- Insertion of Alkylidene Carbenes into B-H Bonds
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We have developed a protocol for insertion of alkylidene carbenes into the B-H bonds of amine-borane adducts, enabling, for the first time, the construction of C(sp2)-B bonds by means of carbene-insertion reactions. Various acyclic and cyclic alkenyl borane-amine adducts were prepared from readily accessible starting materials in good to high yields and were subsequently subjected to a diverse array of functional group transformations. The unprecedented spiro B-N heterocycles prepared in this study have potential utility as building blocks for the synthesis of pharmaceuticals. Preliminary mechanistic studies suggest that insertion of the alkylidene carbenes into the B-H bonds of the amine-borane adducts proceeds via a concerted process involving a three-membered-ring transition state.
- Yang, Ji-Min,Guo, Feng-Kai,Zhao, Yu-Tao,Zhang, Qiao,Huang, Ming-Yao,Li, Mao-Lin,Zhu, Shou-Fei,Zhou, Qi-Lin
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supporting information
p. 20924 - 20929
(2020/12/23)
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- Vinyl carbocations: Solution studies of alkenyl(aryl)iodonium triflate fragmentations
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Generation of vinyl cations is facile by fragmentation of alkenyl(aryl)iodonium trifluoromethanesulfonates. Kinetics and electronic effects were probed by 1H NMR spectroscopy in CDCl3. Products of fragmentation include six enol triflate isomers in addition to iodoarenes. The enol triflates arise from direct reaction of a triflate anion with the starting iodonium salts as well as triflate reaction with rearranged secondary cations derived from those salts. G2 calculations of the theoretical isodesmic hydride-transfer reaction between secondary vinyl cation 7 and primary vinyl cation 6 reveal that cation 6 is 17.8 kcal/mol higher in energy. Activation parameters for fragmentation of (Z)-2-ethyl-1-hexenyl(3,5-bis-trifluoromethylphenyl)iodonium triflate, 17e, were calculated using the Arrhenius equation: Ea = 26.8 kcal/mol, ΔH? = 26.2 kcal/mol, and ΔS? = 11.9 cal/mol·K. Added triflate increases the rate of fragmentation slightly, and it is likely that for most β,β-dialkyl-substituted vinylic iodonium triflates enol triflate fragmentation products are derived from three competing mechanisms: (a) vinylic SN2 substitution; (b) ligand coupling (LC); and (c) concerted aryliodonio departure and 1,2-alkyl shift leading to secondary rather than primary vinyl cations.
- McNeil,Hinkle,Rouse,Thomas,Thomas
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p. 5556 - 5565
(2007/10/03)
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