BONDING ISOMERS OF TRIORGANOSTANNYL ENOLATES ANALYZED BY 119Sn NMR SPECTROSCOPY

Kobayashi, Kazuo; Kawanisi, Mituyosi; Hitomi, Torazô; Kozima, Sinpei

doi:10.1246/cl.1984.497

Cited by 24 publications

(15 citation statements)

References 2 publications

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“…Tributyltin enolates are useful radical mediators [47], although they generally exist in equilibrium with a-tributyltin ketones [48]. Three-component coupling reactions proceed readily to give functionalized ketones in good to excellent yields, where an equilibrium shift to provide tin enolates operates efficiently (Scheme 6.28) [49].…”

Section: Multicomponent Coupling Reactions Mediated By Group 14 Radicalsmentioning

confidence: 99%

Free‐Radical‐Mediated Multicomponent Coupling Reactions

Tojino

Ryu

2005

Multicomponent Reactions

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Section: Multicomponent Coupling Reactions Mediated By Group 14 Radicalsmentioning

confidence: 99%

Free‐Radical‐Mediated Multicomponent Coupling Reactions

Tojino

Ryu

2005

Multicomponent Reactions

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“…After the mixture had been stirred for 1 h at ambient temperature, volatiles were removed under reduced pressure, diethyl ether (100 cm3) and aqueous NH,F (1 5%; 40 cm3) were added and washed with water (50 cm3 x 2), dried (MgSO,) and evaporated. The residue was flash chromatographed (eluted by benzene-hexane, 4: 1) to give oxirane 9 The 13C NMR spectroscopic data showed a small amount of cis-isomer, some of whose absorbances were paired with those of 11. This cis-isomer could not be isolated.…”

Section: -Acetonyl-c-2-bromocyclohexan-r-1-ol7mentioning

confidence: 99%

Facile control of regioselectivity in the reaction of tin enolates with α-halogeno carbonyls by additives

Yasuda¹,

Oh-hata²,

Shibata³

et al. 1993

J. Chem. Soc., Perkin Trans. 1

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Tin enolates 1 reacted with a-halogeno ketones 2 and esters 10 to give a variety of 1.4-diketones 3 and y-keto esters 11, respectively, in the presence of appropriate additives such as hexamethylphosphoric triamide (H M PT), tributylphosphine oxide and tetrabutylammonium bromide, while complexation of these additives with tributyltin bromide allowed catalytic production of 0keto oxiranes 4 instead of 3. The reaction mechanism for the preparation of 1,4-diketone 3 is discussed.

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“…Our own efforts in the area of enantioselective reactions with enolates have led to the recent discovery of a {Cr(salen)}‐catalyzed asymmetric α‐alkylation of cyclic tin enolates to provide 5‐, 6‐, and 7‐membered‐ ring ketones that contain α quaternary stereocenters (salen= N , N ′‐bis(salicylidene)ethylenediamine dianion; Scheme ) 11. While the preparation of acyclic α,α‐disubstituted tin enolates leads inevitably to mixtures of E and Z isomers, tin enolates are known to undergo tautomerization between their O ‐stannyl and C ‐stannyl forms in solution 12. We were intrigued by the possibility that enantioselective alkylation of acyclic enolates might be achievable by a dynamic mechanism such as that outlined in Scheme , wherein mixtures of acyclic tin enolates might undergo reaction selectively through one geometric isomer under the {Cr(salen)}‐catalyzed alkylation conditions.…”

Section: Methodsmentioning

confidence: 99%

Enantioselective Alkylation of Acyclic α,α‐Disubstituted Tributyltin Enolates Catalyzed by a {Cr(salen)} Complex

Doyle

Jacobsen

2007

Angewandte Chemie

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Catalytic a-carbonyl CÀC bond-forming reactions represent a proven strategy for the construction of quaternary stereocenters.[1] Most approaches involve the addition of enolates to carbon-centered electrophiles in which both the p-facial selectivity and the enolate geometry govern the stereoselectivity of the C À C bond-forming event.[2-8] Unfortunately, the synthesis of stereodefined enolates from simple a,a-disubstituted carbonyl compounds that lack either tethered substituents or specific chelating functionality remains a significant challenge in organic synthesis.[9] As a result, successful examples of asymmetric catalytic enolate addition reactions that generate a-carbonyl quaternary stereocenters are limited to b-cyano esters, b-keto esters, and cyclic ketones, and relatively little progress has been made with simple acyclic systems.[10] Our own efforts in the area of enantioselective reactions with enolates have led to the recent discovery of a {Cr(salen)}-catalyzed asymmetric a-alkylation of cyclic tin enolates to provide 5-, 6-, and 7-memberedring ketones that contain a quaternary stereocenters (salen=N,N'-bis(salicylidene)ethylenediamine dianion; Scheme 1).[11] While the preparation of acyclic a,a-disubstituted tin enolates leads inevitably to mixtures of E and Z isomers, tin enolates are known to undergo tautomerization between their O-stannyl and C-stannyl forms in solution. [12] We were intrigued by the possibility that enantioselective alkylation of acyclic enolates might be achievable by a dynamic mechanism such as that outlined in Scheme 2, wherein mixtures of acyclic tin enolates might undergo reaction selectively through one geometric isomer under the {Cr(salen)}-catalyzed alkylation conditions. Herein, we report our progress to this end, and discuss how these studies have enhanced our understanding of the mechanism of the catalytic reaction.Initial studies were performed using the tributyltin enolate of 3-methyl-2-pentanone 2 a, which was prepared as a 1.8:1 mixture of E and Z isomers.[13] Treatment of 2 a with allyl bromide and the [Cr(salen)Cl] complex 1 a at 4 8C afforded the alkylation product 3 a in 80 % yield and 21 % ee (Table 1, entry 1). Variation of the substituents on the salen ligand revealed that the OTIPS derivative 1 b was both more reactive and more enantioselective than the tBu derivative (Table 1, entry 2, 84 % yield, 36 % ee).[14] A significant effect of the catalyst counterion on enantioselectivity and conversion was observed (Table 1, entries 2-4), with the iodide complex 1 d catalyzing the alkylation reaction in 93 % yield and 56 % ee (3.6:1 e.r.). [15] This result established unambiguously that the enantioselectivity of the alkylation product 3 a was not limited by the E/Z ratio of enolate isomers of 2 a (see below). Interestingly, a small increase in enantioselectivity was observed in reactions run with 5 mol % Bu 3 SnOMe as an additive (Table 1, entries 5 and 6), perhaps as a result of acceleration of enolate tautomerization and equilibration. Further variation of the r...

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BONDING ISOMERS OF TRIORGANOSTANNYL ENOLATES ANALYZED BY 119Sn NMR SPECTROSCOPY

Abstract: The bonding isomers of triorganostannyl enolates were analyzed by 119Sn NMR spectroscopy. In some cases the existence of an equilibrium between the O-stannyl enolate and C-stannyl derivative was confirmed by variable temperature 119Sn NMR spectra.

Cited by 24 publications

References 2 publications

Free‐Radical‐Mediated Multicomponent Coupling Reactions

Free‐Radical‐Mediated Multicomponent Coupling Reactions

Facile control of regioselectivity in the reaction of tin enolates with α-halogeno carbonyls by additives

Enantioselective Alkylation of Acyclic α,α‐Disubstituted Tributyltin Enolates Catalyzed by a {Cr(salen)} Complex

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