Allylic stannanes carbonyl addition reactions

BINOL-Ti complexes (1) has been shown to serve as efficient asymmetric catalysts for the carbonyl addition reaction of allylic stannanes and silanes 152,53]. The addition reactions to glyoxylates of ( )-2-butenylsilane and -stannane proceed smoothly to afford the corresponding syn-product with high enantiomeric excess (Scheme 8C.21) [52]. 

The chiral titanium complex BINOL-T1CI2 also catalyzes the asymmetric carbonyl addition reaction of allylic silanes and stannanes. Thus the addition reaction of glyoxylate with (E)-2-butenylsilane and -stannane proceeds smoothly to give the syn product in high enantiomeric excess (eq 12). The syn product thus obtained can be readily converted to the lactone portion of verrucaline A. The reaction of aliphatic and aromatic aldehydes with allylstannane is also catalyzed by BINOL-TiC to give... 

The use of InCls for Lewis acid activation of aldehyde substrates leads to rapid transmetalation of the allylic stannane, followed by carbonyl addition reactions of an allyl indium reagent. Premixing of InCb and the allylic stannane in the absence of aldehyde often produces precipitation and poor results. On the other hand, allyl indium reagents have been independently prepared by several procedures, including reductive metalations. Several important reviews describe the methods of preparation and the reactivity of allyl indium compounds. This discussion will be limited to key factors regarding the transmetalation of allylic stannanes in the presence of aldehydes. Stereochemical events leading to the production of anti adducts as major products are illustrated in Scheme 5.2.55. 

While the chiral aldehydes or allyl nucleophiles are based on stoichiometric amounts for the control of diastereoselectivity [74, 77], it has been found that catalytic amounts of titanium complexes derived from BINOL can mediate the enantioselective addition of allyl stannanes to aldehydes, giving the homoallyl alcohols high enantioselectivity. Mikami reported that the BINOL-Ti complexes prepared in situ in the presence of 4A molecular sieves (MS) catalyze the carbonyl addition reaction of allyl silanes or stannanes to afford the syn product in high enantiomeric excess [78] (Scheme 14.21). 

Studies directed toward the selection and application of an exogenous nucleophile for use with this SiCLj/phosphoramide system first focused on the use of allyltri-butylstannane 20 [35]. The stannane is a competent nucleophile in a wide variety of Lewis acid-promoted carbonyl addition reactions [13, 36-38], and in this system it would provide an effective trap for the ionized chloride through formation of tributyltin chloride. Initial experiments using the optimal phosphoramide catalyst (/ )-10 for the SiCL-mediated opening of meso-epoxides gave good yield and a moderate level of enantioselectivity in the addition of the allyl stannane to... 

The Lewis acid-mediated addition of allyl silanes and allyl stannanes to carbonyl compounds has been a topic of interest for many years [103,104]. Stereochemical studies of reactions of allylsilanes and allylstannanes with aldehydes have included FeCl3-mediated intramolecular reactions (Sch. 20) [105],... 

The Lewis-acid-catalyzed carbonyl-ene reaction represents an important alternative method for the addition of an allyl group to a carbonyl group (Equation 23). The resulting secondary homoallylic alcohols are amenable to a number of structural modifications and constitute useful synthetic building blocks. Because the olefin of the products can be a surrogate for a carbonyl functionality, these homoallylic alcohol are formally the synthetic equivalent of aldol addition products. Powerful asymmetric versions of the carbonyl-ene reaction [196, 197] provide an important alternative to the more traditional allylation methods, which primarily employ silanes, boranes, or stannanes (see Chapter 5). 

Substitution at the terminal position of the allylstannane, as in crotonyltributyl stannane, however, is not tolerated, because hydrogen abstraction from the allylic position is a competing reaction [21], An extension of the method involves the coupling of the anomeric radical precursors 28 with the allyltributyltin reagent 29 [14], In the reagent 29 the double bond is activated toward addition of nucleophilic radicals by the electron-withdrawing t-butoxy carbonyl group. The obtained product 30 has been useful en route to 3-deoxy-D-marmo-2-octulosonic acid (KDO). 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

Cyclopentyl radicals flanked with /1-substituents on both sides are formed in reactions of 3-alkyl-2-phenylselenocyclopentanones36. After photochemical initiation, the cyclopentyl radical is formed through abstraction of the phenylseleno group. Addition to tributyl(2-propenyl)stannane occurs preferentially trans to the / -alkyl substituent and consecutive elimination of the stannyl radical gives the final allylation product. It has also been reported37 that addition of the /1-methyl cyclopentyl radical to 2-propenenitrile occurs with d.r. (transjeis) 92 8, hence it seems likely that the carbonyl group adjacent to the radical center reduces the selectivity. 

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