig. 4) [30]. It was shown that these columns are suited for the acetylation of alcohols, acetalization of carbonyl compounds, Sakurai-Hosomi allylation reactions, and Mukaiyama aldol reactions. 

Fluorous solid catalyst 8 is highly effective for the Mukaiyama aldol reaction [Eq. (9)] and Sakurai-Hosomi allylation reaction [Eq. (lO)j. These reactions have been performed at -78 °C and room temperature, respectively, under heterogeneous conditions. Post-reaction, 8 has been recovered in high yield by decanting the liquids at room temperature. 

Enantioselective Mukaiyama-aldol and Sakurai-Hosomi allylation reactions catalyzed by chiral Lewis acid are currently of great interest because of their utility for the introduction of asymmetric centers and functional groups. 

The catalytic activities of polystyrene-bound tetrafluorophenylbis(triflyl)methane in several reactions (direct esterification, Friedel-Crafts acylation, acetalization, the Mukaiyama aldol reaction, and Sakurai-Hosomi allylation) were superior to those of Nafion SAC-13 which is perfiuororesinsulfonic acid (Table 2.3). 

It has also been shown that the Sakurai-Hosomi reaction of methallylsilanes with glyoxylates is catalyzed by the BINOL-Ti complex (1) to give the products, surprisingly, in an allylic silane (ene product) form with high enantioselecdvity (Scheme 8C.22) [53],... 

Most desirable, however, would be chiral catalysts for the addition of the more readily available and less toxic allyl silanes, but so far the efforts towards an enantioselective variant of the Sakurai-Hosomi reaction have been less successful. Some time ago Ketter and Herrmann [3a] obtained 24 % ee for the addition of allyl silane to aldehydes catalyzed by the dichlorotitanate 1. 

The Lewis acidic character of reactive pentacoordinate silicon compounds has been unequivocally confirmed by Corriu, Sakurai and Hosomi [90]. Allylsilicates prepared from allylsilanes and catechol can undergo allylation reaction with aldehydes in the absence of Lewis acid promoter (Sch. 51). 

We have also found that BINOL-Ti (1) catalyzes the Sakurai-Hosomi reaction of methallylsilanes with glyoxylates (Sch. 6) [36]. Surprisingly, however, the products were obtained in the allylic silane (ene product) form (vide infra), with high enantio-selectivity. 

Since the discovery of the Hosomi-Sakurai allylation reaction [332] much attention... 

In 1976 we reported that aldehydes and ketones are efficiently allylated with allyl-trimethylsilane in the presence of a substoichiometric amount of I iLf, [332]. Subsequently, BU4NF, a fluoride ion source, was found to be an effective catalyst of this allylation reaction [333]. After these initial reports of the Hosomi-Sakurai reaction, several allylsilanes, including highly functionalized compounds, were used for regio- and stereoselective allylation of a variety of carbon electrophiles [6, 13, 14, 334]. In the nineteen-eighties, some Lewis acids (TMSOTf [335], TMSI [336]... 

The use of C2-symmetric 1,2- and 1,3-diols as chiral auxiliaries is a reliable method for asymmetric allylation of acetals [382]. Acyclic acetals derived from homochiral 1-phenylethanol undergo the Hosomi-Sakurai allylation with high diastereoselectivity [383]. Tietze et al. have, on the other hand, reported that the TMSOTf-catalyzed successive acetalization-allylation reaction of aliphatic aldehydes with homochiral silyl ethers 123 and allyltrimethylsilane gives the corresponding homoallyl ethers with complete diastereocontrol these ethers can be readily converted into enantiomerically pure homoallyl alcohols without epimerization (Scheme 10.135) [384]. This method is applicable to asymmetric allylation of methyl ketones [385]. 

Since the discovery of thermally promoted allylation of aldehydes [9], allylstannanes have been widely used in organic synthesis as stable and stereodefined reagents for C-C bond formation. Although it had been reported that activated aldehydes [10] or allylstannanes with chloride on the tin [11] could be used for allylation, remarkably innovative technology for allylation was advanced by Naruta and by Sakurai and Hosomi [12]. They disclosed that allylation was promoted by addition of a Lewis acid this substantially expanded the versatihty of the aUylstaimane procedure. Because many allylation reactions have already been documented [1], the most recent progress in this field will be described after brief description of fundamental aspects. 

Sakurai allylation reaction (Hosomi-Sakurai reaction)... 

Asymmetric allylation is a valuable method for constructing chiral functionalized structures, and therefore many chiral allylmetal reagents directed toward a high level of asymmetric induction have been designed and synthesized. Although some of them have exhibited good to excellent enantio- and diastereoselectivities in reactions with achiral aldehydes, Yamamoto et al. developed the first method for a catalytic process in 1991 [43a]. CAB 3a has a powerful activity in the Sakurai-Hosomi allyla-... 

Stereoselective aldol reactions, allylations, and aziridinations of imines, including activated imines, have been reported." The Sakurai-Hosomi reaction of aUylsilane with imine in the presence of tetrabutylammonium fluoride is found not to be strictly catalysed by fluoride rather, a fluoride-triggered autocatalytic route is suggested. 

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... 

The reaction of carbonyls with alcohols or TMS ethers under catalysis by (la) or Mes Sil provides an efficient, rapid access to oxocarbenium ions. The oxocarbenium ions generated in situ have frequently been utilized for the Hosomi-Sakurai allylation (Scheme 9.14) [2, 56]. This tandem acetalization-allylation reaction is quite convenient to the synthesis of homoallyl ethers. 

If one compares the reactions of allyl/crotylboronates and Sakurai-Hosomi addition of allyl/crotylsilanes bearing chiral auxiliaries with one-point attachment (225, 226 Figure 10.9), the enantiomeric excesses of homoallylic products so-formed are much higher in the former than for the products in the latter reactions. ... 

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... 

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