- Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins
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Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer. This journal is
- Guo, Xingwei,Pfund, Bj?rn,Schreier, Mirjam R.,Wenger, Oliver S.
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p. 8582 - 8594
(2020/09/07)
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- Ether-directed ortho-C-H olefination with a palladium(II)/monoprotected amino acid catalyst
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Weak coordination is powerful! A PdII-catalyzed olefination of ortho-C-H bonds of arenes directed by weakly coordinating ethers is developed by using monoprotected amino acid (MPAA) ligands. This finding provides a method for chemically modifying ethers, which are abundant in natural products and drug molecules. HFIP=hexafluoroisopropanol. Copyright
- Li, Gang,Leow, Dasheng,Wan, Li,Yu, Jin-Quan
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supporting information
p. 1245 - 1247
(2013/03/13)
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- Iron-catalyzed cross-coupling of alkyl sulfonates with arylzinc reagents
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Iron-catalyzed cross-coupling reactions of primary and secondary alkyl sulfonates with arylzinc reagents proceed smoothly In the presence of excess TMEDA and a concomitant magnesium salt. The arylzinc reagents are prepared from the corresponding aryllithium or magnesium reagents with ZnI2. The In situ formation of alkyl Iodides and consecutive rapid cross-coupling avoids discrete preparation of the unstable secondary alkyl halides and also achieves high product selectivity.
- Ito, Shingo,Fujiwara, Yu-Ichi,Nakamura, Eiichi,Nakamura, Masaharu
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supporting information; experimental part
p. 4306 - 4309
(2009/12/26)
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- Chelates as intermediates in nucleophilic additions to alkoxy ketones according to Cram's rule (cyclic model)
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Chelates have been considered intermediates in the often highly stereoselective reactions of α-alkoxy and similarly substituted ketones for over 30 years,10 but without mechanistic evidence. It is now shown, by stop-flow ("rapid injection") NMR kinetics,15 that the specific rates of reaction of ketones C6H5COCH(OR)CH3 with Me2Mg, where R = (i-Pr)3 ("TIPS"), t-BuPh2Si, t-BuMe2Si, Et3Si, Me3Si, and Me, parallel the diastereoselectivity of the reaction; i.e., the fastest reacting compound (R = Me) is the one which gives the highest proportion of the product predicted by Cram's chelate rule. The major product of the slowest reacting compound (R = TIPS) is not in accord with Cram's chelate rule, and this compound reacts at the same specific rate as the parent, C6H5COCH2CH3. This is in accord with earlier work indicating that TIPSO does not chelate. Compounds intermediate in the series react at intermediate rates and give the two diastereomeric products in proportions which can be calculated by assuming two competing reactions (cf. Figure 2): one proceeding via the chelated transition states giving the product predicted by the chelate rule and one not involving chelation which gives the same product composition as the R = TIPS compound. Direct steric effects on carbonyl reactivity due to the remote bulky silyloxy substituents have been excluded by the study of carbon analogues bearing similar bulky groups. Thus, the kinetic effect in the above series appears to be due to steric hindrance to chelation; hence, the parallel of specific rate and stereoselectivity demonstrates that high stereoselectivity is associated with strong chelation, as postulated by Cram and Kopecky in 1959.10.
- Chen, Xiangning,Hortelano, Edwin R.,Eliel, Ernest L.,Frye, Stephen V.
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p. 1778 - 1784
(2007/10/02)
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- THE EFFECT OF para-SUBSTITUENTS ON THE CONFORMATIONAL BEHAVIOUR OF AROMATIC SIDECHAINS
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Proton nmr data are reported for some 2-aryl-1-propyl methyl esters, 2-aryl-1-propyl acetates and trifluoroacetates, 2-aryl-1-propanols and 4-aryl-2,2-dimethylpentanes.The vicinal coupling constants show that there is an increasing population of gauche Aryl/OR rotamers in the first three series (R = Me, R = COMe and R = COCF3) as electron donor groups on the aromatic ring are replaced by electron withdrawing groups.The results are ascribed to a gauche Aryl --- O interaction, the possible origins of which are discussed.
- Cook, Michael J.,Khan, Tauqeer A.,Nasri, Khalida
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p. 249 - 258
(2007/10/02)
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