Allenylboronic esters

Chiral allenylboronic esters.1 The enantioselectivity in synthesis of homo-propargylic esters by the reaction of aldehydes with chiral allenylboronic esters (11, 181) is markedly increased by use of bis-2,4-dimethyl-3-pentyl esters of d- or vAaxtaric acid rather than the diethyl ester. Yields in the reaction of various saturated aldehydes are 70-90%, and optical yields are consistently greater than 90% and even higher (97-99%) when the aldehyde is present in excess. However, yields are poor in reactions with aryl and a,p-unsaturated aldehydes. This modified procedure was used in a synthesis of (S)-(—)-ipsenol (2) from d-(—)-bis(2,4-di-methyl-3-pentyl) tartrate (1) (equation I). 

Haruta, R., Ishiguro, M., Ikeda, N., Yamamoto, H. Chiral allenylboronic esters a practical reagent for enantioselective carbon-carbon bond formation. J. Am. Chem. Soc. 1982, 104, 7667-7669. 

The observation that allenylboronic esters react with carbonyl compounds to afford homopropargylic compounds was first made by Favre and Gaudemar [132]. 

Homopropargylic alcohols are available by reaction of carbonyl compounds with allenyl silanes.105 Full details of this chemistry include preparations of variously substituted allenysilanes together with methods for their reaction using TiCl4. The same compounds are available in optically active form by use of chiral allenylboronic esters.106 This method is rather similar to the chemistry outlined in Scheme 27 for the preparation of homoallylic alcohols. A third method for the preparation of homoallylic... 

Another new method for the preparation of 3-acetylenic alcohols uses chiral allenylboronic esters, derived from dialkyl tartrate esters and alleneboronic acid,... 

Table 9.65 Synthesis of silylated allenylboronates from propargylic halides and esters.

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). ... 

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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