Stannyl enol ether

The reaction mechanism proposed for the addition of organostannanes [29] is similar to that for organoboronic acids. An example of the reaction of methyl vinyl ketone 42 is outlined in Scheme 3.15. The catalytic cycle involves a cationic rhodium complex G, phenylrhodium H, and oxa-n -allylrhodium I. Stannyl enol ether 44 is formed by the reaction of oxa-n -allylrhodium I with Me3SnBF4, which upon hydrolysis gives the ketone 43. The lower yields in the absence of water were explained by the further reaction of 44 with methyl vinyl ketone 42. The rapid hydrolysis with water may prevent such oligomerization. 

One example involves the addition of stannyl enol ethers to benzaldehyde in the presence of silver triflate and the chiral 2,2,-bis(diphenylphosphino)-l,l/-binaphthyl... 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

Hong, F.-T., Paquette, L. A. Olefin metathesis in cyclic ether formation. Direct conversion of olefinic esters to cyclic enol ethers with Tebbe-type reagents. Copper(l)-promoted Stille cross-coupling of stannyl enol ethers with enol triflates construction of complex polyether frameworks. Chemtracts t997, 10,14-19. 

Aldol reaction. The jyn-selectivity for the condensation of cyclohexenyl tributyl-stannyl ether with benzaldehyde is the best among various triflates M(OTf) (M= Ag, Cu, Zn, Sn, Y, Sc, and TMS). Stannyl enol ethers seem to be more reactive than the corresponding silyl enol ethers as donors for aldol reactions, at least in a system catalyzed by a complexed CuCOTOj. 

Protic-acid-catalyzed Michael additions (59) are subject to most of the limitations of base-catalyzed Michael additions (regioselectivity and stereoselectivity of enol generation, polyaddition, etc.), and hence, the stereochemistry has been little studied (60). At low temperatures silyl and stannyl enol ethers,+ ketene acetals, and allyl species are unreactive to all but the most reactive activated olefins. However, it was discovered by Mukaiyama and co-workers that enol ethers and ketene acetals react with a,/f-unsaturated carbonyl compounds in the presence of certain Lewis acids (4,61,62). Sakurai, Hosomi, and co-workers found that allylsilanes behave similarly (5,63,64). 

P,P] Vinylthionium ions such as 40.1 are potent Michael acceptors. Additions of silyl and stannyl enol ethers to vinylthionium ions such as 40.1 have been reported by Mukaiyama and co-workers (84-86). Two methods were employed for generation of the Michael acceptors elimination of ethyl sulfide from 40.2 under the action of a cationic trityl species (method A) and hydride abstraction from 40.3 and 40.4 by a trityl salt (method B) to give 40.1. Although both procedures use a strong Lewis acid to promote reaction, they differ substantially from the prior examples because an acceptor-acid... 

Vinylthionium ions are at the same oxidation state as a,/ -unsaturated esters. They are much more reactive than the corresponding enoates, however, reacting readily with silyl and stannyl enol ethers. 

A novel application of the radical cascade for construction of the indolizidinone skeleton focused on the initial formation of (9-stannyl ketyls. The tributyl tin radical was found to react with the carbonyl group of 507 to give ketyl 508 (Scheme 87) (04T8181). Consecutive 6-endo- and 5-exo-trig cyclizations then furnished stannyl enol ether 510. Eventual hydrolysis of the enol ether provided indolizidinone 511 in 36% yield as a Id-mixture of diastereomers. Again, the predominant isolation of the thermodynamic favored products derived from a 6-endo-trig cyclization can be... 

Traditionally, aldol reactions were carried out under protic conditions, such that the enolate was formed reversibly (see Volume 2, Chapter 1.5). An added measure of control is possible if one uses a sufficiently strong base that the enolate may be quantitatively formed prior to addition of the electrophile. The renaissance that has occurred in the aldol reaction in the last two decades has been mainly due to the development of methods for the formation and use of preformed enolates. The simplest enolates to prepare are those associated with lithium and magnesium, and there now exists a considerable literature documenting certain aspects of lithium and magnesium enolate aldol chemistry. This chapter summarizes the aldol chemistry of preformed enolates of these Group I and Group II metals. Other chapters in this volume deal with boron enolates, zinc enolates, transition metal enolates and the related chemistry of silyl and stannyl enol ethers. 

Diketones. The diketene-tin oxide adduct is a useful Michael donor that reacts with enones to give 1,5-diketones. Note that simple stannyl enol ethers do not undergo the same reaction thus the coordination of the 0-stannyl group by the ester carbonyl must be important. 

The coupling of the enol triflate 74 with the stannyl enol ether 75 proceeded rapidly at room temperature to give 76 in good yield in the presence of (2uCl as an additive, which plays a crucial role for the success of the coupling. The reaction has been utilized extensively for the construction of polyether systems of marine natural products such as maitotoxin [35]. 

Despite this background reaction, the reaction rate is substantially accelerated by Lewis bases, which provides an opportunity for developing an asymmetric variant. The required enoxytrichlorosilanes 21.97 can be generated in various ways, e.g., from the corresponding trimethylsilyl enol ethers on reaction with silicon tetrachloride, catalysed by mercury(ii) acetate from carbonyl compounds or trimethylsilyl enol ethers on treatment with trichlorosilyl triflate from cx-chloroketones on reaction with trichlorosilane and triethylamine or from the corresponding tributyl-stannyl enol ethers, etc. ... 

Although it is mechanistically different from the Tsuji-Trost allylation, indirect allyla-tions of ketones, aldehydes, and esters via their enolates are briefly summarized here. Related reactions are treated in Sect V.2.1.4. Pd-catalyzed allylation of aldehydes, ketones, and esters with aUyhc carbonates is possible via the Tr-allylpaUadium enolates of these carbonyl compounds. Tr-AUylpalladium enolates can be generated by the treatment of silyl and stannyl enol ethers of carbonyl compounds with allyl carbonates, and the allylated products are obtained by the reductive elimination of the Tr-allylpalladium enolates. 

Similarly, the Pd-catalyzed allylation of stannyl enol ethers can be optimized to produce the desired products in good yields, as indicated by the resnlts shown in Scheme 8. It should be noted, however, that the Pd-catalyzed allylation of stannyl enol ethers often fail to display high regio- and stereospecificity levels, which can be attributable, to a considerable extent, to the lower intrinsic reactivity of stannyl enol ethers. Specifically,... 

The cross-coupling of a-stannylated enol ether with 2,4-dimethoxy-5-iodopyrimidine in aqueous ethanol yielded 5-acetyl derivative (Scheme 54). ]... 

The reaction of silyl and tin enolates with nitrosobenzene, the so-called nitroso aldol reaction, was studied by Yamamoto and coworkers aiming at an overall enantioselective hydroxylation [254, 255]. This approach faces, however, the problem that in a noncatalyzed reaction, the nucleophilic silyl and stannyl enol ethers 514 attack the nitrogen atom of the ambident electrophile nitrosobenzene 515 so that the formation of hydroxyamino ketones 516 results [254a]. Fortunately, the authors developed suitable procedures wherein, under catalysis by chiral silver-bisphosphane complexes, aminooxy ketones 517 result in high ambidoselectivity. Alternatively, a controlled attack at nitrogen under formation of hydroxyamino ketones also became feasible by tuning of the catalytic system [254b,c] (Scheme 5.127). 

Stannyl enol ethers are prepared typically from the corresponding metal enolates and tin electrophiles. A convenient protocol is the reaction of enol carboxylate with tin alkoxide (Scheme 3-190). In contrast to silyl enol ethers, stannyl enol ethers usually exist as mixtures of C-Sn (ketone) and O-Sn (enol ether) forms. For example, acetone and carboxylate esters exist mostly as a C-Sn (ketone) form. In general, the 0-Sn (enol ether) tautomers are more nucleophilic to react with electrophiles faster than the C-Sn (ketone) tautomers. 

Stannyl enol ethers are useful reagents as nucleophiles for mild and selective C-C bond formation. A variety of electrophiles such as aldehydes, ketones, enones, haloalkanes, and -allylpalladium complexes reacts with stannyl enol ethers in the presenee or absence of additives or catalysts. 

Stannyl enol ethers have an adequate reactivity toward aldehydes to give aldol adducts without any additive or catalyst (Scheme 3-194). In order to enhance the reactivity and selectivity of stannyl enol ethers, a variety of catalysts like Lewis acids, Lewis bases, and radical initiators is applied to this reaction. In contrast to silyl enol ethers, the reaction proceeds through acyclic or cyclic transition states, depending on the reaction conditions. 

Scheme 3-194. Reaction of stannyl enol ethers with aldehydes.

Stannyl enol ethers undergo the aldol-type reaction with nitroso compounds to give a-aminoxy ketones or a-hydroxylamino ketones (Scheme 3-196). Regioselective and enantioselective synthesis of a-hydroxy ketones and a-amino ketones is of great synthetic significance, as these are readily converted to a variety of heterocycles. -... 

Alkylation of a stannyl enol ether is specifically initiated by an alkyl radical generated from an alkyl halide and a radical initiator such as 2,2 -azobisisobutyronitrile (AIBN) (Scheme 3-197). In addition, the three-component reaction of a stannyl enol ether, simple iodoalkane and an electron-deficient alkene proceeds successfully with anti-stereochemistry. 

Scheme 3-197. Radical reaction using stannyl enol ethers triggered by AIBN.

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). 

When carbon monoxide is present in the three-component coupling, carbonylative four-component coupling reaction takes place. Thus, alkyl halides, carbon monoxide, electron-deficient alkenes, and stannyl enol ethers are combined to give rise to 1,5-... 

Scheme 3-199. Four-component reaction of alkyliodides, carbon monoxide, acrylonitriles, and stannyl enol ethers.

In the presence of a nucleophile like a bromide ion, stannyl enol ethers are converted into anionic pentacoordinate stannates, which nucleophilically react with organic halides to give or-alkylated carbonyl compounds. In contrast, such penta-coordinated tin species hardly react with carbonyl electrophiles. The higher coordinated species is also generated by treatment of tin(IV) enol ethers with not only Bu4NBr but an aprotic polar solvent like Such pentacoordinate stannates add in a similar... 

Competitive reaction of acetophenone stannyl enol ether, which is in a tautomeric equilibrium of keto and enol forms, with cyclohexanone and active organic bromides are summarized in Table 3-11. In the absence of a halide ion, a reaction of the enol ether with cyclohexanone takes place (entries 1,3,5). In contrast, in the presence of Bu4NBr,... 

Table 3-11. Competitive reaction of stannyl enol ethers between cyclohexanone and...

Stannyl enol ethers react with electron-deficient alkenes like or, -unsaturated esters and alkynes in the presence of AIBN to give radicalic carbostannylation adducts, y-stannyl ketones, " " in sharp contrast to the Bu4NBr-mediated reaction, which gives simple Michael adducts. In addition, 1,6-enynes, when applied to this carbostannylation, produce five-membered cyclic adducts. For this particular transformation, electron-deficient groups are not needed (Scheme 3-222). 

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