Categories: C=C Bond Formation >
Synthesis of alkenes by isomerizations
Catalytic access to thermodynamically less stable Z-alkenes have relied upon kinetic control of the reaction. A mild and simple orthogonal approach proceeds via photochemically catalyzed isomerization of the thermodynamic E-alkene to the less stable Z-isomer via a photochemical pumping mechanism.
K. Singh, S. J. Staig, J. D. Weaver, J. Am. Chem. Soc., 2014, 136, 5275-5278.
A riboflavin-mediated highly (Z)-selective and operationally simple, catalytic isomerization has been applied to a variety of enone-derived substrates. The reaction can also be used for the synthesis of the medically relevant 4-substituted coumarin scaffold.
J. B. Metternich, R. Gilmour, J. Am. Chem. Soc., 2015, 137, 11254-11257.
A ruthenium complex is a very efficient catalyst for selective mono-isomerization of various multifunctional alkenes to afford E-products. Many reactions are complete within 10 min at room temperature. Even sensitive enols and enamides susceptible to further reaction can be generated. Very low catalyst loadings can be employed.
C. R. Larsen, D. B. Grotjahn, J. Am. Chem. Soc., 2012, 134, 10357-10360.
Hot water as a mildly acidic catalyst efficiently promoted 1,n-rearrangement (n = 3, 5, 7, 9) of allylic alcohols. In some cases, the rearrangement reactions joined isolated C-C double or triple bonds to generate conjugated polyene or enyne structure motifs. The polyene natural product navenone B has been constructed by iterative use of a Grignard reaction, a 1,3-rearrangement of the resulting allylic alcohol, and subsequent oxidation.
P.-F. Li, H.-L. Wang, J. Qu, J. Org. Chem., 2014, 79, 3955-3962.
Commercially available and very inexpensive benzoic acids catalyze an efficient and simple isomerization of readily prepared allylic alcohols to yield cyclic products, unusual enyne, and dienols. The catalysts can be tuned for reactivity and substrate sensitivity.
J. A. McCubbin, S. Voth, O. V. Krokhin, J. Org. Chem., 2011, 76, 8537-8542.
O3ReOSiPh3 promotes the 1,3-isomerization of various allylic alcohols. Two different strategies allow the selective formation of a single isomer. The first strategy utilizes the formation of a conjugated alkene to ensure a high selectivity. The second strategy employs N,O-bis(trimethylsilyl)acetamide (BSA) as an additive to remove the product from the reaction equilibrium and works well for the isomerization of tertiary allylic alcohols.
C. Morrill, R. H. Grubbs, J. Am. Chem. Soc., 2005, 127, 2842-2843.
Lithium diisopropylamide (LDA) promotes virtually quantitative conversion of allylic ethers to (Z)-propenyl ethers with very high stereoselectivity in THF at room temperature. The reaction time for the conversion increases with more sterically hindered allylic ethers.
C. Su, P. G. Williard, Org. Lett., 2010, 12, 5378-5381.
Substituted benzofurans were synthesized from their corresponding substituted 1-allyl-2-allyloxybenzenes using ruthenium-catalyzed C- and O-allyl isomerization followed by ring-closing metathesis.
W. A. L. van Otterlo, G. L. Morgans, L. G. Madeley, S. Kuzvidza, S. S. Moleele, N. Thornton, C. B. de Koning, Tetrahedron, 2005, 61, 7746-7755.
A hydroxyl group-directed, highly regio- and stereoselective transposition of allylic alcohols based on rhenium catalysis is suitable for a direct isomerization of acetals into the thermodynamically preferred isomer as long as one of the hydroxyl groups is allylic. This method will expand the scope of rhenium-catalyzed alcohol transpositions for complex molecule synthesis.
A. T. Herrmann, T. Saito, C. E. Stivala, J. Tom, A. Zakarian, J. Am. Chem. Soc., 2010, 132, 5962-5963.
Ruthenium hydrides promote the positional isomerization of 1,3-dienes into more highly substituted 1,3-dienes in a stereoconvergent manner. The reaction can also be conducted in one pot starting with an ene-yne metathesis of terminal alkynes and alkenes and a subsequent decomposition of the Grubbs catalyst into a ruthenium hydride, which promotes the dienyl isomerization.
J. R. Clark, J. R. Griffiths, S. T. Diver, J. Am. Chem. Soc., 2013, 135, 3327-3330.
A biphenyl-2-ylphosphine with a basic amino group at the 3′ position possesses orthogonally positioned "push" and "pull" forces, that enable a gold(I)-catalyzed soft propargylic deprotonation and permit the bridging of a difference of >26 pKa units (in DMSO) between a propargylic hydrogen and a protonated tertiary aniline. This design led to efficient isomerization of alkynes into versatile 1,3-dienes with synthetically useful scope under mild reaction conditions.
Z. Wang, Y. Wang, L. Zhang, J. Am. Chem. Soc., 2014, 136, 8887-8890.
Pd(PPh3)4-catalyzed isomerization of methylenecyclopropanes (MCPs) proceeds smoothly at 80°C in acetic acid and toluene to give 1-substituted or 1,1-disubstituted dienes in good to excellent yields. The mechanism is discussed.
M. Shi, B.-Y. Wang, J.-W. Huang, J. Org. Chem., 2005, 70, 5606-5610.
Aryl-substituted cyclopropyl carbinol derivatives undergo a facile stereoselective rearrangement catalyzed by In(OTf)3 in dichloromethane under sonication to produce the substituted conjugated all-trans-butadienes.
B. C. Ranu, S. Banerjee, Eur. J. Org. Chem., 2006, 3012-3015.
Co complexes with large-bite angle phosphine ligands promote an unusual isomerization of E/Z-mixtures of 1,3-dienes to yield almost exclusively the Z-isomer.
Y. N. Timsina, S. Biswas, T. V. RajanBabu, J. Am. Chem. Soc., 2014, 136, 6215-6218.