Aldehydes, reaction with allylboranes

Attempted reaction of 1,3-pentadiene with the optically active diboron derived from dialkyl tartrate in the presence of a phosphine-free platinum catalyst gave poor diastereoselectivity (20% de).63 Better selectivity has been attained with a modified platinum catalyst bearing a PCy3 ligand (Scheme 6).64 The reaction of allylborane thus obtained with an aldehyde followed by oxidation with basic hydrogen peroxide affords the corresponding diol derivative with moderate ee. 

A stereoselective route to 2-(phenylthio)-l,3-butadienes such as 327 or 328 was developed by Pearson et al. [167] with allylboranes as crucial intermediates. Addition of 9-BBN to allenyl sulfide 324 gives the allylborane intermediate 325, which subsequently adds to aldehydes (Scheme 8.89). Typical of Peterson olefinations, this reaction can also be terminated by two different work-up procedures, either acidic conditions leading to anti-elimination, which affords Z-configuration of dienes 327, or basic work-up resulting in a syn-elimination to form (E)-dienes 328. 

Although this general principle of asymmetric induction has not been demonstrated for boron enolates, the related addition reactions of allylboranes to aldehydes (eq. [115]) (131) have been examined in this context. The reaction of chiral diol 175 with either triallyl-borane or tri- -methallylborane afforded the boronic esters 176 (Ri = H, Me) in yields exceeding 95% (132a). The addition reactions of 176 to representative aldehydes are summarized in Table 40. In all cases reported, the sense of asymmetric induction from the chiral... 

The observation that aldehyde diastereoface selection is interrelated with allylborane geometry has important implications for the related aldol processes. The reactions of (-)-180a and (-)-180b with both enantiomers of aldehyde 181 revealed both consonant and dissonant double stereodifferentiation. For the Cram-selective ( )-crotyl... 

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

Organoboranes do not normally react with carbonyl compounds in Grignard-like fashion with the exception of allylboranes. Aromatic aldehydes reacted with dialkylboron chloride derivatives in the presence of base to generate arylalkylmethanols in good yields (Equation (132)). On the other hand, reactions of aromatic aldehydes with dialkylboron chlorides in the presence of oxygen resulted in chlorination (Equation (133)).595... 

Similarly, allylboranes 7, easily prepared by lithiation of allyl(dipenylamine) 5 followed by reaction with (-)-fi-methoxydiisopinocampheylborane (6) and subsequent addition of boron trifluoride-diethyl ether complex, react with aldehydes to give, after alkaline hydrogen peroxide workup, enantiomerically pure (> 95% ee) cyclopropylamine derivatives 8 as single diaste-reomers (diastereoselectivity > 95 5). However, yields of 8 are low (25-28% for alkyl aldehydes, 10-15% for aryl aldehydes) and vicinal anti-j8-diphenylamino alcohols 9 (> 95% ee, 23-48% yield) are always the main products. ... 

The stereoselectivity of the reactions of aldehydes with allylboranes and -boronates bearing chiral substituents on the boron atom depends upon the terminal substituent of the allylic double bond. If the C-3 carbon is symmetrically substituted, dien the products are enantiomers, and if it is unsymmetrically substituted, then diastereoisomers will be formed (Figure 6.42). 

The reactions of boronates 2.68 (Re = Rz = H, R = i-Pr) are somewhat less selective [698], However, by using the arenechromium tricarbonyl complex of benzaldehyde or dicobalt hexacarbonyl complexes of a-alkynylaldehydes, homoal-lyl alcohols are obtained with a high selectivity after decomplexation [722, 1203] (Figure 6.44). These selectivities are interpreted by distorted chair transition states (Figure 6.44). In the reactions of allylboranes, the approach of the aldehyde minimizes both the steric interactions with the boron substituents and the eclipsing 1,3-interactions of the aldehyde C-R bond with the B-C bond. In the case of boronates 2.68, repulsive interactions between the oxygen lone purs are also avoided [698,1204] (Figure 6.44). 

Corey and coworkers have described Ae preparation of allylborane (289) by the reaction of bromobo-rane (2W) and allyltributylstannane and shown that (289) undergoes highly stereoselective reactions with )x>th achiral (95-97% ee) and chiral aldehydes (Scheme 54). The corresponding methallyl, ( )-crotyl and 2-chloro- and 2-bromo-allyl reagents were prepared by similar methods and shown to give excellent results in reactions with achiral aldehydes (84-99% ee in most cases). 

Allylic organostannanes react with aldehydes under thermal conditions with a high degree of stereocontrol. Like the reactions of allylboranes or boronates, a cyclic transition state has been invoked to explain the preferential formation of the anti diastereomer from the -allylstannane (1.148) and the syn diastereomer from the Z-allylstannane. In contrast, the reaction of allylic organosilanes with aldehydes is sluggish under thermal conditions. 

Lewis acids catalyse the addition of allylic organostannanes or organosilanes to aldehydes. In contrast to the thermal reactions of allylboranes or allylstannanes, the use of a Lewis acid promotes reaction via an acyclic transition state. With a y-substituted allylsilane, such as crotyltrimethylsilane 156, the E-isomer reacts with excellent selectivity for the syn product (1.149). The corresponding Z-isomer (of 156) also favours the syn product, although with reduced selectivity (64 36). The transition state is thought to involve the ahgnment of the two tt-bonds 180° to one another (1.150). 

Z-Allylboranes give predominantly erythro-sAcohoh on reaction with aldehydes, whereas fAreo-alcohols are obtained from the corresponding E-isomers, and some new applications of this reaction giving variously substituted alcohols have been described. ... 

Of all the chiral auxiliaries that have been studied so far, a-pinene-based allylboranes have proven to be one of the most effective, inexpensive, and versatile reagents based on high optical purity observed for the product homoallylic alcohols. " a-Pinene is highly inexpensive and readily obtained from pine trees. Both enantiomers of a-pinene are available in nature (-)-a-pinene is more common in Europe, and the (+)-a-isomer is found in North American pine trees. Moreover, a-pinene-based allylboranes were found to exhibit excellent levels of reagent-controlled stereoselectivity in their reaction with a wide variety of chiral and achiral aldehydes. These factors greatly simplify the synthetic design of complex natural products as the requisite chiral centers can be... 

Lithiated a-methylene-P-stannylsilane 195 on reaction with Ipc2BCl 25 provides allylborane 22 that upon treatment with aldehydes and oxidative workup to yield a-methylene-y-hydroxysilanes 196 (Scheme 25.30). ... 

Allylborane 235 exhibits excellent reagent control and provides high ee s for a wide range of carbonyl compounds regardless of the substrate stereochemistry. For example, in the reaction of a-chiral aldehydes S-237 and / -237 with allylboranes (-f-)-235 (-)-235, the prodnct stereochemistry entirely depends on the antipode of the reagent used and not on the substrate stereochemistry (Scheme 25.37). 

Synthesis of Hydroxy-acids.— The allylborane (15) (Scheme 10) is readily obtained from an optically active propargyl acetate by hydroboration and rearrangement (with sodium hydroxide). Its reaction with an aldehyde then gives (16) in 63% yield and 80% enantiomeric excess subsequent ozonolysis and oxidation then... 

Ever since the discovery that allylboranes could add in a nucleophilic fashion to aldehydes and ketones in 1964,carbonyl allylboration reactions have been thoroughly utilized in organic chemistry. Modification of allylic boron reagents and the substrates with which they can react has been the focus of many research groups over the past three decades. The application of allylbor-onates in the context of aldehyde allylation, which results in the formation of homoaUylic secondary alcohols via an allyl transfer reaction with aldehydes, has become an invaluable tool to synthetic chemists (Figure 3). ... 

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