Aldehydes allylic boron compounds

Unlike aldehydes and ketones, allylic boron compounds are not ubiquitous, commercial organic substrates. There are several methods for the preparation of allylic boronates, however, and many of these have been developed in the past decade. This topic has been reviewed recently " so only the most common methods are emphasized in this section. As a result of the lesser stability of allylic boranes, methods to access these reagents are more limited and it is generally easier to prepare allylic boronates with a wide range of functional groups. 

Figure 5.1 lists a number of auxiliaries for asymmetric allyl addition to aldehydes. Substituted allyl boron compounds have also been used in reactions with achiral aldehydes. Table 5.1 lists several examples of 2- and 3-substituted allyl boron compounds, and the products derived from their addition. Note that for the E- and Z-crotyl compounds, the enantioselectivity indicated is for the isomer illustrated. In some cases, there was more than one of the other three possible isomers found as well. 

Table 5.1. Asymmetric addition of substituted allyl boron compounds to aldehydes. Ligands are illustrated in Figure 5.1.

The boron-oxygen mesomeric effect described in the previous section explains the lower reactivity of allylic boronates towards carbonyl compounds compared to that of allylic boranes. The use of Lewis acids, however, allows boronate derivatives, including hindered ones, to react at temperatures comparable to the analogous boranes. As described above (see section Mechanism and Stereochemistry ), the most reactive allylic boronates are those with the most electrophilic boron centers.The nucleophilicity of the y-position of an allylic boron reagent (the position that forms the new C-C bond with the aldehyde) is also important to the reactivity of the reagent. For example, allylic boronates with... 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

Alkoxy-l-boronobutadiene reacted with maleimides, acrylates, and acrylamides to provide cyclic allyl boronates which undergo allylboration with aldehydes (Equation (148)).409 Analogous syntheses of cyclic allylboron compounds via [2+4]-cycloaddition is discussed in Section 9.05.2.2.1. 

Figure 5.1. Chiral boron compounds for asymmetric allyl addition to achiral primary, secondary, and tertiary alkyl, vinyl, and aryl aldehydes, and their typical enantioselectivities (a-e at -78°, g-j at -100°). (a) [17] (b) [18] (c) [19] (d) [19] (e) [20] (f-h) [21-24] (i-j) [25].

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

Extension of this reaction to electrophiles other than aldehydes was unsuccessful [22, 23], However, propargylic boronates were found to react with allylic halides and various carbonyl compounds [23], The boronates were prepared by lithiation of a methyl-substituted alkyne with t-butyllithium followed by treatment with a trialkylborane. The propargylic boronate preferentially reacts with the electrophile at the y-position to yield propargylic products (Eq. 9.20). The methodology has also been applied to alanates with comparable results. 

Although the allylation reaction is formally analogous to the addition of allylboranes to carbonyl derivatives, it does not appear to occur through a cyclic transition state. This is because, in contrast to the boron in allyl boranes, the silicon in allylic silanes has no Lewis acid character and would not be expected to coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic transition state. Both the E- and Z-stereoisomers of 2-butenyl(trimethyl)silane give the product in which the newly formed hydroxyl group is syn to the methyl substituent.64 The preferred orientation of approach by the silane minimizes interaction between the aldehyde substituent R and the methyl group. 

---