Enol stannanes

Alkylation of allylic acetates. Rcgioselccti ve monoalkylation of allylic acetates is possible by use of enol stannanes (prepared by reaction of lithium enolates with chlorotri-n-butyltin) in the presence of this Pd complex. The less substituted end of the allyl group is alkylated with formation of the (E)-isomer.4 Examples ... 

Initial reports on the use of simple enolates as nucleophiles in TT-allylpalladium chemistry met with only limited success.77 106 The enolate of acetophenone reacted with allyl acetate in the presence of Pd(PPh3)4, but gave predominantly dialkylated product.106 The use of the enol silyl ether of acetophenone gave only monoalkylated product with allyl acetate and Pd° catalysis, but substituted allyl acetates did not function in this reaction.106 Enol stannanes, however, have been found to give monoalkylated products with a wide variety of allyl acetates (equation 19).106 In situ generation of enol stannanes from lithium enolates and trialkylstannyl trifluoroacetates followed by Pd°-catalyzed allylation has been demonstrated.107... 

Yamamoto has recently described a novel catalytic, asymmetric aldol addition reaction of enol stannanes 19 and 21 with aldehydes (Eqs. 8B2.6 and 8B2.7) [14]. The stannyl ketones are prepared solvent-free by treatment of the corresponding enol acetates with tributyltin methoxide. Although, in general, these enolates are known to exist as mixtures of C- and 0-bound tautomers, it is reported that the mixture may be utilized in the catalytic process. The complexes Yamamoto utilized in this unprecedented process are noteworthy in their novelty as catalysts for catalytic C-C bond-forming reactions. The active complex is generated upon treatment of Ag(OTf) with (R)-BINAP in THF. Under optimal conditions, 10 mol % catalyst 20 effects the addition of enol stannanes with benzaldehyde, hydrocinnamaldehyde, or cinnamaldehyde to give the adducts of acetone, rerf-butyl methyl ketone (pinacolone), and acetophenone in good yields and 41-95% ee (Table 8B2.3). 

The addition of acyclic substituted enolates 19d-e and cyclic enolates 21a-c were also examined in this study. Stannanes 21a-c afforded the anti adduct with excellent simple diastereoselectivity and up to 96% ee (Eq. 8B2.6). In contrast, the use of acyclic Z-enol stannanes 21 provided the complementary syn adducts in equally high levels of diastereoselectivity and enantioselectivity. The correlation of enolate geometry with simple induction (K-enolates yield antiadducts, whereas Z-enolates yield syn adducts) has led Yamamoto to invoke a cyclic transition-state structure 22. 

The range of carbon nucleophUes that can be used in the catalytic aUyUc substitution reaction is not unlimited. The use of more basic reagents, such as alkylUthiums or cuprates, usually results only in reduction of the aUylic substrate. Malonate derivatives, enol stannanes, enamines, and stable enols, however, can give allylation products. 

The BINAP silver(I) complex can be further applied as a chiral catalyst in the asymmetric aldol reaction. Although numerous successful methods have been developed for catalytic asymmetric aldol reaction, most are the chiral Lewis acid-catalyzed Mukaiyama aldol reactions using silyl enol ethers or ketene silyl acetals [32] and there has been no report which includes enol stannanes. Yanagisawa, Yamamoto, and their colleagues found the first example of catalytic enantioselective aldol addition of tributyltin enolates 74 to aldehydes employing BINAP silver(I) complex as a catalyst (Sch. 19) [33]. 

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. 

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. 

For the addition of enol stannanes to vinylthionium ions see Hashimoto, Y. Sugumi, H. Okauchi, T. Mukaiyama, T. Chem. Lett. 1987, 1695-1698. 

Yamamoto has also examined the reactions of substituted E- and Z-enol stan-nanes derived from cycloalkanones and acyclic ferf-butylethyl and -propyl ketones (Eq.49). The addition reactions of cyclic enolates afforded the adducts in excellent yields (92-96%) and 89/11 to 93/7 anti/syn diastereomeric ratio with the enantiopurity of the major diastereomer 234 uniformly high (92-95% ee). The use of acyclic Z-enol stannanes delivered the complementary iyn adducts in superb diastereoselectivities (<1/99 anti/syn) and enantioselectivity (91-95% ee). The high degree of stereospecificity of the addition process with respect to the... 

Enol Stannanes (Tabic 23, Entries 3-6) General Procedure15 ... 

Under the influence of the same catalyst, enol stannanes smoothly alkylate allylic acetates to give y,5-olefinic ketones of E -geometry e.g. Scheme 71). Alkylation occurs with high regioselectivity at the less substituted end of the allyl moiety, and -isomers result irrespective of the stereochemistry of the reactant. 

The role of palladium in organic synthesis continues to be explored and exploited. Enol stannanes are monoalkylated by allylic acetates in the presence of tetrakis(triphenylphosphine)palladium, Enol stannanes give higher selectivity for monoalkylation than enolate ions or silyl enol ethers. High regioselec-tivity is observed for alkylation at the less substituted end of the allyl moiety. Olefins, after complexation to palladium(ll), alkylate enolate anions. The organopalladium product may be converted into saturated ketones, or into enones by /3-elimination, or acylated with carbon monoxide (Scheme... 

Enol stannanes of cyclohexanone and propiophenone have been indicated to take part in r/treo-selective aldol reactions with benzaldehyde at low temperatures e.g. —78 °C), but to be erythro-seAsciiwe at higher temperatures ca 45 °C). Two complementary methods have been described for stereoselection in aldol-type reactions. Whilst a-mercurio-ketones show eryr/wo-selection in their reactions with aldehydes in the presence of boron trifluoride diethyl etherate, pre-formed lithium enolates and aldehydes, in the presence of simple trialkyl-boranes, lead to mixtures that are rich in the more stable threo-d do product. Aldol-type products arise from 1,3-alkyl migrations of alk-l-enyl alkyl acetals and ketals, in a reaction that is catalysed by boron trifluoride diethyl etherate (Scheme 52). Diastereoselection is possible, since (.E)-alkenyl acetals give the... 

In diastereo- and enantiocontrolled aldol addition, the use of enol borinates or enol stannanes has emerged as a standard methodology (Evans or Oppolzer aldol addition) [105]. These intermediates are generated in situ from the parent carbonyl compounds, which typically are auxiliary-substituted amides or esters. The counter ion, which is attached to the enolate, plays a dominant role in the stereochemical outcome. 

Asymmetric Processes. BINAP-AgOTf complex has been used in several catalytic reactions since its first appearance in 1996. Aldol reactions afford -hydroxy ketones, which is a common structure found in natural products. New ways for forming such a motif are always of synthetic utility. Enol stannanes, existing in the O-Sn form and/or C-Sn form, reacted with aldehydes... 

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