Allenylboronic acid

Allyltributyltin, 10 Vinyltrimethylsilane, 343 Homoallylic ethers Allyltrimethylsilane, 11 Crotyltrimethylsilane, 86 Trityl perchlorate, 339 Homopropargylic alcohols Allenylboronic acid, 36 D imethoxy [ 1 -trimethylsilyl-1,2-buta-dienyljborane, 218 Lithium acetylide, 44 Triisopropoxy[l-(trimethylsilyl)-l,2-buta-dienyl]titanium, 218 Hydroperoxides Oxygen, singlet, 228 Hydroxy acids a-Hydroxy acids... 

Triethylaluminum, 204 Triisobutylaluminum, 205 Trimethylaluminum, 22, 205 Vilsmeier reagent-Lithium tri-r-butoxy-aluminum hydride, 342 Boron Compounds Alkyldimesitylboranes, 8 Allenylboronic acid, 36 9-Borabicyclo[3.3.1]nonane, 92 Borane-Dimethylamine, 42 Borane-Dimethyl sulfide-Sodium borohydride, 25... 

Chiralallenylboronic esters.1 These reagents can react with aldehydes to provide /(-acetylenic alcohols enantioselectivity (60-95% ee). Thus, allenylboronic acid 1, prepared as shown, reacts with aldehydes in the presence of ( + )-DET or ( + )-DlPT to give (S)-alcohols (2), whereas (R)-alcohols (2) are obtained in the presence of (-)-DF.T or ( —) DIPT, both in optical yields of 85-95%. The selectivities are lower in reaction with aryl and z,/3-unsaturated aldehydes. 

PhCHO) being less effective substrates than their aliphatic counterparts. The relative complexity and low yields (40-60%) of the starting allenylboronic acid coupled with the requirement that the chiral boronic esters have to be freshly prepared immediately before their use in the allenylboration detract somewhat from the overall value of the process as do the long reaction times (20 h, -78 °C), the hydrolysis of the chiral boron ligation upon work-up and the relative inefficiency of chirality transfer for aromatic substrates. 

Allenylboronic acid, readily accessible from propargylic halides, on esterification with tartaric acid esters can also be induced to add to aldehydes with high enantioselectivity to give homopropargylic alcohols (Scheme 82). ... 

Ikeda, N., Omori, K., Yamamoto, H. Complete 1,3-asymmetric induction in the reactions of allenylboronic acid with 3-hydroxy ketones. Tetrahedron Lett. 1986, 27, 1175-1178. 

Reaction of allenylboronic acid with 3-hydroxy ketones in anhydrous ether at room temperature in the presence of SA molecular sieves for 20 h, followed by treatment with basic hydrogen peroxide, yields 1,3-diols with high 1,3-asymmetric induction (>99%) (equation 9). ... 

PhCHO) being less effective substrates than their aliphatic counterparts. The relative complexity and low yields (40-60%) of the starting allenylboronic acid coupled with the requirement that the chiral boronic... 

Palladium-catalyzed annulation reactions of conjugate acceptors and allenyl boronic ester provide substituted cyclopentenes in high yields and diastereose-lectivities (Scheme 6.24). These reactions are hypothesized to commence by the conjugate addition of a nucleophilic propargyl-palladium complex. Transmetalation of allenylboronic acid pinacol ester with a Pd(II) catalyst proceeds via an SE2 mechanism to provide the propargyl-palladium complex, which on conjugate attack on the electrophile furnishes an allene intermediate. Finally, endo carbopalladation of the pendant allene and protodepalladation generates the cyclopentene [28]. 

Catalytic enantioselective allylic substitutions of aiyl- and alkyl- substituted phosphates (63) with commercially available allenylboronic acid pinacol ester (64) resulted in the formation of tertiary or quaternary C-C bonds. Reactions were promoted by sulfonate-bearing chiral bidentate Af-heterocyclic carbene (NHC) complexes of copper (66), which exhibited the unique ability to furnish chiral products arising from the Sj.,2 mode of addition. Allenyl-containing products (65) were generated in up to 95% yield, >98% 8 2 selectivity and 99 1 enantiomeric ratio (Scheme 21). ... 

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