- Bromination of tri(isopropyl)boroxine and asymmetric synthesis of (2-cyano-3,3-dimethylcyclopropyl)boronic esters
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Bromination of triisopropylboroxine to tris(1-bromo-1-methylethyl) boroxine is far more facile than α-bromination of sec-alkylboronic esters. Normal fluorescent room light is sufficient to initiate the free radical reaction, which can be carried out as a titration. The steric environment for replacement of the bromine by lithioacetonitrile and subsequent asymmetric insertion of a chloromethyl group into the carbon-boron bond is more highly hindered than what has been studied previously, and the pinanediol ester proved to be the only useful chiral boronic ester in such circumstances. Cyclization of the cyano-substituted boronic ester to the corresponding cyclopropylboronic ester yielded a mixture of diastereomers, presumably the result of base-induced epimerization of the initially formed major isomer having the boronic ester and cyano groups trans.
- Matteson, Donald S.,Fernando, Dilinie
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p. 100 - 105
(2007/10/03)
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- Oxo(trisyl)borane (Me3Si)3C-BO as an Intermediate
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The acyclic trisylboranes R-B(OSiMe3)-Cl (4a) and R-B(OH)-H (5a) and thecyclic boranes (-RB-O-CO-CO-O) (1a) and (-RB-O-RB-O-SO2-O-) (6a) [R=(Me 3Si)3C, Trisyl ] are thermolyzed in the gasphase to give well-defined products. The tris(trisyl)boroxine (-RB-O-)3 (2a) is formed from 4a and 5a at 140 and 160°C, respectively, besides Me3SiCl and H2, respectively, whereas the six-membered ring [-BMe-CH(SiMe3)-SiMe2-O-SiMe2-CH2-](8) is the product from 1a and 6a at 600 and 700°C, respectively , besides CO/CO2 and SO3, respectively. The oxoborane R-BO is presumablya common intermediate. It is stabilized at the lower temperature by cyc lotrimerization to give 2 and at the higher temperatur by a sequence of several intramolecular steps: a 1,3-silyl shift along the chain C-B-O, an exchange of Me and Me3SiO along the chain Si-C-B, and a C-H addition to the B=C double bond; the steps can be rationalized by analogous known reactions. The gas-phase thermolysis at 600°C of the dioxaboracyclohexenes (-BR-O-CH'=CH-CRR'-O-) (7b-d; R=Me, iPr, tBu; R'=Me) yields the boroxines (RBO)3 and the enones Me-CO-CH=CHR-Me; the cyclohexene 7e (R=Me; R'=CF3) is not decomposed at 600°C.
- Paetzold, Peter,Neyses, Stephan,Geret, Laurence
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