Enolates, stannyl

Stannyl enolates give good addition yields in the presence of a catalytic amount of m-(C4II9)4N 1 lir, 315 The bromide ion plays an active role in this reaction by forming a more reactive species via coordination at the tin atom. 

Arylacetate esters have been generated by coupling aryl bromides with stannyl enolates generated from silyl ketene acetals. 

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... 

Phosphoramide Catalysis. In a series of elegant studies, Denmark has described a process utilizing trichlorosilyl enolates 27 and 28 as the nucleophilic component in catalytic, enantiose-lective aldol addition reactions (Scheme 8B2.3) [151. These enoxysilanes are prepared by treatment of stannyl enolates with SiCl4. Although trichlorosilyl enolates are sufficiently reactive entities to undergo addition to aldehydes rapidly at -78°C, Denmark documented that... 

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... 

Miura et al. built on Ryu s previous work to achieve four-component radical cascades leading to diketones (Scheme 63) [174]. This outstanding result relies on initial carbonylation of alkyl radicals to form acyl radicals, such as 196. The nucleophilicity of acyl radicals allowed them to react with electron-deficient olefins to form ct-cyano radicals (197), whose phihcity is now reversed. Thus, they were able to add onto stannyl enolates and led to ketyl radicals such as 198. Those latter radicals underwent / -elimination of trib-utylstannyl radicals. This key elimination regenerated the mediator for the initial dehalogenation. This very fine tuning of the radical reactivities is the key element that makes the whole process work. 

Application of triorganostannyl enolates to organic synthesis was investigated on O-stannyl and C-stannyl enolates. " Sn NMR spectra showed that tribntylstannyl enolate is a mixture of both enolate kinds at lower temperatnre. The O- to C-stannyl enolate ratio is temperature-dependent and it was confirmed to be reversible. 

Metal enolates as synthons in organic chemistry TABLE 6. Arylations via cyclohexanone stannyl enolates (68)... 

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. 

The use of C unsubstituted and substituted stannyl enolates has been studied by Yamamoto in a series of elegant reports involving a novel bisphosphine Ag(I) complex 64 as a catalyst for C-C bond formation [30]. The addition of methyl ketone and acetate-derived enolates furnishes adducts in up to 96% ee. The use of E-stannyl enolates yields the 2-anti diastereomer as the major product in up to 96% ee. The use of acyclic Z-enol stannanes provided the complementary syn-substituted adducts as the major adduct in equally high diastereoselectivity and enantioselectivity. The observed correlation between enolate geometry and the simple diastereoselectivity of the product (E-enolates yield anti adducts while Z-enolates yield syn adducts) has led Yamamoto to postulate the involvement of a closed, cyclic transition-state structure. 

Stannyl enolates (equation 3-21, M = R SnX3. ) will take part in substitution or addition reactions with electrophiles under milder conditions than will the protic enols (M = H see Section 14.3.2).7... 

Table 14-6 Enol keto proportions of stannyl enolates in CDCI3 solution at room temperature.

A number of methods are now available for preparing the tin enolates, which are often not isolated, but are generated and caused to react further in situ. Tributyltin methoxide reacts exothermically with enol acetates without a solvent, to give the corresponding stannyl enolate, and methyl acetate which can be removed under reduced pressure or by slow distillation (b.p. 57 °C) (e.g. equation I4-54).37-104-105... 

The (3-keto stannyl enolates will give aldol reactions with aldehydes (Scheme 14-5),121 and are more reactive than the simple enolates in that they also show Michael additions with unsaturated carbonyl compounds (equation 14-67).122... 

Stannyl enolates derived from aromatic ketones were found to add to alkynes in the presence of AIBN (20 mol%), giving (3-(acylmethyl)alkenylstannanes (Scheme 5.7.13). Acylmethylstannylation products... 

Tin-oxygen bonds are also of great synthetic use. Because tin is more electropositive than sihcon, a stannyloxy group is a more powerful electron-donating group. Hence, stannyl enolates occasionally work as better nucleophiles when use of the silyl analogs is not satisfactory [7]. This chemistry is discussed in Section 12.6. 

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. ... 

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