Allyltributylstannane, allylation

Addition of allylic zinc bromides to nitrones, generated in situ from allylbro-mides and zinc powder in THF (670), allyltributylstannane (671) and lithiated allyl ferf-butyldimethylsilyl ether (672), proceeds regioselectively in good yields and is used to synthesize homoallyl hydroxylamines (Scheme 2.189). The latter were subjected to an iodo cyclization reaction (see Scheme 2.186). 

The ability to conduct radical reactions without the use of tin reagents is important. Allylic triflones have been used to conduct allylation reactions on a range of substrates (39) as a replacement for allyltributylstannane (Scheme 28). The main limitation was that unactivated or trisubstituted triflones failed to undergo reactions. In other nontin radical methods, arenesulfonyl halides have been used as functional initiators in the CuCl/4,4 -dinonyl-2, 2 -bipyridine-catalysed living atom-transfer polymerization of styrenes, methacrylates, and acrylates.The kinetics of initiation and propagation were examined with a range of substituted arylsulfonyl halides with initiator efficiency measured at 100%. 

Certain organostannanes have been shown to react with DMAD to give simple addition products. Thus, the reaction of allyltributylstannane with DMAD gives the 1 1 adduct 508 [Eq. (75)] whereas crotyltributylstannane gives the adduct 509, occurring through an allylic type of rearrangement [Eq. (76)]. ... 

In addition to allylsilanes, CM can also be applied to allylstannanes, which serve as valuable reagents for nucleophilic additions and radical reactions.To date, only eatalyst 1 has been shown to demonstrate CM reactivity in the preparation of 1,2-disubstituted allylstannanes, as ruthenium catalysts were found to be inactive in the presence of this substrate class.Poor stereoselectivities were generally observed, with the exeeption of one instance of >20 1 Z-selectivity in the reaction of allyltributylstannane with an acetyl-protected allyl gluco-side. 

A sequence of reactions that was recently reported by Hanessian and Alpegiani nicely illustrates how the allylstannane method is useful for functionalization of complex, sensitive substrates and, more generally, how stereochemistry can be controlled in radical addition reactions (Scheme 40).138 Dibromo- 3-lac-tam (25) can be monoallylated with a slight excess of allyltributylstannane and then reduced with tributyltin hydride to provide 3-allylated (3-lactam (26) (the acid salt of which shows some activity as a 3-lactamase inhibitor). Stereochemistry is fixed in the reduction step hydrogen is delivered to the less-hindered face of the radical. Alternatively, monodebromination, followed by allylation, now delivers the allyl group from the less-hindered face to provide stereoisomer (27). Finally, allylation of (25) with excess allylstannane produces the diallylated product (not shown). 

In 1998, Yamamoto et al. reported the first catalytic enantioselective allylation of imines with allyltributylstannane in the presence of a chiral 7i-allylpalladium complex 23 (Scheme 9) [15]. The imines derived from aromatic aldehydes underwent the allylation with high ee values. Unfortunately, the allylation reaction of aliphatic imines resulted in modest enantioselectivities. They proposed that a bis-Jt-allylpalladium complex is a reactive intermediate for the allylation and reacts with imines as a nucleophile. The bis-Jt-allylpalladium complex seemed the most likely candidate for the Stille coupling [16]. Indeed, the Stille coupling reaction takes place in the presence of triphenylphosphine even if imines are present, whereas the allylation of imines occurs in the absence of the phosphine [17]. They suggested the phosphine ligand played a key role in controlling the... 

An interesting enantioselective addition-allyl-transfer sequence of an electron-deficient alkene 163 with alkyliodides and allyltributylstannane 164 was described... 

It is well-established that 7r-allylpalladium is electrophilic, and no reaction with electrophiles has been observed. However, there is an evidence that bis-7r-allylpalladium (172), generated in situ, could be amphiphilic. Typically, formation of the 2-substituted 3,6-divinylpyran (175) by the reaction of butadiene with aldehyde can be explained by the amphiphilic nature of the bis-7r-allylpalladium 173 generated in situ as an intermediate, which reacts with the electrophilic carbon and the nucleophilic oxygen in the aldehyde as shown by 174 [86]. As a similar reaction, piperidone is obtained by the reaction of butadiene with isocyanate [87]. The reaction of allyltributylstannane (176), allyl chloride and benzalmalononitrile (177) in the presence of PdCl2(Ph3P)2 (3 mol %) afforded the diallylated product 178 in high yield. 

Aldehydes can be allylated with allyltributylstannane using cerium(III) chloride in acetonitrile, a method particularly suitable for substrates bearing acid-sensitive groups.185... 

Carboxylic acids promote the allylation of aldehydes by allyltributylstannane.187 In (g) the case of crotylation, some regioselectivity can be achieved by an appropriate choice of acid. 

However, it could be shown that in principle simple imines can be used successfully as well for asymmetric aminoalkylation reactions catalyzed by chiral Lewis acids. The asymmetric allylation of simple imines 16 with allyltributylstannane (Scheme 6) catalyzed by the / -pinene derivative 17 [32a], for example furnished comparatively good results (for related asymmetric catalytic allylations of simple imines, see [32b-d]). Moreover, it was demonstrated on the basis of several Strecker-type syntheses [33-35] that catalysts such as the chiral aluminum complex 18 (Scheme 6) [33a—b] are also well suited for enantioselective aminoalkylations with simple imines. The mechanism indicated in Scheme 6 shows that the... 

A palladium-catalyzed intramolecular 1,4-dialkoxylation of (2-(cyclohexa-l,5-dienyl)phenyl)methanol provides a stereocontrolled route to isochromans 542 (Equation 224) <1998TL1223>. A tandem nucleophilic allylation-alkoxyal-lylation reaction of an ortio-alkynylbenzaldchydc 543, allyl chloride and allyltributylstannane in the presence of allylpalladium chloride furnishes the isochroman 544 via a (i-mdo-diycycl ization process (Equation 225) <2002TL7631 >. 

Another way in which dihydrobenzo[c]furans can be made is by reacting allylic halides having an aldehyde in the same molecule with allyltributylstannane in the presence of a catalytic amount of Pcydba CHClj <02CL158>. 

A-allylic amines are conveniently prepared in high yields by the reaction of azides with allylindium reagents in the presence of sodium iodide at ambient temperature (Equation (74)).307 Stannylation with tributylchloro-stannane occurs exclusively at the a-carbon, yielding allyltributylstannanes (E, Z)-isomerization of the allylic double bond depends largely upon the substitution pattern on the allylic moiety (Equation (75)).26 Allyl and propargyl bromides react with diphenyl diselenides in aqueous media to give allyl and propargyl selenides (Equation (76)).308 309... 

Corey s asymmetric allylation methodology was utilized in the total synthesis of amphidinolide T3 (95), a marine natural product that exhibits significant antitumor properties37 (Scheme 3.1gg). The asymmetric allylation of the aldehyde 96 was carried out successfully with chiral allylborane reagent generated in situ from allyltributylstannane and (R,R)-82 to furnish the homoallylic alcohol desired (97) in 85% yield with excellent diastereoselectivity. Subsequent conversion of the alcohol to the tosylate ester followed by treatment with potassium hydroxide resulted in formation of the trisubstituted tetrahydrofuran 98. 

A nucleophilic allylation-heterocyclization via bis-Jt-allylpalladium complexes with allyltributylstannane and an o-chloroallyl benzaldehyde generates allyl-vinyl-substituted phthalans in good yields (Equation 144) <2002CL158>. No Stille coupling products have been observed in these reactions. 

The direct allylation of radical precursors (e.g., alkyl hahdes, thioacyl derivatives) with allyltributylstannane in the presence of an initiator represents a well-established protocol for carbon-carbon bond formation [29,30]. This methodology provides a convenient means for introducing an allyl group to the anomeric carbon of carbohydrates [31]. hi their recent work on the preparation of building blocks for C-glycoside synthesis, Postema and coworkers... 

The addition of allyltributylstannane to aldehydes can also be effected with equimolar amounts of MeSiCl3 or MeSiCl(OMe)2 (Eq. 13) [21]. The initial product is the silyl ether which is hydrolyzed in the aqueous work-up. An allylic silane intermediate was shown not to be involved in the addition. The reaction with benzaldehyde could be accomplished with 0.33 equiv. of trichlorosilane but at a much slower rate. The product of this addition was cleaved by treatment with KF or aqueous acetic acid in THE... 

Keck asymmetric allylation The reaction of aldehydes with allyltributylstannane in the presence of Lewis acid catalysts to form homoallylic alcohols. 236... 

Denmark has spectroscopically examined the reaction of both allyl- and 2-bute-nylstannanes with aldehydes using the Lewis acids SnCU and BF3-OEt2 [73, 82]. First, the metathesis of both allyltributylstannane and tetraallyltin with SnCl4 was determined (by C NMR spectroscopy) to be instantaneous at -80 °C. The reaction of allyltributylstannane with a complexed aldehyde was detemiined to be significantly more complicated. When a molar equivalent of SnCU per aldehyde was employed, metathesis was determined to be the preferred pathway for aldehydes. When one half a molar equivalent of SnC per aldehyde is used, the reaction pathways and product distribution become very sensitive to both the aldehyde structure and addition order. A spectrum of mechanistic pathways was documented ranging from direct addition (acetaldehyde) to complete metathesis (pivalalde-hyde) to a competitive addition and metathesis (4-t-butylbenzaldehyde). The results obtained with a molar equivalent of SnCl4 are most relevant, as this reagent stoichiometry is most commonly used in the addition reactions. 

The reaction of methallyltri-n-butylstannane 117 with achiral aldehydes is also effectively promoted by the binol-Ti complex [89 c]. In all but one case (cyclo-hexanecarboxaldehyde), the yields and enantioselectivities observed with the methallylstannane are identical or higher than those obtained in the reactions with allyltributylstannane with only 10 mol% of the binol-Ti complex (Scheme 10-50). Insight into the nature of the titanium catalyst is provided by the observation of asymmetric amplification [89 b] and chiral poisoning [89 g]. An intruiging hypothesis on the origin of enantioselection in allylation and related reactions [89 h]. 

The asymmetric allylation of achiral aldehydes with a novel silver complex has recently been reported (Scheme 10-51) [90]. Initially, it was shown that the silver-promoted reaction of allyltributylstannane with benzaldehyde could be accelerated by triphenylphosphine. A survey of various chiral phosphine reagents and silver salts identified the combination of binap and AgOTf as optimal. The reaction of benzaldehyde and allyltributylstannane promoted by 5 mol% of the binap-AgOTf... 

Chiral rhodium catalyst 118, pioneered by Nishiyama, has been put to use in the addition of allyltributylstannane to achiral aldehydes [91], This catalyst is relatively insensitive to water and can even be purified by silica gel chromatography. The optimized allylation conditions employ 1 equiv of the aldehyde, 1.5 equiv of allyltributylstannane, and 5 mol% of 118 (Scheme 10-53). The reactions with many different aldehydes can all be performed at room temperature to provide good yields of the desired homoallylic alcohols albeit in moderate to poor enan-tioselectivity. 

The a-siloxyalkyl radical intermediate can be used for carbon-carbon bond formation by intra- and intermolecular trapping (Scheme 10.233). In the BUjSriH-mediated system, a bromoalkenoylsilane is efficiently converted into a bicyclic compound by tandem radical cyclization [601]. When allyltributylstannane is used instead of BuaSnH, the a-siloxyalkyl radical generated from a 5-bromoalkanoylsi-lane undergoes homolytic allylation to provide a homoallyl silyl ether [602]. 

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