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.
Scheme 1 Synthesis of β,γ-Unsaturated Ketones[1]
| R1 | R2 | R3 | Temp(°C) | Yield(%) | Ref |
(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.
Scheme 2 Synthesis of α-Oxoamides by Reaction of Stannanecarboxamides with Acyl Chlorides[3]
| R1 | Temp(°C) | Time(h) | Yield(%) | Ref |
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).
Scheme 3 Synthesis of 3-(Trialkylstannyl)alk-2-enamides by Carbamoylstannylation of Terminal Alkynes[4]
| R1 | Yielda(%) | Regioselectivity(%) | Ref |
| a GC yield based on the amount of 4 used. |
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.
Scheme 4 Synthesis of 1,4-Dicarbonyl Compounds by Acylstannylation of Alk-2-ynoate Esters[5]
| R1 | R2 | R3 | R4 | Time (h) | Ratio (10/11) | Yield (%) | Ref |
| Ph | Me | (CH2)4Me | Me | 2.5 | 88:12 | 66 | [5] |
| Ph | Me | TMS | Et | 3 | 98:2 | 85 | [5] |
| Ph | Me | Me | Me | 1.5 | 91:9 | 56 | [5] |
| Ph | Me | Ph | Et | 24 | 66:34 | 58 | [5] |
| Et | Bu | (CH2)4Me | Me | 24 | 79:21 | 47 | [5] |
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.
Scheme 5 Synthesis of 1,4-Dicarbonyl Compounds by Acylation of Enones[5]
| R1 | R2 | R3 | Time (h) | Yield (%) | Ref |
| R1 | R2 | R3 | R4 | Time (h) | dr | Yield (%) | Ref |
| Ph | Bu | Me | 4-F3CC6H4 | 1.5 | 73:27 | 64 | [5] |
| Et | Bu | Me | 4-F3CC6H4 | 1 | 51:49 | 62 | [5] |
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.
Scheme 6 Synthesis of ε-Oxoallylstannanes by Acylstannylation of 1,3-Dienes[6]
| R1 | R2 | R3 | R4 | Time (h) | Ratio (18/19) | Yield (%) | Ref |
| Ph | Me | Me | H | 2 | 33:67 | 74 | [6] |
| Ph | Me | Ph | H | 2 | 47:53 | 68 | [6] |
| 1-piperidyl | Bu | Me | Me | 2 | – | 73 | [6] |
5-Oxoalk-2-enylstannanes 18 and 19 (R1 = Ph);...
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