Allenylboronate chiral

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

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). 

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. 

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. 

Table 10-22. Reaction of chiral allenylboronate 229 with aldehydes [134b]. -P Y /-Pr...

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... 

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... 

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

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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