Stannanes preparation

Various nonracemic allenylstannanes have been prepared from nonracemic propargylic mesylates and (Bu3Sn)2CuLi. The stereochemistry of the displacement was shown to be anti by correlation with an allenic stannane prepared through Claisen orthoester rearrangement of a propargylic alcohol of known configuration (Scheme 33)80. 

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 ... 

Butyllithium reagents, with chromium, 5, 382 Butyl(tri-2-pyridyl)stannane, preparation, 3, 827 ft-Butylvanadium(III) complex, preparation, 5, 19... 

C—H bond alkenylations, 10, 222—223 in cross-coupling polymerization, 11, 687 N-Heteroaromatic compounds, hydrogenation, 1, 792 Heteroaromatic stannanes, preparation, 3, 828 Heteroaryl halides, aminocarbonylatioon, 11, 527 Heteroarylstannanes, preparation, 3, 826 Heteroatoms... 

Allylic stannanes, prepared by stannanolysis of allylic sulphones, are hydroxy-methylated with trioxan-BF3 Et20 to give homoallylic alcohols. The regio-specihc sequence has been used to prepare ( )-lavandulol (50), an important perfumery compound (Scheme 66). ... 

The greater metallic nature of tin is clearly indicated here for tin forms only one hydride, stannane, SnH4. It is best prepared by the... 

Vinylation can also be done by Pd-catalysed cross-coupling in which one component is used as a halide or triflate and the other as a stannane (Stille reaction) or boronic acid (Suzuki reaction). Entry 9, Table 11.3, is an example of the use of a vinylstannane with a haloindole. lndole-3-boronic acids, which can be prepared by mcrcuration/boration, undergo coupling with vinyl triflates (Entry 10). 

The Suzuki coupling of arylboronic acids and aryl halides has proven to be a useful method for preparing C-aryl indoles. The indole can be used either as the halide component or as the boronic acid. 6-Bromo and 7-bromoindolc were coupled with arylboronic acids using Pd(PPh3)4[5]. No protection of the indole NH was necessary. 4-Thallated indoles couple with aryl and vinyl boronic acides in the presence of Pd(OAc)j[6]. Stille coupling between an aryl stannane and a haloindole is another option (Entry 5, Table 14.3). 

The above procedure was used for the preparation of all compounds except 104p, which was obtained from 104r by palladium-catalyzed coupling with tributylvinyl-stannane followed by palladium-catalyzed cyclopropanation of the resulting vinyl intermediate with diazomethane (Scheme 32) (99BMC3187). 

A large number of publications appeared on these aspects5, but most of these studies did not address stereochemical questions. In most cases, a given synthetic problem can be better solved by other allylmetals. Grignard reagents have some importance as intermediates for the preparation of allylboronates (Section D.1.3.3.3.3.2.1.), allylsilanes (Section D.1.3.3.3.5.2.L), allyl-stannanes (Section D. 1.3.3.3.6.2.1.1.), or allyltitanium derivatives (Section D.I.3.3.3.8.2.). 

Two approaches for the synthesis of allyl(alkyl)- and allyl(aryl)tin halides are thermolysis of halo(alkyl)tin ethers derived from tertiary homoallylic alcohols, and transmetalation of other allylstannanes. For example, dibutyl(-2-propenyl)tin chloride has been prepared by healing dibutyl(di-2-propenyl)stannane with dibutyltin dichloride42, and by thermolysis of mixtures of 2,3-dimethyl-5-hexen-3-ol or 2-methyl-4-penten-2-ol and tetrabutyl-l,3-dichlorodistannox-ane39. Alternatively dibutyltin dichloride and (dibutyl)(dimethoxy)tin were mixed to provide (dibutyl)(methoxy)tin chloride which was heated with 2,2,3-trimethyl-5-hexen-3-ol40. 

Both allylstannane transmetalation and thermolysis of homoallyl stannoxanes have been used to prepare 2-butenyltin halides as (E)j(Z) mixtures44-45. The reaction between 2-butenyl-(tributyl)stannane and dibutyltin dichloride initially provides dibutyl(l-methyl-2-propenyl)tin chloride as the kinetic product by an SE2 process, but this isomerizes under the reaction conditions to give a mixture containing the (Z)- and (E)-2-butenyl isomers46. 

Tributyl(l-ethoxy-2-propenyl)stannane (2) was first prepared from tributyl[chloro(ethoxy)-methyl]stannane and vinylmagnesium bromide, and found to be unstable with respect to 1,3-migration of the tributyltin substituent101. Tributyl(l-ethoxy-3-methyl-2-butenyl)stannane (1) was found to be more stable, and has since been prepared directly from (tributyl)(diethoxy-methyl)stannane and 2-methylpropenylaluminum dichloride generated in situ102. 

Treatment of a-isomer 8, which was prepared from the corresponding stannane in a similar way with methyl vinyl ketone under identical conditions, afforded 9 in 75% yield. 

In contrast, a-alkoxycuprate 10, prepared from the corresponding stannane, gave the conjugate addition product with only 0-20% ee upon treatment with 2-cyclohexenone in the presence ofchlorotrimethylsilane67. However, the conjugate addition of 10 to ethyl propynoatc in the presence of chlorotrimethylsilane provided 11 in 85% yield with nearly 96% ee67. 

A variety of reductions designed to prepare the stannane CF3SnH3 by reaction with, e.g., LiAlH4, failed (24). In each instance, SnH4 was produced, but little CFaSnHg was isolated. 

To examine whether the breakdown in equilibration arose from a slow rate of equilibration or if the equilibrium does not favor the equatorial alkyllithium,the time course of equilibration was investigated (Scheme 34). The diastereomeri-cally pure a-alkoxystannanes 198 were prepared in analogy to Linderman s procedures 170], and their equilibrations were examined. Transmetallation and alkylation under kinetic control (entries 1 and 2) provided the expected equatorial and axial adducts, respectively, with no detectable minor isomer. Attempted equilibrations of the axial stannanes (entries 4 and 6) led to mixtures of axial and equatorial products. The equatorial stannane gave only the equatorial adduct under both equilibration conditions (entries 3 and 5). From these data, the authors concluded that the equilibration does take place under these conditions, and that the equilibrium lies essentially completely toward the equatorial alkyllithium. The observed product ratios in entries 4 and 6 can be attributed to a slow rate of equilibration. 

Very recently, the coupling of tributyl(phenyl) and tributyl(vinyl) stannanes with cyclohexene derivatives was successfully carried out using a similar catalytic cocktail prepared from Pdj(dba)3 CHCl, IPr HCl and Cs CO as base. Outstanding results... 

Allylic stannanes can be prepared from allylic halides and sulfonates by displacement with or LiSnMe3 or LiSnBu3.146 They can also be prepared by Pd-catalyzed substitution of allylic acetates and phosphates using (C2H5)2AlSn... 

Another major route for synthesis of stannanes is reaction of an organometallic reagent with a trisubstituted halostannane, which is the normal route for the preparation of aryl stannanes. 

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