- EPR and Preparative Studies of 5- endo Cyclizations of Radicals Derived from Alkenyl NHC-Boranes Bearing tert-Butyl Ester Substituents
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Radical H atom abstraction from a set of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two tert-butyl ester substituents was studied by EPR spectroscopy. The initial boraallyl radical intermediates rapidly ring closed onto the O atoms of their distal ester groups in 5-endo mode to yield 1,2-oxaborole radicals. Unexpectedly, two structural varieties of these radicals were identified from their EPR spectra. These proved to be two stable rotamers, in which the carbonyl group of the tert-butyl ester was oriented toward and away from the NHC ring. These rotamers were akin to the s-trans and s-cis rotamers of α,β-unsaturated carbonyl compounds. Their stability was attributed to the quasi-allylic interaction of their unpaired electrons with the carbonyl units of their adjacent ester groups. EPR spectroscopic evidence for two rotamers of the analogous methyl ester containing NHC-oxaborole radicals was also obtained. An improved synthetic procedure for preparing rare NHC-boralactones was developed involving treatment of the alkenyl NHC-boranes with AIBN and tert-dodecanethiol.
- Dai, Wen,Curran, Dennis P.,Walton, John C.
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- Synthesis and Spectroscopic Studies of 1,1'-(1,8-Naphthylene)di-1H-1,2,3-triazoles
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The first 1,8-diheteroaromatic naphthalenes, 1,1'-(1,8-naphthylene)di-1H-1,2,3-trizoles (1), were synthesized by 1,3-dipolar cycloadditions of 1,8-diazidonaphthalene to acetylenic esters.The spectral properties of these compounds were studied and compared with those of the corresponding 1-(1-naphthyl)-1H-1,2,3-triazoles (2).The two triazole rings at the peri-positions in 1 are in a face-to-face arrangement according to the results of 1H NMR spectra.The UV spectra of 1 are almost identical with each other and show a red shift from those of corresponding 2.No significant spectral differences between 1 and 2 were observed in the IR spectra.The fragment ion with two azirine groups at the peri-position in the naphthalene ring was observed in the MS spectra of 1.
- Nagawa, Yoshinobu,Honda, Koichi,Nakanishi, Hiroshi
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p. 2931 - 2936
(2007/10/02)
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