Science of Synthesis Knowledge Updates 2014 Vol. 4 (eBook)

5.2.17.9 Acylstannanes (Including S, Se, and Te Analogues) (Update 2014)

P. B. Wyatt

5.2.17.9.1 Applications of Acylstannanes in Organic Synthesis

5.2.17.9.1.1 Method 1: Synthesis of β,γ-Unsaturated Ketones by Acylation of Allylic Esters with Acylstannanes

Acylstannanes 1 react with allyli esters 2 in the presence of palladium(II) trifluoroacetate as catalyst to provide good yields of β,γ-unsaturated ketones 3 (▶ Scheme 1).[1] Similar transformations may be achieved using acylsilanes in place of acylstannanes;[2] however, for introduction of the benzoyl group, the tin reagents provide much higher yields than their silicon counterparts, as well as a greatly reduced risk of isomerization to form the α,β-un-saturated isomers of the ketone products.

(E)-1,4-Diphenylbut-3-en-1-one (3, R1 = R3 = Ph); Typical Procedure:[1]

A mixture of acylstannane 1 (R1 = Ph; R2 = Me; 121 mg, 0.50 mmol; as reported), allylic trifluoroacetate 2 (R3 = Ph; 115 mg, 0.50 mmol), Pd(OCOCF3)2 (8.0 mg, 0.025 mmol), and THF (0.25 mL) was stirred under argon at rt for 8 h. The product 3 was isolated following column chromatography (silica gel, hexane/EtOAc 9:1); yield: 50%.

5.2.17.9.1.2 Method 2: Synthesis of α-Oxoamides by Reaction of Stannanecarboxamides with Acyl Chlorides

The stannanecarboxamide 4 readily couples with acyl chlorides 5 to form α-oxoamides 6 (▶ Scheme 2); no catalyst is needed.[3] Use of dichlorides, such as oxalyl chloride, allows polycarbonyl compounds to be prepared.

N,N-Diisopropyl-2-oxopropanamide (6, R1 = Me); Typical Procedure:[3]

At rt, to a soln of iPr2NCOSnMe3 (4; 1.1 mmol) and docosane (0.37 mmol, internal standard for GC analysis) in benzene (2 mL) (CAUTION: carcinogen) was added AcCl (1.0 mmol) over 10 min and the soln was stirred at rt for 1 h. Volatiles were evaporated and the residue was subjected to column chromatography (silica gel, hexane then CH2Cl2) to give 2-oxoamide 6 (R1 = Me) as a colorless oil; yield: 83%.

5.2.17.9.1.3 Method 3: Synthesis of 3-(Trialkylstannyl)alk-2-enamides by Carbamoylstannylation of Terminal Alkynes

Terminal alkynes 7 undergo carbamoylstannylation upon treatment with the stannane-carboxamide 4 in the presence of a rhodium catalyst [Rh(acac)(CO)2] (▶ Scheme 3).[4] This process is highly regioselective; in the products 8, the carbamoyl group has been added to the terminus of the original alkyne; the reaction is also highly stereoselective (syn addition).

5.2.17.9.1.4 Method 4: Synthesis of 1,4-Dicarbonyl Compounds by Acylstannylation of α,β-Unsaturated Carbonyl Compounds

syn Addition of acylstannanes to alk-2-ynoate esters 9 occurs in the presence of bis(cycloocta-1,5-diene)nickel(0) [Ni(cod)2] as catalyst (▶ Scheme 4). The major products 10, corresponding to acylation at the more electrophilic β-carbon of the C≡C bond, are favored over regioisomers 11.

Acylation of enones 12 is also possible (▶ Scheme 5); in this case tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] has higher catalytic activity than bis(cycloocta-1,5-diene)nickel(O) and tin-containing products are not isolated. It has been proposed[5] that the reaction generates a transient metal enolate species 13 (e.g., M = PdSnBu3), which is protonated by stoichiometric quantities of added water to give 1,4-diketones 14, or which may alternatively be trapped by an added aldehyde to give aldol products 15.

5.2.17.9.1.5 Method 5: Synthesis of ε-Oxoallylstannanes by Acylstannylation of 1,3-Dienes

Acylstannanes 16 add to 1,3-dienes 17 in a 1,4-sense, in the presence of bis(cycloocta-1,5-diene)nickel(0) as catalyst, to yield ε-oxoallylstannanes 18 and 19 (▶ Scheme 6). Regioselectivity is modest in examples where unsymmetrical dienes 17 are used.

5-Oxoalk-2-enylstannanes 18 and 19 (R1 = Ph);...