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Synthesis of alkenes by isomerizations
An effective Pd-catalyzed isomerization of terminal olefins with 2-PyPPh2 as the ligand provides trans-2-olefins bearing various functional groups with high regio- and stereoselectivity under mild reaction conditions. The ligand is crucial for the reaction.
W. Ren, F. Sun, J. Chu, Y. Shi, Org. Lett., 2020, 22, 1868-1873.
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.
A β-dialdiminate-supported, high-spin cobalt(I) complex can convert terminal alkenes, including allylbenzenes, to Z-2-alkenes with unprecedentedly high regioselectivity and stereoselectivity. Deuterium labeling studies indicate that the catalyst operates through a π-allyl mechanism.
D. Kim, G. Pillon, D. J. DiPrimio, P. L. Holland, J. Am. Chem. Soc., 2021, 143, 3070-3074.
A stepwise PCET activation of thermodynamically stable olefins is mediated by an excited-state oxidant and a Brřnsted base to afford an allylic radical that is captured by a Cr(II) cocatalyst to furnish an allylchromium(III) intermediate. In situ protodemetalation of this allylchromium complex by methanol is highly regioselective and affords an isomerized and less thermodynamically stable alkene product.
K. Zhao, R. R. Knowles, J. Am. Chem. Soc., 2022, 144, 137-144.
An operationally simple, atom-economical, and efficient cobalt-catalyzed migrational isomerization of styrenes provides trisubstituted alkenes. Even with very low catalyst loadings, the reaction could be conducted in neat and completed in 1 h with excellent conversion and high E stereoselectivity.
J. Zhao, B. Cheng, C. Chen, Z. Lu, Org. Lett., 2020, 22, 837-841.
Cobalt complexes catalyze an isomerization of 1,1-disubstituted alkenes as an alternative stereoselective approach to trisubstituted alkenes. This protocol exhibits a good functional group tolerance and scalability. The high stereoselectivity of the reaction is attributed to both a π-π stacking effect and the steric hindrance between substrate and catalyst.
S. Zhang, D. Bedi, L. Cheng, D. K. Unruh, G. Li, M. Findlater, J. Am. Chem. Soc., 2020, 142, 8910-8917.
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.
The use of an amido-diphosphine ligand enables an efficient cobalt-catalyzed Z to E isomerization of β-substituted styrenes with good functional tolerance and high stereoselectivity. The reaction could be scaled up to gram-scale with a catalyst loading of 0.1 mol %.
H. Liu, M. Xu, C. Cai, J. Chen, Y. Gu, Y. Xia, Org. Lett., 2020, 22, 1193-1198.
A rhodium complex catalyzes the synthesis of E-alkenes from E/Z mixtures of alkenes in the presence of B2pin2 under mild reaction conditions. The reaction offers broad functional group tolerance and has great application potential.
H. Yang, W. Dong, W. Wang, T. Li, W. Zhao, Synthesis, 2020, 52, 2833-2840.
Hexaisobutyryl reduced cercosporin (HiBRCP) is a highly efficient organic photosensitizer for the promotion of E to Z isomerization of alkenes. Moreover, an energy-transfer-mediated nickel catalysis in the presence of HiBRCP enabled cross-coupling of aryl bromides and carboxylic acids.
Y. Zhang, M. Xia, M. Li, Q. Ping, Z. Yuan, Y. Liu, H. Yin, S. Huang, Y. Rao, J. Org. Chem., 2021, 86, 15284-15297.
The combination of a CoCl2 precatalyst with an amido-diphosphine-oxazoline ligand catalyzes the geometrical isomerization of E/Z mixtures of 1,3-dienes to afford (E) isomers in high stereoselectivity. This facile transformation offers a broad substrate scope with good functional group tolerance and could be scaled up to gram scale.
W. Wang, S. He, Y. Zhong, J. Chen, C. Cai, Y. Luo, Y. Xia, J. Org. Chem., 2022, 87, 4712-4723.
A cobalt-catalyzed multipositional isomerization of conjugated dienes is operationally simple and atom-economical using readily available starting materials with an E:Z mixture to access disubstituted 1,3-dienes with excellent yields and good E,E stereoselectivity. A mechanism via alkene insertion of cobalt hydride species and β-H elimination of a π-allyl cobalt intermediate is proposed.
J. Zhao, G. Xu, X. Wang, J. Liu, X. Ren, X. Hong, Z. Lu, Org. Lett., 2022, 24, 4592-4597.
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.
Zwitterionic organocatalysts with an amide anion/iminium cation charge pair catalyze the isomerization of maleic acid diesters to give fumaric acid diesters. Whereas the cooperative effect of the charges lower the activation barrier for a Michael addition of the catalyst to the substrate, the key interaction results from a nonclassical hydrogen bond.
Y.-P. Lam, Z. Lam, Y.-Y. Yeung, J. Org. Chem., 2021, 86, 1183-1190.
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.
C. Morrill, R. H. Grubbs, J. Am. Chem. Soc., 2005, 127, 2842-2843.
An efficient Z-selective oxidative isomerization process of allyl ethers catalyzed by a cobalt(II) (salen) complex using N-fluoro-2,4,6-trimethylpyridinium trifluoromethanesulfonate (Me3NFPY•OTf) as an oxidant provides thermodynamically less stable Z-enol ethers in excellent yields with high geometric control. Diallyl ethers can also be isomerized at room temperature.
G. Huang, M. Ke, Y. Tao, F. Chen, J. Org. Chem., 2020, 85, 5321-5329.
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.
A unique palladium hydride complex generated from a simple Pd source and boric acid [B(OH)3] enables a redox neutral rearrangement of an allene to a 1,3-diene.
Y. Al-Jawaheri, M. Turner, M. C. Kimber, Synthesis, 2018, 50, 2329-2336.
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.
The Crabtree’s reagent catalyzes the isomerization of N-sulfonyl 2,2-disubstituted aziridines to allyl amines under mild conditions without activation of the catalyst by hydrogen.
A. Cabré. G. Sciortino, G. Ujaque, X. Verdaguer, A. Lledós, A. Riera, Org. Lett., 2018, 20, 5747-5751.
Efficient alkene isomerizations of readily available 1,1-disubstituted alkenylboronate esters provide synthetically valuable trisubstituted alkenylboronates. Either stereoisomer of the target alkenylboronate motif can be obtained by employing different isomerization catalysts.
L. Segura, I. Massad, M. Ogasawara, I. Marek, Org. Lett., 2021, 23, 9194-9198.
A cobalt-catalyzed regioselective olefin isomerization reaction with fine-tunable NNP-pincer ligand structures features high kinetic control of regioselectivity. This mild catalytic system enables the isomerization of 1,1-disubstituted olefins bearing a wide range of functional groups in excellent yields and regioselectivity.
X. Liu, W. Zhang, Y. Wang, Z.-X. Zhang, L. Jiao, Q. Liu, J. Am. Chem. Soc., 2018, 140, 6873-6882.