Synthesis of vinylboronic acids and vinylboronates
Aliphatic, aromatic, heteroaromatic, vinyl, or allylic Grignard reagents eact with pinacolborane at ambient temperature in tetrahydrofuran to afford the corresponding pinacolboronates. The initially formed dialkoxy alkylborohydride intermediate quickly eliminates hydridomagnesium bromide and affords the product boronic ester in very good yield. This reaction also can be carried out under Barbier conditions.
J. W. Clary, T. J. Rettenmaier, R. Snelling, W. Bryks, J. Banwell, W. T. Wipke, B. Singaram, J. Org. Chem., 2011, 76, 9602-9610.
Methylpentanediolborane can be prepared easily by reaction of hexyleneglycol with borane dimethylsulfide or B2H6 generated from NaBH4 and I2. MPBH hydroborates stereo- and regioselectively highly functionalized alkynes. MPBH compares favorably with pinacolborane in terms of reactivity. The obtained vinylboronic esters are air- and chromatography-stable.
N. PraveenGanesh, S. d'Hondt, P. Y. Chavant, J. Org. Chem., 2007, 72, 4510-4514.
Reliable, operationally simple, catalytic α-selective hydroalumination reactions proceed in the presence of diisobutylaluminum hydride and (Ni(dppp)Cl2), and, unlike uncatalyzed transformations, generate little or no alkynylaluminum byproducts. The derived α-vinyl halides and boronates can be synthesized through direct treatment with the appropriate electrophiles.
F. Gao, A. H. Hoveyda, J. Am. Chem. Soc., 2010, 132, 10961-10963.
4-Substituted 1,2-oxaborol-2(5H)-ols were prepared through copper-catalyzed carbomagnesation of propargyl alcohol, followed by the transmetallation of magnesium to boron in a one-pot procedure. The Suzuki-Miyaura cross-coupling of these new 2,2-disubstituted alkenylboronic acids with aryl halides afforded stereodefined 2,3-disubstituted allyl alcohols in good to excellent yields.
G.-H. Fang, Z.-J. Yan, M.-Z. Deng, Synthesis, 2006, 1148-1154.
A new method for the synthesis of organoborates starting from α,β-unsaturated acetals was developed. The available alkoxy-functionalized butadienyl- and styrylboronic esters cross couple effectively with a broad spectrum of aryl halides under mild conditions. This method provides an effective approach to aromatic ketones.
P. B. Tivola, A. Deagostino, C. Prandi, P. Venturello, Org. Lett., 2002, 4, 1275-1277.
The Cross-coupling reaction of bis(pinacolato)diboron with β-(trifluoromethanesulfonyloxy)-α,β-unsaturated carbonyl compounds was carried out in the presence of PdCl2(PPh3)2-2PPh3 (3 mol%) and KOPh in toluene or K2CO3 in dioxane for the synthesis of cyclic and acyclic β-boryl-α,β-unsaturated esters, amides, and ketones in high yields.
J. Takagi, A. Kamon, T. Ishiyama, N. Miyaura, Synlett., 2002, 1880-1882.
A Cu(O-t-Bu)-Xantphos catalyst system was effective for the preparation of various allenylboronates with different substitution patterns, those with functional groups, and an axially chiral one. A Lewis acid promoted stereoselective addition of allenylboronates to aldehydes leads to homopropargylic alcohols.
H. Ito, Y. Sasaki, M. Sawamura, J. Am. Chem. Soc., 2008, 130, 15774-15775.
Alkynylboration of alkynes with alkynyl(pinacol)boranes in the presence of nickel catalysts gave cis-1-borylbut-1-en-3-yne derivatives. Some boryl-substituted enynes were reacted with sp2 halides under Suzuki-Miyaura coupling conditions.
M. Suginome, M. Shirakura, A. Yamamoto, J. Am. Chem. Soc., 2006, 128, 14438-14439.
In the presence of a chiral phosphoramidite ligand, the palladium-catalyzed diboration of allenes can be executed with high enantioselectivity with a range of aromatic and aliphatic allenes. A mechanism is proposed based on isotopic-labeling experiments, stereodifferentiating reactions, kinetic analysis, and computational experiments.
H. E. Burks, S. Liu, J. P. Morken, J. Am. Chem. Soc., 2007, 129, 8766-8773.
Pd-catalyzed diboration of prochiral allenes occurs exclusively at the internal position and is accelerated in the presence of Lewis basic ligands. A chiral ligand was employed, and the enantiomeric excess of a variety of diboration products was found to be in the range of 86-92% ee.
N. F. Pelz, A. R. Woodward, H. E. Burks, J. D. Sieber, J. P. Morken, J. Am. Chem. Soc., 2004, 126, 16328-16329.
Bromoboration of propyne with BBr3 proceeds in syn-selectivity to produce (Z)-2-bromo-1-propenyldibromoborane, which is prone to stereoisomerization. Treatment with pinacol yields the stable and storable pinacolboronate. Negishi coupling gives trisubstituted (Z)-alkenylpinacolboronates in good yields. Iodinolysis of the boronates affords alkenyl iodides in good yields.
C. Wang, T. Tobrman, Z. Xu, E.-i. Negishi, Org. Lett., 2009, 11, 4092-4095.
A stereoselective synthesis of tetrasubstituted alkenylboronates, which are useful synthetic intermediates for further functionalizations, was established via the lithiation/nucleophilic addition reaction of 1,1-organodiboronates to carbonyl compounds.
K. Endo, M. Hirokami, T. Shibata, J. Org. Chem., 2010, 75, 3469-3472.
A number of alkynyl pinacolboronates were stereoselectively reduced to the cis-alkenyl pinacolboronates via hydroboration with dicyclohexylborane followed by chemoselective protodeboronation using acetic acid. Treatment with potassium hydrogen fluoride smoothly converted these to the corresponding potassium organotrifluoroborates.
G. A. Molander, N. M. Ellis, J. Org. Chem., 2008, 73, 6841-6844.
The cobalt-catalyzed Diels-Alder reaction of alkynyl pinacol boronic esters with various dienes generates cycloadducts in very good regioselectivity. A reaction sequence (Diels-Alder reaction, Suzuki coupling, DDQ oxidation) was successfully performed as a one pot operation without isolation of the intermediates.
G. Hilt, K. I. Smolko, Angew. Chem. Int. Ed., 2003, 42, 2795-2797.
In situ generation of allylic boronates by iridium-catalyzed borylation of cyclic alkenes in the presence of additives, such as methylimidazole and DBU, followed a reaction with aldehydes allows the synthesis of stereodefined homoallylic alcohols. Cycloalkenes without additives as well as acyclic substrates gave vinylic boronates, which were coupled with organohalides in a Suzuki-Miyaura sequence.
V. J. Olsson, K. J. Szabó, J. Org. Chem., 2009, 74, 7715-7723.
1-Alkenylboronic acid pinacol esters have been synthesized via a palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron with 1-alkenyl halides or triflates in toluene at 50°C in the presence of KOPh in high yields with complete retention of configuration of the double bonds. A one-pot synthesis of unsymmetrical 1,3-dienes via the borylation-coupling sequence has been developed.
J. Takagi, K. Takahashi, T. Ishiyama, N. Miyaura, J. Am. Chem. Soc., 2002, 124, 8001-8006.
Pd-catalyzed enantioselective diborations of prochiral allenes followed by allylation reactions with primary imines provide vinyl boronates which may be oxidized to give nonracemic Mannich products. Alternatively, enantiomerically enriched homoallylic amine derivatives may be obtained by protonation and Suzuki cross-coupling of the vinyl boronate.
J. D. Sieber, J. P. Morken, J. Am. Chem. Soc., 2006, 128, 74-75.