Allyltributylstannanes, addition

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). 

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si—H moiety has been reported as a key step in several unimolecular chain transfer reactions [11]. In particular, the 1,5-hydrogen transfer of radicals 8-11 (Reaction 3.4), generated from the corresponding iodides, was studied in competition with the addition of primary alkyl radicals to the allyltributylstannane and approximate rate constants for the hydrogen transfer have been obtained. Values at 80 °C are in the range of (0.4-2) x 10" s, which correspond to effective molarities of about 12 M. 

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). 

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

A chiral BINOL-indium(in) complex has been used to catalyse the addition of allyltributylstannane to aldehydes in high ee.184... 

The enantioselective addition of allyltributylstannanes to aldehydes and ketones has been performed in the presence of various chiral indium(III) complexes derived from (R)-BINOL,136 (S )-BINOL,137,138 and PYBOX.139,140... 

In 1995, Porter et al. [34] reported the first excellent results for free radical addition to an electron-deficient alkene by use of chiral zinc complexes. Reaction of the oxa-zolidinone 9 with tert-butyl iodide and allyltributylstannane 30 in the presence of Zn(OTf)2 and a chiral bis(oxazoline) ligand 12 gave the adduct 44 in 92 % yield with 90 % ee (Sch. 18). The chiral bis(oxazoline) complexes derived from ZnCl2 or Mg(OTf)2 gave racemic products. In this reaction, lower allyltin/alkene ratios gave substantially more telomeric products, and a [3 + 2] adduct 45 of the oxazolidinone 9 and the allylstannane 30 was obtained at temperatures above 0 °C. 

Fhal and Renaud have examined the alkylation of a radical generated from the a-iodoimide 333 with a variety of Lewis acids, as shown in Sch. 42 [70]. The stereogenic step in this process would be hydrogen atom transfer from tin to a Lewis acid-com-plexed radical generated from 333. Initial screening was performed for the reaction of allyltributylstannane and imide 333, which was conducted by precomplexation of the imide with the Lewis acid and then addition of the stannane in the presence of AIBN under irradiation at 10 °C. The Lewis acid prepared from BINOL was ineffective whereas that prepared from the bis-sulfonamide 337 was slightly superior to that from the TADDOL ligand 339. 

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... 

While selective reaction of aldehydes takes place with the typical Lewis acids TiCL, SnCl4, TMSOTf, etc., lanthanide triflates [Ln(OTf)3] are unique Lewis acids that change the reaction course dramatically aldimine reacts selectively in the coexistence of aldehydes [70]. Among a series of Ln(OTf)3 tested, Yb(OTf)3 exhibited the most prominent chemoselectivity in addition to high chemical yields. The silyl enol ethers of ketones, allyltributylstannane and Me3SiCN are all applicable as chemoselective nucleophiles (Table 2-9). Preferential formation of Yb(OTf)3-aldimine complexes was postulated by C NMR spectral analysis in the presence of PhCHO and Y-benzylideneaniline. 

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 Lewis acid-promoted addition of allyltributylstannane and ( )-85 to /i-alk-oxy aldehydes has also been investigated [84c]. The Lewis acid-promoted reaction of a 9-siloxy aldehyde 106 with ( )-85 affords almost entirely the syn homo-... 

The addition of allyltributylstannane to a chirally modified aldehyde 111 proceeds in high yield and moderate diastereoselectivity to give the horaoallylic alcohols 112 and 113 (Scheme 10-46) [86]. When the methyl group on the sugar is not present, the reaction proceeds in higher diastereoselectivity (95%) and excellent yield. The formation and reaction of a 2/1 complex (aldehyde/Lewis acid) with the allylstannane can account for the observed selectivity. 

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. 

During the past two decades the homogeneous and heterogeneous catalytic enan-hoselective addition of organozinc compounds to aldehydes has attracted much attention because of its potential in the preparation of optically active secondary alcohols [69]. Chiral amino alcohols (such as prolinol) and titanium complexes of chiral diols (such as TADDOL and BINOL) have proved to be very effective chiral catalysts for such reactions. The important early examples included Bolm s flexible chiral pyridyl alcohol-cored dendrimers [70], Seebach s chiral TADDOL-cored Frechet-type dendrimers [28], Yoshida s BINOL-cored Frechet-type dendrimers [71] and Pu s structurally rigid and optically active BlNOL-functionalized dendrimers [72]. All of these dendrimers were used successfully in the asymmetric addition of diethylzinc (or allyltributylstannane) to aldehydes. 

A plausible mechanism for this asymmetric allylation has been formulated (Fig. 7). The first step involves the transmetallation of allyltributylstannane to palladium. The resulting bis-7t-allylpalladium complex 69 would react with im-ine 67 to give the 7t-allylpalladium complex 70 this coordination stage is the key step for the asymmetric induction observed in this reaction. The addition step would produce the 7t-allylpalladium amide 71, and another transmetallation of allyltributylstannane to palladium would lead to the formation of the desired product and the regeneration of complex 69. 

The catalytic process was also applied to enantioselective additions of meth-allyltributylstannane and allenyltributylstannane to aldehydes [13,14]. The methyl group of methallytin compound does not affect the chemical yield or enantioselectivity in the reaction [13]. A positive nonlinear effect was observed for... 

Functionalized allyltributylstannanes can also be used in the BINOL/Ti-cata-lyzed reaction and addition of P-substituted allyl groups with heteroatoms in the side chain to aldehydes has been achieved with a high degree of enantiocon-trol [ 16]. The catalytic asymmetric allylation has been successfully applied to to -tal syntheses of the macrohdes (l )-(-i-)-ricinelaidic acid lactone and (-)-gloe-osporone [21]. 

Most commonly used chiral Lewis acids have been derived from main group and early transition series elements. An initial attempt at utilizing optically active catalysts of late transition metal complexes for the enantioselective addition of allyltributylstannane to aldehydes was made by Nuss and Rennels [30]. Employment of Rh(COD)[(-)-DIOP]BF4 (11) as a catalyst, however, resulted in only a small degree of asymmetric induction (17% ee). 

Addition of allyltributylstannane to aldehydes is also catalyzed hy Cul in DMF at room temperature. ... 

Conjugate addition. Allyl transfer from allyltributylstannane to enals, enones, and... 

Iodine transfer addition to allyltrimethylsilane provides a more environmentally friendly alternative to allyltributylstannane. In these allylations, which are exemplified in Scheme 6, the initially formed I-transfer product undergoes spontaneous loss of TMSI to generate the observed allylation product. Guindon has shown that allylation of the Lewis acid complexes of ) -alkoxy esters in this manner can lead to products with high anti stereoselectivity [22]. It is also believed that the presence of Lewis acids enhances the electrophilicity of the radical. Allylations of this type can also prove successful when Br-transfcr or PhSe-transfer reactions are employed. 

Addition to N-tosylaldimines. Protected a-amino acids are obtained from the addition to ethyl glyoxylate under the influence of CuPFg-lp-tolyllBINAP. Indole and allyltributylstannane are typical addends. The a-hydroxyamine precursor of the imine can be used in the reaction with enol silyl ethers, but a higher (room) temperature is needed. 

Exposure of 14 (m = 3, R = rcrf-Bu) to cyclohexyl iodide, allyltributylstannane, and AIBN leads to a macrocycle 15 with two new stereogenic centers. The allyl group provides additional functionality for further transformations and also creates a new stereocenter in the process. In order to effect the desired macrocyclization, addition of the first-formed radical to the proximal acrylamide moiety must be faster than addition to the chain transfer agent allyltributylstannane, a requirement that can be fulfilled under appropriate reaction conditions. Premature chain transfer in this particular system, under conditions that discourage bimolecular reaction between two templates, leads to two simple n = 1 products (vide infra). 

Allylation. The catalyzed addition of an allyl group to aldehydes by using allyltrimethylsilane, and a similar reaction on imines with allyltributylstannane, proceed at room temperature or below. Homoallylic alcohols and amines are formed, respectively. 

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