Allylboranate

Reaction of the allylborane 147 with 148 then workup with KOH gave the E isomer, while the workup with sulfuric acid gave the Z isomer 149. Heating the azidodiene 149 in CDCI3 in a sealed tube afforded 150 in one step. The stereoselectivity of the reaction was found to be slightly dependent on the geometry of the diene (89TL6661) (Scheme 29). 

I.3.3.3.3.I. Allylboranes and AUylboronates Lacking Stereogenic Centers Within the AUyl Unit... 

L Preparation of Allylboranes and Allylboronates Lacking Stereogenic Centers... 

L3.3.3.3.1.1.1. From Allylic Organometallic Reagents and Electrophilic Boranes (a) Allylboranes and Allylboronates... 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

Quaternary centers have also been generated, often with good to excellent stereoselectivity, by the addition of y.y-disubstituted allylboranes or y,) -disubstituted aiiylboronates to aldehydes 50,97. 

Allylboranes and Allylboronates With Stereogenic Centers to Boron... 

The synthesis of chiral a-substituted allylborane 2 via the hydroboration of 1,3-cyclohexa-diene with diisopinocampheylborane has been reported21. 

Finally, it has been reported that allylboranes 4 and 5 may be prepared by the hydroboration of 4-trimethylsilyl-2,3-alkadienes by using either 9-borabicyclo[3.3.1]nonane or dicyclo-... 

Several studies of reactions of configurationally unstable a-substituted allylboranes have also been reported19,29. The reactions of dialkyl[( )-l-alkyl-2-butenyl]boranes and aldehydes at — 78 °C provide a mixture of syn- and an/i -diastereomers. reflecting reactions by both the Z-and /f-isomers. When generated and used at — I00°C, however, the ff/m-diastereomer is obtained with >95% diastercoselectivity and >90% selectivity for the /T-olefin isomer by way of a transition state analogous to 429. This result suggests that the allylboron isomerization is slow at —100 JC relative to carbonyl addition. 

In 2009, Ritter and coworkers reported a selective Fe-catalyzed hydroboration of 1,3-dienes to produce linear ( )-y-disubstituted allylboranes under mild conditions when a combination of Ll-FeCl2 and magnesium metal as a catalyst was used. Tbe branched ( )-allylboranes were obtained by using L2-FeCl2 instead of Ll-FeCl2 (Scheme 26) [88]. For the synthesis of 2-borylallylsilanes, this method was superior to the previously reported silaboration of allenes [89]. 

In 1987, Vaultier and coworkers [27] developed a combination of a [4+2] cycloaddition of a bora-1,3-diene to provide an allylborane, which then reacts with an aldehyde to give a highly functionalized alcohol. The Lallemand group, as well as Hall and colleagues, has recently used this procedure. In an approach for the synthesis of the antifeedant natural product clerodin (4-83), Lallemand and coworkers performed a three-component domino reaction of 4-80, 4-81 and methyl acrylate to give 4-82 (Scheme 4.18) [28]. 

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. 

The nucleophilic attack on an acceptor-substituted allene can also take place at the acceptor itself, especially in the case of carbonyl groups of aldehydes, ketones or esters. Allenic esters are reduced to the corresponding primary alcohols by means of diisobutylaluminum hydride [18] and the synthesis of a vinylallene (allenene) by Peterson olefination of an allenyl ketone has also been reported [172]. The nucleophilic attack of allenylboranes 189 on butadienals 188 was investigated intensively by Wang and co-workers (Scheme 7.31) [184, 203, 248, 249]. The stereochemistry of the obtained secondary alcohol 190 depends on the substitution pattern. Fortunately, the synthesis of the desired Z-configured hepta-l,2,4-trien-6-ynes 191 is possible both by syn-elimination with the help of potassium hydride and by anti-elimination induced by sulfuric acid. Analogous allylboranes instead of the allenes 189 can be reacted also with the aldehydes 188 [250]. 

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. 

Allylsilanes and allylboranes are allyl anion equivalents, which are stable enough to be included in subsequent allylation reactions of aldehydes under... 

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... 

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

Although the class of bis(isopinocampheyl)allylboranes often leads to better levels of double diastereoselectivity, the basic oxidative work-up required in these allylations is not compatible with all substrates. This is true for the crotylation of the aldehyde derived from the terminal alkene of 168 (Scheme 18),... 

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