Diarylborinic acids

Significant features of these active borinic acid catalysts are that they are strong Lewis acids and have a hydroxy group on the boron atom. Dehydration is strongly favored in THF. The reaction usually proceeds smoothly, and a,/3-enones are obtained in high yields as ( ) isomers. In reactions of a-substituted-/3-hydroxy carbonyl compounds, a,/3-enones are preferentially obtained from anti aldols, whereas most syn aldols are recovered. This dehydration thus represents a useful and convenient method for isolating pure syn aldols from syn anti isomeric mixtures (Eq. 106). 

The mechanism we have proposed to explain borinic acid-catalyzed dehydration is also depicted in Eq. (106). Reaction of the 8-hydroxy function with the diarylborinic acid leads to a cyclic intermediate, which should be susceptible to dehydration. Subsequent transformation to a,/3-enones occurs via an enolate intermediate resulting from selective abstraction of a pseudo-ayaaX a-proton perpendicular to the carbonyl face. A cyclic intermediate formed from a syn aldol and a diarylborinic acid would be thermodynamically less stable than the cyclic intermediate. Thus, dehydration to ( )-a,/3-enones occurs selectively for anti aldols. 

Several arylboron compounds bearing electron-withdrawing aromatic groups have been examined as catalysts for the OPP oxidation of (5)-perillyl alcohol. Catalysis has been performed with 1 to 2 mol % catalyst in the presence of 6 equiv. pivalaldehyde as hydride acceptor in toluene or benzene solution. Representative results are summarized in Table 1. 

The generality and scope of the (C6F5)2BOH-catalyzed OPP oxidation has been explored using various primary and secondary alcohols. The results are summarized in Table 2. All the allylic alcohols used are oxidized to a,/3-enals and a,/5-enones in high yield (entries 1-5). Unfortunately, however, (E)I Z) isomerization occurs between 

In the oxidation of a diastereomeric mixture of carveol (syrr.anti = 42 58), the syn alcohol is stereoselectively oxidized and the anti alcohol is recovered in 98 % diastereomeric purity. This shows that the catalytic activity of (C6p5)2BOH is very sensitive to steric hindrance in the alcohols (Eq. 108). In oxidations of equimolar mixtures of geraniol and j8-citronellol, geranial is obtained in 96 % yield and most of the /8-citro-nellol is recovered imchanged (Eq. 109). The selective conversion of allylic alcohols in the presence of saturated alcohols is particularly noteworthy. 

---

Dioxaborinium salts, isoelectronic with pyrylium salts, have been prepared from 1,3-diketones and boron compounds such as arylboronic acids of diarylborinic acids or esters thereof. A number of 1,3,2-dioxaborinium perchlorates (202) have been isolated (70RRC635). 

SEE (Scheme 10.4) [29]. Diarylborinic acids such as bis(pentafluorophenyl)borinic... 

Although symmetrical dialkylborinic acids can be obtained from boron alkoxides or boronic esters and organolithium or RMgX reagents " , these compounds usually are more conveniently synthesized, either by partial hydrolysis of trialkylboranes , or by hydroboration of alkenes with monohalogenoboranes followed by hydrolysis (see also refs. 1-8, 5.3.2.4 and ref. 2, 5.3.2.4.1). Diarylborinic acids are prepared by the reaction of 2 mol of ArMgX with trialkoxyborane at low The products are... 

The reactions of dialkyl- or diarylborinic acid esters and alkylboronic acid esters with organolithium, Mg or Al compounds may serve to prepare unsymmetrical BRjR and BArjAr organoboranes " Synthesis of such organoboranes usually proceeds without complications ... 

In the aromatic series, treatment of the intermediate Li triarylalkoxyborate, with hydrochloric acid results in displacement of the aryl group even more easily than for Li diaryldialkoxyborates. This difficulty can be overcome using amines instead of hydrochloric acid or starting with the amine complexes of diarylborinic acids ... 

W. Kliegel, G. Lubkowitz, J. O. Pokriefke, S. J. Rettig, J. Trotter, Structural studies of organoboron compounds, part 72 - nitrones and oximes of bifunctional carbonyl compounds and their reaction products with diarylborinic acids. Crystal and molecular structure of examples of five-, six-, and seven-membered boron chelates, Canadian Journal of Chemistry - Revue Canadienne De Chimie 2000, 78,1325. 

Chan L, Taylor MS (2011) Regioselective alkylation of carbohydrate derivatives catalyzed by a diarylborinic acid derivative. Org Lett 13 3090-3093... 

Arylboron compounds with electron-withdrawing aromatic groups such as triarylborons, diarylborinic acids, and arylboronic acids represent a new class of air-stable and water-tolerant Lewis acid or Bronsted acid catalysts in organic synthesis. In particular, arylboronic acids are showing to be powerful tools in the design of chiral boron catalysts. 

Diarylborinic acids are stronger Lewis acids than the corresponding boronic acids [25]. Ishihara and Yamamoto et al. have designed BLA 13, which is prepared from [3,5-(CF3)2QH3 2B(OH) and a chiral triol in dichloromethane in the presence of MS 4A (activated powder) at room temperature [24dj. Diels-Alder reaction of cyclopen ta-diene and various a,P-enals proceeds smoothly in the presence of 5 mol% of 13, and good enantioselectivities have been observed for the exo adducts (Figure 10.7). 

Diarylborinic acids and arylboronic acids bearing electron-withdrawing substituents are useful as air-stable Lewis acid catalysts for various organic transformations. In particular, the potential of 3,5-(CF3)2C6H3B(OH)2 as a Lewis acid catalyst has strikingly increased since Yamamoto and Ishihara s initial study [32b]. Continued exploratory research on the application of arylboron compounds as air-stable and reusable catalysts is expected to provide powerful and practical methods for various Bronsted-Lewis acid-catalyzed organic transformations. 

The synthesis of diarylborinic acids and their derivatives was accomplished using a Grignard approach (Scheme 6.35 and Example 6.5) [76]. In some cases, the diarylborinic... 

---