Tributylstannyl enolates

Isolated tributylstannyl enolates react with benzaldehyde under the influence of metal salts... 

In a novel departure from the traditional approach to the asymmetric Mukaiyama aldol, Denmark has reported a Lewis base-catalyzed aldol addition reaction of enol trichlorosilanes and aldehydes. These unusual silyl ketene acetals are readily prepared by treatment of the tributylstannyl enolates 246 with SiC (Eq. 51). In the initial ground-breaking studies, the methyl acetate-derived trichlorosilyl ketene acetal 247 was shown to add rapidly to a broad range of aldehydes at -80 C to give adducts (89-99% yield, Eq. 52). 

Trichlorosilyl enol ethers. The reaction of tributylstannyl enol ethers with SiCl results in trichlorosilyl analogs that are highly reactive as donors in aldol reactions without catalysts. Asymmetric synthesis in the presence of chiral phosphoramides is realized. 

Catalytic asymmetric aldol addition of tributylstannyl enol ethers to aldehydes is achieved in the presence of (R)-BINAP AgOTf catalyst.The catalyst is considered to enhance the electrophilic reactivity of aldehydes to give diastereomeric adducts with high selectivities from -enol ethers, awft -aldol products are produced, whereas yn-aldol products result from Z-enol ethers. The stereochemical outcome is understood in terms of 6-membered cyclic transition states (Scheme 3-195). 

Arylacetate esters have been generated by coupling aryl bromides with enolates generated from O-silyl ketene acetals in the presence of tributylstannyl fluoride. 

Palladium-catalyzed reactions are the most studied. Eor example, 5-tributylstannyl-l,4-dioxene 130 underwent a cross-coupling reaction with the enol ttiflate 131 in refluxing THF in the presence of LiCl and a catalytic amount of Pd(PPh3)4 (Equation 19) <1996TL7013>. 

Transformation of a substrate into its ion radical enhances the species reactivity. Sometimes, this can overcome steric encumbrance of the substituent to be removed. Thus, l,4-di-iodo-2,6-dimethybenzene expels only one, sterically congested, iodine from position 1 upon the action of the tributylstannyl radical. Upon the action of the enolate ion of pinacolone (Me3CCOCH2) in the photoinitiated SrnI process, both iodines (from positions 1 and 4) are substituted (Branchi and co-authors 2000). 

A Michael addition reaction of BusSnLi to enones can be used for the generation of lithium enolates of / -tributylstannyl ketones. The two examples shown in Scheme 13 employed such a route for the generation of ketone ,/i-dianions13. 

A preparation of y-oxygenated allyhc stannanes in which the chirality resides in the alkoxy function has been described (Eq. 41) [61]. The starting alcohol is derived from tri-O-acetyl D-glucal. Acid-catalyzed addition of (Z)-l-methoxy-3-tributylstannyl-l-propene afforded the mixed acetal which was converted to the (Z) enol ether with TMSI and hexamethyldisilazane (HMDS). 

These results clearly show that the diastereoselectivity depends on the geometry of the enol stannane, and that cyclic transition-state structures (A and B, Fig. 1) are probable models. Thus, from the (i )-enolate, the and-aldol product can be obtained via a cyclic transition state model A, and another model B connects the (Z)-enolate to the sy -product. Similar six-membered cyclic models containing a BlNAP-coordi-nated silver atom instead of tributylstannyl group are also possible alternatives when transmetalation to silver enolate is sufficiently rapid. 

Oxidative coupling. Methods for ketone synthesis by trapping electrophilic species generated by CAN oxidation with silyl enol ethers are quite useful. 2-Tributylstannyl-l,3-dithiane and l-trimethylsiloxy-l,3-butadiene are exemplary substrates. 

Addition of tributylstannyl-lithium to crotonaldehyde and protection of the resulting alcohol with chloromethyl methyl ether gives the stannane (192), which reacts with both alkyl and aryl aldehydes RCHO to form specifically the t/rr o-hydroxy-enol ethers (193). These latter compounds have been used to prepare tra/i5-4,5-disubstituted butyrolactones by hydrolysis and subsequent oxidation. Palladium-catalysed carbonylation of RX in the presence of organotin species constitutes a useful synthesis of unsymmetrical ketones, and in the example reported this year RX is an arenediazonium salt. The reaction, which is basically an aromatic acylation, proceeds in good to excellent yield. Another Pd-catalysed reaction of aromatics, this time aryl bromides, is their reaction with acetonyltributyltin (194), prepared from methoxytributyltin and isopropenyl acetate, to give the arylacetones (195). ... 

An alkyl radical is produced by the reaction of tributylstannyl radical and iodoalkanes to trigger the reaction. An alkyl radical reacts with stannyl enol ethers via Sh2 mechanism to give an fl -alkyl ketone and regenerate a stannyl radical. In the presence of an electron-deficient alkene like fumarate and maleate, an alkyl radical adds to the alkene to generate a radical whose conformation is governed by a dipole-dipole repulsion between the two ester carbonyl groups. Subsequent reaction with the stannyl enol ether occurs preferentially in the opposite side to R to give an anti (erythro) adduct preferentially (Scheme 3-198). 

Kobayashi et al. developed the method for the catalytic asymmetric reaction (Scheme 8.24). The aldol reaction of silyl enol ethers with aldehydes by using chiral diamine-coordinated tin(II) triflate was promoted in the presence of tributylstannyl fluoride. With exo-olefin 145, the a-nonsubstituted aldol adduct 147 was obtained in high enantioselectivity. On the other hand, Z-enol ether 148 was reacted to give syn adduct 150 in excellent stereoselectivity. 

Ketyl radicals are also available using tributylstannyl radicals as oxophilic species. O-tributylstannyl ketyls are produced under neutral radical conditions by reaction of the carbonyl group with TBTH and AIBN (142 143). The overall process (142—>146, Scheme 25.68) leads to compound 146, which is formed by a cascade process involving the ring opening of cyclopropylmethyl radical 143, cyclization of the resulting 144, and protonation of enolate 145 after its radical reduction. 

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