Synthesis of cyclobutenes
The facile preparation of organoboronates and their remarkable stability and functional group tolerance enable straightforward syntheses of four- and five-membered carbo- and heterocycles via in situ transmetalation of sensitive organometallics with boron alkoxides.
A. N. Baumann, M. Eisold, A. Music, D. Didier, Synthesis, 2018, 50, 3149-3160.
Gold catalysis enables an efficient intermolecular [2+2] cycloaddition between two different types of chloroalkynes and unactivated alkenes. Monosubstituted unactivated alkenes can be converted with excellent regioselectivities. With 1,2-disubstituted unactivated alkenes, the reaction is largely stereospecific.
Y.-B. Bai, Z. Luo, Y. Wang, J.-M. Gao, L. Zhang, J. Am. Chem. Soc., 2018, 140, 5860-5865.
The use of sterically hindered cationic Au(I) complexes as catalysts enables a regioselective, intermolecular reaction of terminal alkynes with alkenes to give cyclobutenes.
V. López-Carrillo, A. M. Echavarren, J. Am. Chem. Soc., 2010, 132, 9292-9294.
A catalytic intermolecular [2 + 2] cycloaddition of terminal alkynes with electron-deficient alkenes proceeds in the presence of 8-quinolinolato rhodium/phosphine catalyst system to give cyclobutenes from various substrates in high yields and with complete regioselectivity.
K. Sakai, T. Kochi, F. Kakiuchi, Org. Lett., 2013, 15, 1024-1027.
An In(tfacac)3-TMSBr catalyzed selective [2 + 2]-cycloaddition enables an efficient synthesis of cyclobutenes from aryl alkynes and acrylates with high chemo- and stereoselectivity. Densely functionalized decalin skeletons can be produced through a dearomatizing cascade reaction. The obtained cyclobutenes could be easily converted into cyclobutanes as well as synthetically useful 1,4- and 1,5-diketones with high chemo- and stereoselectivity.
L. Shen, K. Zhao, K. Doitomi, R. Ganguly, Y.-X. Li, Z.-L. Shen, H. Hirao, T.-P. Loh, J. Am. Chem. Soc., 2017, 139, 13570-13578.
A nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with electron-deficient alkenes as well as electronically neutral norbornene leads to cyclobutenes. The use of conjugated enynes circumvented possible side rections, such as oligomerizations and cyclotrimerizations. η3-Butadienyl coordination is the key for the selective formation of cyclobutenes.
A. Nishimura, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc., 2012, 134, 15692-15695.
Cobalt-catalyzed transformations of cyclic alkenes such as cyclopentene and cycloheptene with internal alkynes led to a chemoselective Alder-ene or a [2 + 2] cycloaddition reaction depending on the electronic nature of the alkyne and the bite angle of a bidentate phosphine ligand used.
G. Hilt, A. Paul, J. Treutwein, Org. Lett., 2010, 12, 1536-1539.