Boronates allenylboronates

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]. 

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... 

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. 

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]. 

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