Synthesis of cyclopropenes
A novel Rh(II)-catalyzed enantioselective [2 + 1]-cycloaddition of ethyl diazoacetate to terminal acetylenes and olefins with high enantioselectivity is described. The catalyst, which is readily available, robust, and effective, can be recovered and reused.
Y. Lou, M. Horikawa, R. A. Kloster, N. A. Hawryluk, E. J. Corey, J. Am. Chem. Soc., 2004, 126, 8916-8918.
Silver triflate is an efficient catalyst for the cyclopropenation of internal alkynes using donor-/acceptor-substituted diazo compounds as carbenoid precursors to yield highly substituted cyclopropenes, which cannot be synthesized directly via rhodium(II)-catalyzed carbenoid chemistry.
J. F. Briones, H. M. L. Davies, Org. Lett., 2011, 13, 3984-3987.
Dianions generated from alkyllithium reagents and cycloprop-2-ene carboxylic acids can be functionalized by electrophiles at the vinylic position. Due to reproducibility issues for reactions that were carried out in Et2O, a detailed study revealed that THF was a superior solvent. Additionally, it was found that the addition of NMO has a beneficial effect both on the stability of the dianions and on the rates of alkylation.
L. A. Fisher, J. M. Fox, J. Org. Chem., 2008, 73, 8474-8478.
A new cyclopropenation reaction, which involves Cα-Si bond insertion of alkylidene carbenes derived from α-silyl ketones, features excellent selectivity for insertion into Cα-Si bonds rather than insertion into Cγ-H bonds or addition to γ,δ-double or -triple bonds. The selectivity trend clearly indicates that the α-oxygen in the tether significantly promotes Cγ-H insertion.
J. Li, C. Sun, D. Lee, J. Am. Chem. Soc., 2010, 132, 6640-6641.
Sodium bromodifluoroacetate (BrCF2CO2Na) is an effective difluorocarbene source for high-yielding synthesis of gem-difluorocyclopropanes and gem-difluorocyclopropenes under mild conditions.
K. Oshiro, Y. Morimoto, H. Amii, Synthesis, 2010, 2080-2084.
The application of continuous flow technology enabled a controlled generation of difluorocarbene from TMSCF3 and a catalytic quantity of NaI. The in situ generated electrophilic carbene reacts smoothly with a broad range of alkenes and alkynes to provide the corresponding difluorocyclopropanes and difluorocyclopropenes within 10 min residence time at high reaction concentrations.
P. Rullière, P. Cyr, A. B. Charette, Org. Lett., 2016, 18, 1988-1991.