Reactions of Allylboronates

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

Several examples of reactions of allylboronates and /f-alkoxy-x-unsubstituted aldehydes ha ve been reported63-64. These reactions do not proceed with exceptional selectivity, however, since the stereocenter of the aldehydic component is remote from the site of the developing C-C bond. 

The reactions of allylboronates 1 (R = H or CH3) may proceed either by way of transition state 3, in which the a-substituent X adopts an axial position, or 4 in which X occupies an equatorial position. These two pathways are easily distinguished since 3 provides 7 with a Z-olefin, whereas 4 provides 8 with an E-olefinic linkage. There is also a second fundamental stereochemical difference between these two transition states 7 and 8 are heterochirally related from reactions in which 1 is not racemic. That is, 7 and 8 arc enantiomers once the stereochemistry-associated with the double bond is destroyed. Thus, the selectivity for reaction by way of 3 in preference to 4, or via 6 in preference to 5 in reactions of a-subsliluted (Z)-2-butenylboronate 2, is an important factor that determines the suitability of these reagents for applications in enantioselective or acyclic diastereoselective synthesis. 

The allyl boronate esters (R,R)- and (S,S)-1 are prepared by reaction of allylboronic acid, CH2=CHCH2B(OH)2 with l- and D-diisopropyl tartrate.1... 

Allylboration, Allenylboration, and Propargylboration 9.05.3.3.1 Synthesis and reactions of allylboron compounds... 

Less is known about the reactions of allylboron compounds however, an ate complex between B-allyl-9-BBN and NaOMe afforded allylarenes in high yields within 0.5-1 h (Equation (209)).898... 

Less is known about the reaction of allylboron compounds however, allylation will occur smoothly in the presence of a base and a palladium catalyst (Scheme 40). The reaction of tri(crotyl)borane with iodobenzene in THF in the presence... 

Roush asymmetric allylation Reaction of allylboronates with aldehydes to give homoallylic alcohols. 386... 

The reactions of ailylboranes, -silanes and -stannanes with carbonyl compounds and imines always take place with double-bond migration, and the structural stability of the reagent is a common problem that is encountered ( 2.7). The reactions of ailylboranes and -boronates occur without catalysis, while those of allylsilanes and -stannanes usually require the presence of a Lewis add [253], The mechanism of the reactions of allylboron derivatives is concerted, and the addition occurs via a six-membered cyclic transition state. A slightly distorted chair transition state model in which the oxygen of the carbonyl group is coordinated to the boron atom is usually invoked (Figure 6.42). Various steric and polar interactions dictate whether the Si or Re face of the prochiral aldehyde is attacked (models C j... 

Reactions of allylboronates (87)-(89) and aldoximes, imines and sulfenimides have been described. These reactions are considerably slower than those of aldehydes, and consequently reagents (87) and (88) are generally used in preference to the less reactive pinacol ester (89). The reactions of imines and (87) proceed at room temperature, while those of (88) and oximes and sulfenimides generally require heating in refluxing CCU or toluene. The reaction of (88) and oximes can be performed at room temperature if a pressure of several kilobars is applied. ... 

Table 4 Relative Diastereoselection in the Reactions of Allylboronates and a-Methyl Chiral Aldehydes Ratios of...

A more complete picture of relative diastereoselection in the reactions of allylboronates and chiral aldehydes is given in Table 5 structures of the products in the glyceraldehyde acetonide (151) series are given in Scheme The data for (150) and (151) reconfirm the conclusion of Table 4 that dia-... 

Electronic effects are also clearly operational in the reactions of allylboronate (144) since the dia-stereofacial preference switches upon moving from an a-methyl chiral aldehyde (145) to an oxygenated aldehyde such as (150) or (151). TTie identity of the major transition state(s) is less clear with (144), however, since at least two reasonable possibilities exist in every case (those resembling (142a) or (142b) in reactions with a-methyl chiral aldehydes and (158) or (159) with oxygenated aldehydes). 

Table 1. Enantioselective Reaction of Allylboronates 1-3 with Achiral Aldehydes...

The reactions of allylboronates with a-methyl-P-alkoxy aldehydes provides propionate adducts which are useful for the synthesis of polypropionate... 

Figure 6.5 General reaction of allylboronates (1) with aldehydes via...

Although most reactions of allylboronates involve aldehydes, other electrophilic partners are also possible. As exemplified in Equation 40, ketones can also react with allylboronates, yielding tertiary homoallylic alcohols [109]. Additions with simple ketones are much slower than similar reactions with aldehydes, and the stereoselectivity in these additions is quite variable, depending on the difference in size between the two substituents on the ketone. This was also noted in a recent study of binaph-thol-derived allylboronates, where high levels of enantioselection (>96% ee) were obtained in the reaction of 3,3 -(CF3)2-BINOL allylboronate 82 with several aromatic ketones (Equation 41) [110]. 

Reactions of allylboronates with imines [119,120] and oximes [119,121-123] have also been documented. These reactions are attractive because they lead to homoallylic amines as products. They are much slower than similar reactions with aldehydes and the additions are often less selective. Prediction of their stereochemical outcome is complicated by the possibilities that they may proceed via boat-like transition states, and the imine or oxime substrate might undergo E-Z isomerization under the harsh conditions of the additions. Wuts and co-workers, however, observed that (E)-3-tri-alkylsilyl-substituted reagents like 85 are particularly effective for additions to N-ben-zyl imine derivatives (Equation 44) [124]. A few examples of enantioselective additions have been reported [125,126], one of the more successful being the addition of the 2-carboxyester allylboronate 86 to imine 87, yielding exo-methylene y-lactam 88 as a pure enantiomer (Equation 45) [127]. 

Hall and coworkers developed the novel enantioselective allylboration of aldehydes catalyzed by chiral Bronsted acid in 2006 (Scheme 1.10) [12]. Moderate to good enantioselectivities (up to 80% ee) were observed in the reactions of allylboronate and both aromatic and aliphatic aldehydes with 10mol% of chiral Bronsted acid (7) [13], a Lewis acid-assisted chiral Bronsted acid (chiral LBA) developed by Yamamoto group. On the basis of their previous studies on allylboration reactions, the authors proposed that the use of the strong chiral Bronsted acid provides the chiral recognition event through B LA-type cyclic transition state with coordination to the oxygen atom of the allylboronate. 

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