Fluoride-catalyzed allylation

The fluoride-catalyzed, allylic C = C bond migration in a perfluoroalk-1-enc was first reported in 1953.27 Whereas pyrolysis of sodium perfluoropentanoate at 290-300 C gives per-fluorobut-1-enc (la) in high yield, the potassium salt decarboxylates at 165-200 C to yield principally perfluorobut-2-ene (2a). Apparently, the potassium fluoride formed in the decarboxylation catalyzes the isomerization. That sodium perfluoropentanoate yields no rearranged product demonstrates that sodium fluoride is ineffective in catalyzing the isomerization. 

Reinvestigation of Fluoride-Catalyzed Allylation of Aldehydes with Allyl-... 

It is interesting to examine the possibility that the reactivities of allylsilanes may be modified by changing substituents on the silicon atom. We have then reinvestigated the effect of substituents on silicon in fluoride-catalyzed allylation. ... 

Table IV shows product distribution in the reaction of allyltrimethylsilane (13) and al-lyldimethylfluorosilane (14) with carbonyl compounds. One of the side reactions in the fluoride-catalyzed allylation is enolization especially for easily enolizable carbonyl com-...

Allylsilanes are very stable thermally with respect to allylic shift of the silyl group and regioisomeric allylsilanes are therefore very reliable in the regiospecificity of their Lewis acid catalyzed reactions (Scheme 1), the electrophile always bonding to the terminus of the allyl unit remote from the silyl group. In contrast, fluoride-catalyzed reactions are not regiospecific (Scheme 2), probably because... 

Allylsilanes are intrinsically nonnucleophilic. We believe that addition of fluoride ion to an allylsilane forms a silicate anion, which can react at either end of the tc-system. Thus from a mechanistic viewpoint, substitution of an allylsilane under nucleophilic conditions is more appropriately considered an addition-elimination reaction with the terms Se2 and Se2 aptly representing the elimination process. For recent evidence of pentacoordinate silicate intermediates in fluoride ion catalyzed allylation using trimethylallylsilane, see a) M. Kira, M. Kobayashi and H. Sakurai, Tetrahedron Lett.. 28. 4081 (1987) b) M. Kira, K. Sato and H. Sakurai. J. Am. Chem. Soc.. 110, 4599 (1988) c) T. Hayashi, Y. Matsumoto, T. Kiyoi, Y. Ito, S. Kohra, Y. Tominaga and A. Hosomi, Tetrahedron Lett., in press d) H. Sakurai, Lewis Acid Character and Selective Reactions of Pentacoordinate Silicon Compounds, this volume. 

More recently, Majetich et al. have proposed pentacoordinate allylsilicate intermediates formed from fluoride ion addition to silicon as an ambient nucleophilic species. However, nothing had been known so far about pentacoordinate allylsilicates, before we have initiated the present study, whereas a number of pentacoordinate organosilicates have been well characterized. It is an interesting problem whether the fluoride-catalyzed reactions actually involve free allyl anions or not. The author will discuss this problem in the next section somewhat in detail. In this section the author will describe preparation of well-defined pentacoordinate allylsilanes and their reactions which have been published recently. ... 

Previously the author reported that an allylic nucleophilic species is generated readily from trimethylallylsilane by the catalysis with fluoride ions. The reaction provides a versatile method of introducing an allyl group into carbonyl compounds.However, the exclusive regiospecificity of allylation, which is characteristic of the Lewis acid-catalyzed allylation with allylsilanes, disappeared. 

In the preceding section, the author has shown a very regioselective allylation reaction with allyltrifluorosilanes in the presence of fluoride ions, which is not only useful as chemose-lective, regiospecific and highly stereoselective allylation but affords an interesting mechanistic probe for the fluoride-catalyzed reaction of aUylsilanes. 

As a Carbon Nncleophile in Fluoride Ion-Catalyzed Reactions. The reactions with aldehydes, ketones (eq 23), and o , -unsaturated esters (eq 24) can also be catalyzed by fluoride ion, usually introduced as tetra-n-butylammonium fluoride (TBAF), or other silicophihc ions such as alkoxide. These reactions produce silyl ether intermediates, which are usually hydrolyzed before workup. The stereochemistry of attack on chiral ketones can sometimes be different for the Lewis acid- and fluoride ion-catalyzed reactions. In addition some electrophiles only react in the fluoride-catalyzed reactions, as with the addition to trinitrobenzene giving an allyl Meisenheimer complex . ... 

Scheme 5.25 Catalytic cycle of the palladium-catalyzed allylic alkylation of silyl enol ethers under activation by fluoride and/or alkoxide.

Mowery and DeShong reported on the use of siloxanes 72 (Figure 16) as versatile transmetallation agents for Pd(dba)3-catalyzed couplings with aryl halides and allylic alcohol derivates, in the presence of TBAF and at high temperature (95 They later used aryl silatrane 73 (Figure 16) as a suitable partner for the fluoride-promoted... 

Intramolecular Sakurai reaction. Allylic and propargylic silanes can undergo a Lewis acid catalyzed intramolecular Sakurai reaction.1 In cyclization to hydrin-danones, the stereochemical outcome can differ from that obtained by fluoride ion catalysis (presumably kinetically controlled cyclization), equation (I).2... 

Intramolecular conjugate allylation (12, 25).2 Fluoride ion catalyzes intramolecular Michael additions of allyltrimethylsilane to a,p-enones as well as a,p-unsaturated esters, nitriles, and amides Lewis acid catalysis is not effective. The method is particularly suited to cyclopentane annelations.2... 

Typically, oc,0-unsaturated esters, a,0-unsaturated aldehydes and a,0-unsaturated nitriles are poor acceptors for the Lewis acid catalyzed silylallylation procedure, but they are excellent acceptors for the complementary fluoride ion mediated allylation procedure (cf. Volume 4, Chapter 1.2, Section 1.2.2.1.7). Other suitable acceptors include 1,4-quinones,70 a,0-unsaturated acyl cyanides (162),718 silyl ot,0-enoates (163)71b and nitroalkenes (Scheme 26) 72 reduction (titanium(III) trichloride) of the intermediate nitronates arising from nitroalkene allylation affords y,8-enones (166). 

Majetich reports a general intramolecular Lewis acid allylation protocol for the synthesis of bicy-clo[5.4.0]undecen-3-ones (168) and bicyclo[4.4.0]decen-3-ones (170), which are 1,6-addition products (Scheme 27). The same precursors, (167) and (169), when submitted to the fluoride ion cyclization protocol, also afford 1,2- and 1,4-addition products.73 Typically, ethylaluminum dichloride, a proton sponge Lewis acid, is used in order to minimize adventitious protonic desilylation. Other 0,y-unsatu-rated silanes also undergo similar intramolecular Lewis acid catalyzed additions for example, the si-lylpropargylic enones (171) undergo intramolecular cyclization to the allenylspiro system (172).74... 

Potassium fluoride can also be used as a base in the Pd(0)-catalyzed reactions of 149 with allylic acetates, as shown in Scheme 33 (94MI1). Sinou and co-workers reported that the acetates of the isomeric geraniol, nerol, and linalool gave similar mixtures of final sulfides 150e (94MI1). 

Fluoride ion (from CsF, CdF2, or AgF) can also catalyze the allylation using AllylSiF3 and AllylSi(OMe)3, respectively [10, 11] the asymmetric version (with <56% ee) requires a combination of Bu4N+[SiPh3F2] with the Lewis acidic complex of CuCl and BINAP [21]. 

A catalytic amount of TBAF is effective in the allylation of aromatic imines with 10 in refluxing THF." Similar to the TBAF-catalyzed carbonyl allylation, the imine allylation is promoted by a fluoride-triggered autocatalytic cycle. 

Ti-F bond [182.5(5) pm], which as expected is shorter than the Ti-Cl bond of 17 [228.4(3) pm] [12]. We were of course interested in the catalytic properties of 24 in comparison with triflate 23. In the presence of 23, allylsilane 5 adds smoothly to benzaldehyde the product is, however, racemic. Most probably the catalyst is tri-methylsilyl triflate, which is formed according to pathway B (Scheme 2, cf. [4a]). On the contrary, no reaction between benzaldehyde and allyl silane 5 is observed in the presence of fluoride 24. As we observed earlier, all cyclopentadienyltitanium TADDOLates studied catalyzed the conversion of benzaldehyde 25 with salt-free (i-PrO),TiCH3 26 to 2-phenylethanol 27 [11]. In this case the titanium fluoride effect was indeed very pronounced, as at -78 °C 0,5 % of 24 afforded 60 % of product 27 in 17 h with 78 % ee with 2 % of 24 the induction is 93 % ee. 

Allyl carbonates (Scheme 5) and diene monoxides (Eq. 32) were also employed in the palladium-catalyzed coupling reaction of arylsilanes 136, 37]. The reaction does not require activation by a fluoride ion or an additional base like a hydroxide ion. 

Conjugate addition to acyclic Michael acceptors. Sakurai and Hosomi (9, 445-446) reported one example of the fluoride ion-catalyzed reaction of allyltrimethylsilane with an acyclic enone. In that case (reaction with C6H5CH=CHCOCH,). 1.2- and 1,4-adducts are obtained in the ratio 2 1.1,4-Addition is enhanced by use of DMF and HMPT as solvent and by increase in the size of the group adjacent to the carbonyl group. 1,4-Addition is the main or predominant reaction with a,3-unsaturated esters or nitriles. In this case, it is superior to or competitive with allylation with lithium diallylcuprate. Yields in 1,4-additions to a,p-enones can compare favorably with those obtained with reactions catalyzed by TiC. ... 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

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