- Heterometallic CuIIFeIII and CuIIMnIII alkoxo-bridged complexes revealing a rare hexanuclear M6(μ-X)7(μ3-X)2 molecular core
-
The novel hexanuclear complexes [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2) were prepared through one-pot self-assembly
- Nesterova, Oksana V.,Nesterov, Dmytro S.,Vranovi?ová, Beáta,Bo?a, Roman,Pombeiro, Armando J. L.
-
-
Read Online
- Selective Alkane C-H Bond Oxidation Catalyzed by a Non-Heme Iron Complex Featuring a Robust Tetradentate Ligand
-
An iron complex, [FeII(BpyPY2Me)(CH3CN)2](OTf)2 (1-Fe, where BpyPY2Me is 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine and OTf is triflate), supported by an oxidatively rugged tetradentate ligand is reported for the catalytic oxidation of unactivated C-H bonds in cyclohexane and adamantane. With the use of m-chloroperbenzoic acid (mCPBA) as the terminal oxidant, the iron catalyst shows an alcohol-to-ketone (A/K) ratio of 7.5 for cyclohexane oxidation with conversion percentages as high as 90% with respect to oxidant. Moreover, catalysis toward adamantane oxidation shows high regioselectivity (3°/2° = 45) favoring tertiary C-H bonds with yields up to 87%. Results, including electrospray ionization mass spectrometry and UV-vis spectroscopy, indicate that a molecular non-heme iron(IV)-oxo intermediate is the catalytically active species.
- Chen, Lizhu,Su, Xiao-Jun,Jurss, Jonah W.
-
-
Read Online
- Methyltrioxorhenium-Catalyzed C-H Insertion Reactions of Hydrogen Peroxide
-
Methyltrioxorhenium catalyzes the C-H insertion reaction of hydrocarbons by hydrogen peroxide.In suitable substrates the reaction is sterospecific with retention.
- Murray, Robert W.,Iyanar, Kaliappan,Chen, Jianxin,Wearing, James T.
-
-
Read Online
- Alkali metal directed assembly of heterometallic Vv/M (M = Na, K, Cs) coordination polymers: Structures, topological analysis, and oxidation catalytic properties
-
The reactions of [VO(acac)2] with bis(salicylaldehyde)- oxaloyldihydrazone (H4L) and an alkali metal carbonate M 2CO3 (M = K, Na, Cs), in EtOH/H2O medium upon reflux, resulted in the generation of three new heterometallic VV/M materials, namely the 1D [(VO2)2(μ4-L) {Na2(μ-H2O)2(H2O) 2}]n (1), 2D [{V(μ-O)2}2(μ 4-L){K2(μ-H2O)2(H 2O)2}]n (2), and 3D [{V(μ-O) (μ3-O)}2(μ8-L){Cs2(μ-H 2O)2(H2O)2}]n (3) coordination polymers. They were isolated as air-stable solids and fully characterized by IR, UV-vis, 1H, and 51V NMR spectroscopy, ESI-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses, the latter showing that 1-3 are constructed from the resembling [(VO2)2(μ4/8-L)]2- blocks assembled by the differently bound aqua-metal [M2(μ-H 2O)2(H2O)2]2+ moieties (M = Na, K, Cs). The main distinctive features of 1-3 arise from the different coordination numbers of Na (5), K (7), and Cs (9) atoms, thus increasing the complexity of the resulting networks from the ladder-like 1D chains in 1 to double 2D layers in 2, and layer-pillared 3D framework in 3. The topological analysis of 2 disclosed a uninodal 4-connected underlying net with a rare kgm [Shubnikov plane net (3.6.3.6)/kagome pattern] topology, while 3 features a trinodal 4,7,8-connected underlying net with an unprecedented topology. Compounds 1-3 also show solubility in water (S25 C ≈ 4-7 mg mL-1) and were applied as efficient precatalysts for the homogeneous oxidation of cyclohexane by aqueous H2O2, under mild conditions (50 C) in MeCN/H2O medium and in the presence of an acid promoter. Total yields (based on substrate) of cyclohexanol and cyclohexanone up to 36% and turnover numbers (TONs) up to 5700 were achieved.
- Gupta, Samik,Kirillova, Marina V.,Guedes Da Silva, M. Fatima C.,Pombeiro, Armando J. L.,Kirillov, Alexander M.
-
-
Read Online
- Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant
-
Oxidations of C–H bonds with m-chloroperoxybenzoic acid (m-CPBA) catalyzed by transition metal complexes are known to proceed through a number of routes, from the non-selective free radical to selective concerted and metal-mediated ones. However, there is a lack of understanding of the m-CPBA oxidative behavior, reaction mechanisms and factors that trigger its activity. An experimental and theoretical investigation of sp3 C–H bond oxidation with m-CPBA in the presence of the heterometallic pre-catalyst [CoIII4FeIII2O(Sae)8]·4DMF·H2O (1) (H2Sae = salicylidene-2-ethanolamine) and HNO3 promoter has been performed herein. The catalytic system 1/HNO3/m-CPBA allows mild hydroxylation of tertiary C–H bonds with 99% retention of stereoconfiguration of model alkane substrates, supported by high TOFs up to 2 s?1 (for cis-1,2-dimethylcyclohexane) and TONs up to 1.4 × 104 (at 50 °C). The catalytic effect of 1 is seen at the ppm level, while 1000 ppm (0.1 mol%) loading allows 1000-fold increase of the initial reaction rate up to 9 × 10?5 M s?1. The reaction mechanism was investigated by means of combined kinetic studies (including isotope effects), isotopic labeling (18O2, H218O, D2O), ESI-MS spectroscopy and DFT theoretical studies. The results suggest that the main oxidation pathway proceeds through a concerted mechanism involving a cobalt-peroxo C–H attacking species or via a cobalt–oxyl species (rebound process), rather than a free-radical pathway. Remarkably, the Co(iii) catalyst does not change its oxidation state during the most energetically favored pathway, consistent with a metal–ligand cooperativity. The chlorobenzene radical is responsible for H abstraction in the non-selective side route, which is efficiently suppressed by the acidic promoter. Finally, signs for slow direct oxygen exchange between m-CPBA and water in the presence of a proton or a metal complex are found, suggesting that the results of 18O-tests should be treated cautiously when m-CPBA is used as the oxidant.
- Kuznetsov, Maxim L.,Nesterov, Dmytro S.,Nesterova, Oksana V.,Pombeiro, Armando J. L.,Shul'pin, Georgiy B.
-
p. 282 - 299
(2022/01/19)
-
- Enhancing Chemo- And Stereoselectivity in C-H Bond Oxygenation with H2O2by Nonheme High-Spin Iron Catalysts- And Role of Lewis Acid and Multimetal Centers
-
Spin states of iron often direct the selectivity in oxidation catalysis by iron complexes using hydrogen peroxide (H2O2) on an oxidant. While low-spin iron(III) hydroperoxides display stereoselective C-H bond hydroxylation, the reactions are nonstereoselective with high-spin iron(II) catalysts. The catalytic studies with a series of high-spin iron(II) complexes of N4 ligands with H2O2 and Sc3+ reported here reveal that the Lewis acid promotes catalytic C-H bond hydroxylation with high chemo- and stereoselectivity. This reactivity pattern is observed with iron(II) complexes containing two cis-labile sites. The enhanced selectivity for C-H bond hydroxylation catalyzed by the high-spin iron(II) complexes in the presence of Sc3+ parallels that of the low-spin iron catalysts. Furthermore, the introduction of multimetal centers enhances the activity and selectivity of the iron catalyst. The study provides insights into the development of peroxide-dependent bioinspired catalysts for the selective oxygenation of C-H bonds without the restriction of using iron complexes of strong-field ligands.
- Das, Abhishek,Jana, Rahul Dev,Paine, Tapan Kanti
-
p. 5969 - 5979
(2021/05/04)
-
- Selective C-H halogenation over hydroxylation by non-heme iron(iv)-oxo
-
Non-heme iron based halogenase enzymes promote selective halogenation of the sp3-C-H bond through iron(iv)-oxo-halide active species. During halogenation, competitive hydroxylation can be prevented completely in enzymatic systems. However, synthetic iron(iv)-oxo-halide intermediates often result in a mixture of halogenation and hydroxylation products. In this report, we have developed a new synthetic strategy by employing non-heme iron based complexes for selective sp3-C-H halogenation by overriding hydroxylation. A room temperature stable, iron(iv)-oxo complex, [Fe(2PyN2Q)(O)]2+ was directed for hydrogen atom abstraction (HAA) from aliphatic substrates and the iron(ii)-halide [FeII(2PyN2Q)(X)]+ (X, halogen) was exploited in conjunction to deliver the halogen atom to the ensuing carbon centered radical. Despite iron(iv)-oxo being an effective promoter of hydroxylation of aliphatic substrates, the perfect interplay of HAA and halogen atom transfer in this work leads to the halogenation product selectively by diverting the hydroxylation pathway. Experimental studies outline the mechanistic details of the iron(iv)-oxo mediated halogenation reactions. A kinetic isotope study between PhCH3 and C6D5CD3 showed a value of 13.5 that supports the initial HAA step as the RDS during halogenation. Successful implementation of this new strategy led to the establishment of a functional mimic of non-heme halogenase enzymes with an excellent selectivity for halogenation over hydroxylation. Detailed theoretical studies based on density functional methods reveal how the small difference in the ligand design leads to a large difference in the electronic structure of the [Fe(2PyN2Q)(O)]2+ species. Both experimental and computational studies suggest that the halide rebound process of the cage escaped radical with iron(iii)-halide is energetically favorable compared to iron(iii)-hydroxide and it brings in selective formation of halogenation products over hydroxylation.
- Rana, Sujoy,Biswas, Jyoti Prasad,Sen, Asmita,Clémancey, Martin,Blondin, Geneviève,Latour, Jean-Marc,Rajaraman, Gopalan,Maiti, Debabrata
-
p. 7843 - 7858
(2018/10/31)
-
- Stereospecific sp3 C–H oxidation with m-CPBA: A CoIII Schiff base complex as pre-catalyst vs. its CoIIICdII heterometallic derivative
-
The mono- and binuclear Schiff base complexes [CoL3]·DMF (1) and [CoCdL3Cl2]·0.5H2O (2) were facilely synthesized using zerovalent cobalt and cadmium chloride (for 2) as starting materials and the pre-formed pro-ligand HL (2-methoxy-6-[(methylimino)methyl]phenol, the product of condensation of o-vanillin and methylamine) in air. The compounds were characterized by single crystal X-ray diffraction analysis and spectroscopic methods in solution and in the solid state. Both complexes demonstrate a profound catalytic activity in the stereoselective oxidation of cis-1,2-dimethylcyclohexane (model substrate) with m-CPBA (m-chloroperbenzoic acid) under mild conditions in the presence of promoters of various acidity (HNO3, TFA and HOAc). The heterometallic binuclear CoIIICdII pre-catalyst (2) was more active than the mononuclear CoIII one (1), exhibiting higher products yields up to 51% and excellent stereospecificity (up to 99.2% retention of stereoconfiguration). This result could be associated with a synergistic effect of two different metals in 2. Based on the large obtained kinetic isotope effect and H218O labeling studies, the overall reaction mechanism was proposed to proceed without the participation of free alkyl radicals. The acidity of the promoter was shown to influence catalytic parameters for both 1 and 2 so that the better parameters are achieved with the acid possessing lower pKa values (a stronger acid). The comparison of the catalytic behaviours of 1 and 2 is discussed in detail considering relevant examples from the literature.
- Nesterova, Oksana V.,Kasyanova, Katerina V.,Makhankova, Valeriya G.,Kokozay, Vladimir N.,Vassilyeva, Olga Yu.,Skelton, Brian W.,Nesterov, Dmytro S.,Pombeiro, Armando J.L.
-
p. 171 - 184
(2018/05/28)
-
- Iron Complex Catalyzed Selective C-H Bond Oxidation with Broad Substrate Scope
-
The use of a peroxidase-mimicking Fe complex has been reported on the basis of the biuret-modified TAML macrocyclic ligand framework (Fe-bTAML) as a catalyst to perform selective oxidation of unactivated 3° C-H bonds and activated 2° C-H bonds with low catalyst loading (1 mol %) and high product yield (excellent mass balance) under near-neutral conditions and broad substrate scope (18 substrates which includes arenes, heteroaromatics, and polar functional groups). Aliphatic C-H oxidation of 3° and 2° sites of complex substrates was achieved with predictable selectivity using steric, electronic, and stereoelectronic rules that govern site selectivity, which included oxidation of (+)-artemisinin to (+)-10β-hydroxyartemisinin. Mechanistic studies indicate FeV(O) to be the active oxidant during these reactions.
- Jana, Sandipan,Ghosh, Munmun,Ambule, Mayur,Sen Gupta, Sayam
-
supporting information
p. 746 - 749
(2017/03/01)
-
- Stereoselective oxidation of alkanes with: M -CPBA as an oxidant and cobalt complex with isoindole-based ligands as catalysts
-
Two complexes with isoindole-core ligands of general formula [M{C6H4C(NH2)NC(ONCMe2)2}2](NO3)2 (M = Co for 1 and M = Ni for 2) were studied as catalysts for the mild stereoselective alkane oxidation with m-chloroperbenzoic acid (m-CPBA) as an oxidant and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) as a main model substrate. Complex 1 disclosed a pronounced activity, with high retention of stereoconfiguration of substrates (>98% for cis-1,2-DMCH) and highest cis/trans ratio of tertiary alcohols (products) of 56, under mild conditions. The best achieved yields of tertiary cis-alcohols were of 13.7 and 50.5%, based on the substrate (cis-1,2-DMCH) and the oxidant (m-CPBA) respectively. Kinetic experiments, high bond and stereoselectivity parameters, kinetic isotope effect of 7.2(2) in the oxidation of cyclohexane, and incorporation of 18O from H218O support the involvement of CoIVO high-valent metal-oxo intermediates as main C-H attacking species.
- Nesterova, Oksana V.,Kopylovich, Maximilian N.,Nesterov, Dmytro S.
-
p. 93756 - 93767
(2016/10/21)
-
- Formation and High Reactivity of the anti-Dioxo Form of High-Spin μ-Oxodioxodiiron(IV) as the Active Species That Cleaves Strong C-H Bonds
-
Recently, it was shown that μ-oxo-μ-peroxodiiron(III) is converted to high-spin μ-oxodioxodiiron(IV) through O-O bond scission. Herein, the formation and high reactivity of the anti-dioxo form of high-spin μ-oxodioxodiiron(IV) as the active oxidant are demonstrated on the basis of resonance Raman and electronic-absorption spectral changes, detailed kinetic studies, DFT calculations, activation parameters, kinetic isotope effects (KIE), and catalytic oxidation of alkanes. Decay of μ-oxodioxodiiron(IV) was greatly accelerated on addition of substrate. The reactivity order of substrates is toluene8]toluene is 95 at -30 °C, which the largest in diiron systems reported so far. The present diiron complex efficiently catalyzes the oxidation of various alkanes with H2O2. Strong anti oxidant: A high-spin μ-oxodioxodiiron(IV) species undergoes transformation from the syn-dioxo to the anti-dioxo form, which cleaves strong C-H bonds of alkanes. The high-spin anti-dioxodiiron(IV) species with a sterically less hindered structure (see figure) is a highly reactive and selective oxidant. These results provide insight into the high reactivity of the active species Q of soluble methane monooxygenases and the development of efficient alkane oxidation catalysts.
- Kodera, Masahito,Ishiga, Shin,Tsuji, Tomokazu,Sakurai, Katsutoshi,Hitomi, Yutaka,Shiota, Yoshihito,Sajith,Yoshizawa, Kazunari,Mieda, Kaoru,Ogura, Takashi
-
p. 5924 - 5936
(2016/04/26)
-
- Catalytic oxidation of alkanes by a (salen)osmium(VI) nitrido complex using H2O2 as the terminal oxidant
-
The osmium(vi) nitrido complex, [OsVI(N)(L)(CH3OH)]+ (1, L = N,N′-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes under ambient conditions using H2O2 as the oxidant. Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover numbers up to 2230. Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H2O2 to [OsVI(N)(L)]+ to generate an [OsVIII(N)(O)(L)]+ active intermediate.
- Chen, Man,Pan, Yi,Kwong, Hoi-Ki,Zeng, Raymond J.,Lau, Kai-Chung,Lau, Tai-Chu
-
supporting information
p. 13686 - 13689
(2015/09/02)
-
- P450-catalyzed regio- and stereoselective oxidative hydroxylation of disubstituted cyclohexanes: Creation of three centers of chirality in a single CH-activation event This paper is dedicated to the memory of Harry H. Wasserman
-
Wild-type P450-BM3 is able to catalyze in a highly regio- and diastereoselective manner the oxidative hydroxylation of non-activated disubstituted cyclohexane derivatives lacking any functional groups, including cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,4-dimethylcyclohexane, and trans-1,4-methylisopropylcyclohexane. In all cases except chiral trans-1,2-dimethylcyclohexane as substrate, the single hydroxylation event at a methylene group induces desymmetrization with simultaneous creation of three centers of chirality. Certain mutants increase selectivity, setting the stage for future directed evolution work.
- Ilie, Adriana,Agudo, Rubén,Roiban, Gheorghe-Doru,Reetz, Manfred T.
-
p. 470 - 475
(2015/02/02)
-
- Evidence that steric factors modulate reactivity of tautomeric iron-oxo species in stereospecific alkane C-H hydroxylation
-
A new iron complex mediates stereospecific hydroxylation of alkyl C-H bonds with hydrogen peroxide, exhibiting excellent efficiency. Isotope labelling studies provide evidence that the relative reactivity of tautomerically related oxo-iron species responsible for the C-H hydroxylation reaction is dominated by steric factors. This journal is The Royal Society of Chemistry.
- Mitra, Mainak,Lloret-Fillol, Julio,Haukka, Matti,Costas, Miquel,Nordlander, Ebbe
-
supporting information
p. 1408 - 1410
(2014/02/14)
-
- Highly efficient alkane oxidation catalyzed by [MnV(N)(CN) 4]2-. Evidence for [MnVII(N)(O)(CN) 4]2- as an active intermediate
-
The oxidation of various alkanes catalyzed by [MnV(N)(CN) 4]2- using various terminal oxidants at room temperature has been investigated. Excellent yields of alcohols and ketones (>95%) are obtained using H2O2 as oxidant and CF3CH 2OH as solvent. Good yields (>80%) are also obtained using (NH4)2[Ce(NO3)6] in CF 3CH2OH/H2O. Kinetic isotope effects (KIEs) are determined by using an equimolar mixture of cyclohexane (c-C6H 12) and cyclohexane-d12 (c-C6D12) as substrate. The KIEs are 3.1 ± 0.3 and 3.6 ± 0.2 for oxidation by H2O2 and Ce(IV), respectively. On the other hand, the rate constants for the formation of products using c-C6H12 or c-C6D12 as single substrate are the same. These results are consistent with initial rate-limiting formation of an active intermediate between [Mn(N)(CN)4]2- and H2O2 or CeIV, followed by H-atom abstraction from cyclohexane by the active intermediate. When PhCH2C(CH3)2OOH (MPPH) is used as oxidant for the oxidation of c-C6H12, the major products are c-C6H11OH, c-C6H10O, and PhCH2C(CH3)2OH (MPPOH), suggesting heterolytic cleavage of MPPH to generate a Mn=O intermediate. In the reaction of H2O2 with [Mn(N)(CN)4]2- in CF 3CH2OH, a peak at m/z 628.1 was observed in the electrospray ionization mass spectrometry, which is assigned to the solvated manganese nitrido oxo species, (PPh4)[Mn(N)(O)(CN)4] -·CF3CH2OH. On the basis of the experimental results the proposed mechanism for catalytic alkane oxidation by [MnV(N)(CN)4]2-/ROOH involves initial rate-limiting O-atom transfer from ROOH to [Mn(N)(CN)4]2- to generate a manganese(VII) nitrido oxo active species, [MnVII(N)(O) (CN)4]2-, which then oxidizes alkanes (R'H) via a H-atom abstraction/O-rebound mechanism. The proposed mechanism is also supported by density functional theory calculations.
- Ma, Li,Pan, Yi,Man, Wai-Lun,Kwong, Hoi-Ki,Lam, William W.Y.,Chen, Gui,Lau, Kai-Chung,Lau, Tai-Chu
-
p. 7680 - 7687
(2014/06/10)
-
- Site-selective oxidation of unactivated C sp 3-H bonds with hypervalent iodine(III) reagents
-
By design: The site-selective oxidation of unactivated secondary C sp 3-H bonds was accomplished with hypervalent iodine(III) reagents and tert-butyl hydroperoxide (see scheme). The preparation and derivatization of the hypervalent iodine(III) reagent are simple, thus allowing the rational design of these reagents to optimize the site selectivity of the oxidation. Copyright
- Moteki, Shin A.,Usui, Asuka,Zhang, Tiexin,Solorio Alvarado, Cesar R.,Maruoka, Keiji
-
supporting information
p. 8657 - 8660
(2013/09/12)
-
- An iron catalyst for oxidation of alkyl C-H bonds showing enhanced selectivity for methylenic sites
-
Many are called but few are chosen: A nonheme iron complex catalyzes the oxidation of alkyl C-H bonds by using H2O2 as the oxidant, showing an enhanced selectivity for secondary over tertiary C-H bonds (see scheme). Copyright
- Prat, Irene,Gomez, Laura,Canta, Merce,Ribas, Xavi,Costas, Miquel
-
supporting information
p. 1908 - 1913
(2013/03/14)
-
- Selective activation of secondary C-H bonds by an iron catalyst: Insights into possibilities created by the use of a carboxyl-containing bipyridine ligand
-
In this work, we report the discovery of a carboxyl-containing iron catalyst 1 (FeII-DCBPY, DCBPY = 2,2′-bipyridine-4,4′- dicarboxylic acid), which could activate the C-H bonds of cycloalkanes with high secondary (2°) C-H bond selectivity. A turnover number (TN) of 11.8 and a 30% yield (based on the H2O2 oxidant) were achieved during the catalytic oxidation of cyclohexane by 1 under irradiation with visible light. For the transformation of cycloalkanes and bicyclic decalins with both 2° and tertiary (3°) C-H bonds, 1 always preferred to oxidise the 2° C-H bonds to the corresponding ketone and alcohol products; the 2°/3° ratio ranged between 78/22 and >99/1 across 7 examples. 18O isotope labelling experiments, ESR experiments, a PPh3 method and the catalase method were used to characterize the reaction process during the oxidation. The success of 1 showed that, in addition to using a bulky catalyst, high 2° C-H bond selectivity could also be achieved using a less bulky molecular iron complex as the catalyst.
- Cheng, Shi,Li, Jing,Yu, Xiaoxiao,Chen, Chuncheng,Ji, Hongwei,Ma, Wanhong,Zhao, Jincai
-
p. 3267 - 3273
(2013/10/01)
-
- Catalyst-controlled aliphatic c-h oxidations with a predictive model for site-selectivity
-
Selective aliphatic C-H bond oxidations may have a profound impact on synthesis because these bonds exist across all classes of organic molecules. Central to this goal are catalysts with broad substrate scope (small-molecule-like) that predictably enhance or overturn the substrate's inherent reactivity preference for oxidation (enzyme-like). We report a simple small-molecule, non-heme iron catalyst that achieves predictable catalyst-controlled site-selectivity in preparative yields over a range of topologically diverse substrates. A catalyst reactivity model quantitatively correlates the innate physical properties of the substrate to the site-selectivities observed as a function of the catalyst.
- Gormisky, Paul E.,White, M. Christina
-
supporting information
p. 14052 - 14055
(2013/10/21)
-
- Highly efficient, regioselective, and stereospecific oxidation of aliphatic C-H groups with H2O2, catalyzed by aminopyridine manganese complexes
-
Aminopyridine manganese complexes [LMnII(OTf)2] having a similar coordination topology catalyze the oxidation of unactivated aliphatic C-H groups with H2O2, demonstrating excellent efficiency (up to TON = 970), site selectivity, and stereospecificity (up to >99%).
- Ottenbacher, Roman V.,Samsonenko, Denis G.,Talsi, Evgenii P.,Bryliakov, Konstantin P.
-
supporting information
p. 4310 - 4313
(2012/11/06)
-
- Combined effects on selectivity in Fe-catalyzed methylene oxidation
-
Methylene C-H bonds are among the most difficult chemical bonds to selectively functionalize because of their abundance in organic structures and inertness to most chemical reagents. Their selective oxidations in biosynthetic pathways underscore the power of such reactions for streamlining the synthesis of molecules with complex oxygenation patterns. We report that an iron catalyst can achieve methylene C-H bond oxidations in diverse natural-product settings with predictable and high chemo-, site-, and even diastereoselectivities. Electronic, steric, and stereoelectronic factors, which individually promote selectivity with this catalyst, are demonstrated to be powerful control elements when operating in combination in complex molecules. This small-molecule catalyst displays site selectivities complementary to those attained through enzymatic catalysis.
- Chen, Mark S.,White, M. Christina
-
scheme or table
p. 533 - 571
(2010/10/05)
-
- Efficient stereo- and regioselective hydroxylation of alkanes catalysed by a bulky polyoxometalate
-
Direct functionalization of alkanes by oxidation of C-H bonds to form alcohols under mild conditions is a challenge for synthetic chemistry. Most alkanes contain a large number of C-H bonds that present difficulties for selectivity, and the oxidants employed often result in overoxidation. Here we describe a divanadium-substituted phosphotungstate that catalyses the stereo- and regioselective hydroxylation of alkanes with hydrogen peroxide as the sole oxidant. Both cyclic and acyclic alkanes were oxidized to form alcohols with greater than 96% selectivity. The bulky polyoxometalate framework of the catalyst results in an unusual selectivity that can lead to the oxidation of secondary rather than the weaker tertiary C-H bonds. The catalyst also avoids wasteful decomposition of the stoichiometric oxidant, which can result in the production of hydroxyl radicals and lead to non-selective oxidation and overoxidation of the desired products.
- Kamata, Keigo,Yonehara, Kazuhiro,Nakagawa, Yoshinao,Uehara, Kazuhiro,Mizuno, Noritaka
-
scheme or table
p. 478 - 483
(2010/09/17)
-
- A highly efficient non-heme manganese complex in oxygenation reactions
-
A non-heme manganese(ii) complex shows a high catalytic activity in the epoxidation of olefins by iodosyl benzene and in the oxidation of olefins, alcohols and alkanes by peracetic acid; a mechanism involving metal-based oxidants is proposed for the oxida
- Nehru, Kasi,Kim, Soo Jeong,Kim, In Young,Seo, Mi Sook,Kim, Youngmee,Kim, Sung-Jin,Kim, Jinheung,Nam, Wonwoo
-
p. 4623 - 4625
(2008/10/09)
-
- Alkane oxygenation with H2O2 catalysed by FeCl 3 and 2,2′-bipyridine
-
The H2O2-FeCl3-bipy system in acetonitrile efficiently oxidises alkanes predominantly to alkyl hydroperoxides. Turnover numbers attain 400 after 1 h at 60°C. It has been assumed that bipy facilitates proton abstraction from a H2O2 molecule coordinated to the iron ion (these reactions are stages in the catalytic cycle generating hydroxyl radicals from the hydrogen peroxide). Hydroxyl radicals then attack alkane molecules finally yielding the alkyl hydroperoxide.
- Shul'pin, Georgiy B.,Golfeto, Camilla C.,Süss-Fink, Georg,Shul'pina, Lidia S.,Mandelli, Dalmo
-
p. 4563 - 4567
(2007/10/03)
-
- Oxidations by the system "hydrogen peroxide-[Mn2L 2O3][PF6]2 (L = 1,4,7-trimethyl-1,4, 7-triazacyclononane)-oxalic acid". Part 6. Oxidation of methane and other alkanes and olefins in water
-
Oxidation of alkanes with hydrogen peroxide in water solution at 10-50 °C is efficiently catalyzed by the cationic dinuclear manganese (IV) derivative [Mn2L2O3]2+ (1, with L = 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN) in the form of the hexafluorophosphate salt ([1][PF6]2) if oxalic acid is present as a co-catalyst. Methane gives methanol and formaldehyde (turnover numbers, TONs, were 7 and 2, respectively, after reduction of the reaction mixture with ascorbic acid) whereas cyclohexane was oxidized with TONs up to 160 affording cyclohexyl hydroperoxide, cyclohexanone and cyclohexanol (the ketone was the main product, although at room temperature almost pure alkyl hydroperoxide was formed). In contrast to the oxidation in acetonitrile, the reaction with linear n-alkanes in water exhibits an unusual distribution of oxygenates. For example, in the oxidation of n-heptane the normalized reactivity of the methylene group in position 4 of the chain is 3-7 times higher than that of the CH2 group in position 2. Dec-1-ene is epoxidized by hydrogen peroxide in water (a biphasic system) catalyzed by [1][PF6] 2 and oxalic acid in the presence of a small amount of acetonitrile with TONs up to 1000 (no epoxidation has been detected in the absence of MeCN).
- Shul'pin, Georgiy B.,Nizova, Galina V.,Kozlov, Yuriy N.,Arutyunov, Vladimir S.,Dos Santos, Ana Cláudia M.,Ferreira, Ana Carolina T.,Mandelli, Dalmo
-
p. 4498 - 4504
(2007/10/03)
-
- Non-heme iron complexes for stereoselective oxidation: Tuning of the selectivity in dihydroxylation using different solvents
-
A new class of functional models for non-heme iron-based dioxygenases, including [(N3Py-Me)Fe(CH3CN)2](ClO4) 2 and [(N3Py-Bn)Fe(CH3CN)2](ClO 4)2 {N3Py-Me = [di(2-pyridyl)methyl]methyl(2-pyridyl) methylamine; N3Py-Bn = [di(2-pyridyl)methyl]benzyl(2-pyridyl)methylamine}, is presented here. NMR, UV and X-ray analyses revealed that six-coordinate low-spin FeII complexes with the pyridine N-atoms and the tertiary amine functionality of the ligand bound to Fe are formed. The two remaining coordination sites located cis to each other are occupied by labile CH 3CN groups that are easily exchanged by other ligands. We demonstrate that the reactivity and stereoselectivity of the complexes investigated depend on the choice of the solvent. The complexes have been examined as catalysts for the oxidation of both alkanes and olefins in CH3CN. In this solvent alkanes are oxidized to alcohols and ketones and olefins to the corresponding cis-epoxides and cis-diols. In acetone as solvent a different reactivity pattern was found, with, as the most striking example, the trans-dihydroxylation of cis-olefins. 18O-labeling studies in CH3CN establish incorporation of 18O from H218O2 and H218O in both the epoxide and the diol implicating an HO-FeV=18O active intermediate originating from an H 218O-FeIIIOOH species. These results are in full agreement with mechanistic schemes derived for other dioxygenase model systems. Based on labeling studies in acetone an additional oxidation mechanism is proposed for this solvent, in which the solvent acetone is involved. This is the first example of a catalyst that can give cis- or trans-dihydroxylation products, just by changing the solvent. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.
- Klopstra, Marten,Roelfes, Gerard,Hage, Ronald,Kellogg, Richard M.,Feringa, Ben L.
-
p. 846 - 856
(2007/10/03)
-
- FeCl3-activated oxidation of alkanes by [Os(N)O 3]-
-
Although the ion [OsVIII(N)(O)3]- is a stable species and is not known to act as an oxidant for organic substrates, it is readily activated by FeCl3 in CH2Cl2/CH 3CO2H to oxidize alkanes efficiently at room temperature. The oxidation can be made catalytic by using 2,6-dichloropyridine N-oxide as the terminal oxidant. The active intermediates in stoichiometric and catalytic oxidation are proposed to be [(O)3OsVIII≡N-Fe III] and [Cl4(O)OsVIII≡N-Fe III], respectively.
- Yiu, Shek-Man,Wu, Zhi-Biao,Mak, Chi-Keung,Lau, Tai-Chu
-
p. 14921 - 14929
(2007/10/03)
-
- Ligand topology tuning of iron-catalyzed hydrocarbon oxidations
-
Quite unexpectedly, the topology of the tetradentate ligand in [FeII-(bpmcn)(OTf)2] (bpmcn = N,N′- bis-(2-pyridylmethyl)-N,N′-dimethyl-trans-1,2-diaminocyclohexane), whether cis-α or cis-β (see picture), determines the course of catalytic alkane hydroxylation and olefin oxidation reactions with H2O2, which afford products with varying stereo-selectivity and dramatically different sources of incorporated oxygen. These results demonstrate the exquisite role ligands can play in the fine tuning of the reactivity of an iron catalyst.
- Costas, Miquel,Que Jr., Lawrence
-
p. 2179 - 2181
(2007/10/03)
-
- Chemospecific chromium[VI] catalyzed oxidation of C-H bonds at -40 °C
-
H5IO6 in the presence of catalytic chromoyl diacetate is a powerful method for oxidation of C-H bonds. Tertiary and oxygen activated C-H bonds are oxidized to tertiary alcohols or ketones at temperatures as low as -40 °C. The putative reagent is neutral dioxoperoxy chromium[VI] which undergoes C-H oxidation with retention of stereochemistry. This reagent appears to be the first reagent capable of oxidation of a C-H bond in the presence of an olefin without concomitant epoxidation. Copyright
- Lee, Seongmin,Fuchs
-
p. 13978 - 13979
(2007/10/03)
-
- Stereoselective alkane hydroxylations by metal salts and m-chloroperbenzoic acid
-
Simple metal (M=Mn, Fe, Co) perchlorates associated with m-chloroperbenzoic acid are able to conduct stereoselective alkane hydroxylations via a mechanism involving metal-based oxidants; the catalytic activity of the metal salts is in the order of Co(ClO4)2>Mn(ClO4)2>Fe (ClO4)2.
- Nam, Wonwoo,Ryu, Ju Yeon,Kim, Inwoo,Kim, Cheal
-
p. 5487 - 5490
(2007/10/03)
-
- Biomimetic alkane hydroxylation by cobalt(III) porphyrin complex and m-chloroperbenzoic acid
-
The catalytic hydroxylation of alkanes by an electron-deficient cobalt(III) porphyrin complex and m-chloroperbenzoic acid yielded alcohols as major products with a high kH/kD value, > 99% retention of stereochemistry, and a high regioselectivity; a high-valent cobalt - oxo porphyrin complex was suggested as a reactive hydroxylating intermediate.
- Nam,Kim,Kim,Kim
-
p. 1262 - 1263
(2007/10/03)
-
- Catalytic oxidation with a non-heme iron complex that generates a low- spin Fe(III)OOH intermediate
-
The antitumor drug bleomycin (BLM) is proposed to act via a low-spin iron(III) hydroperoxide intermediate called 'activated bleomycin'. To gain more insight into the mechanistic aspects of catalytic oxidation by these intermediates we have studied the reactivity of [(N4Py)Fe(CH3CN)](ClO4)2 (1) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) with excess H2O2. Under these conditions a transient purple species is generated, [(N4Py)FeOOH]2+ (2), which has spectroscopic features and reactivity strongly reminiscent of activated bleomycin. The catalytic oxidation of alkanes such as cyclohexane, cyclooctane, and adamantane by 1 with H2O2 gave the corresponding alcohols and ketones in up to 31% yield. It was concluded, from the O2 sensitivity of the oxidation reactions, the formation of brominated products in the presence of methylene bromide, and the nonstereospecificity of the oxidation of cis- or trans-dimethylcyclohexane, that long-lived alkyl radicals were formed during the oxidations. Oxidation of alkenes did not afford the corresponding epoxides in good yields but resulted instead in allylic oxidation products in the case of cyclohexene, and cleavage of the double bond in the case of styrene. Addition of hydroxyl radical traps, such as benzene and acetone, led to only partial quenching of the reactivity. The kinetic isotope effects for cyclohexanol formation, ranging from 1.5 in acetonitrile to 2.7 in acetone with slow addition of H2O2, suggested the involvement of a more selective oxidizing species in addition to hydroxyl radicals. Monitoring the UV/Vis absorption of 2 during the catalytic reaction showed that 2 was the precursor for the active species. On the basis of these results it is proposed that 2 reacts through homolysis of the O-O bond to afford two reactive radical species: [(N4Py)Fe(IV)O]2+ and ·OH. The comparable reactivity of 1 and Fe-BLM raises the possibility that they react through similar mechanistic pathways.
- Roelfes, Gerard,Lubben, Marcel,Hage, Ronald,Que Jr., Lawrence,Feringa, Ben L.
-
p. 2152 - 2159
(2007/10/03)
-
- Hydroxylation of aliphatic hydrocarbons with m-chloroperbenzoic acid catalyzed by electron-deficient iron(III) porphyrin complexes
-
The catalytic hydroxylation of aliphatic hydrocarbons by m- chloroperbenzoic acid (MCPBA) has been studied in the presence of electron- deficient iron(III) porphyrin complexes. High yields of alcohol products were obtained with small amounts of ketone formation under mild reaction conditions. The stereospecificity and regioselectivity of the iron porphyrin complexes have been investigated in hydroxylation reactions as well. The hydroxylation of alkanes has been performed in the presence of isotopically 18O-labeled water, H218O, in order to understand the effects of the electronic nature of iron porphyrin complexes, the concentration of H218O, the C-H bond strength of alkanes, and the reaction temperature on the 18O- incorporation from the labeled water into alcohols. We found that the amounts of 18O incorporated into the alcohol products varied in the reactions; these results were interpreted with that the reaction of oxygen atom transfer from a high-valent iron oxoporphyrin complex to alkanes competes with that of oxygen atom exchange between the intermediate and labeled water that leads to 18O-incorporation from H218O into the alcohol products. Deuterium kinetic isotope effects (KIEs) in the alkane hydroxylations by the iron porphyrin complexes and MCPBA have been studied with a mixture of cyclohexane and cyclohexane-d12. The KIE values obtained in the reactions were found to depend significantly on the nature to the iron porphyrin complexes. The temperature dependence of k(H)/k(D) was also studied from -40 to 25 °C and the parameters of Arrhenius equation (i.e., the pre-exponential factor ratio, A(H)/A(D), and the isotopic difference of C-H and C-D bond activation energies, E(a)(D)-E(a)(H)) were determined.
- Lim, Mi Hee,Lee, Yoon Jung,Goh, Yeong Mee,Nam, Wonwoo,Kim, Cheal
-
p. 707 - 713
(2007/10/03)
-
- Oxidation with the "H2O2 - Manganese(IV) complex - Carboxylic acid" reagent 1. Oxidation of saturated hydrocarbons with peroxy acids and hydrogen peroxide
-
The complex [LMnIV(O)3MnIVL](PF6)2 (1), where L is 1,4,7-trimethyl-1,4,7-triazacyclononane, catalyzes a highly efficient stereoselective oxygenation of saturated hydrocarbons in the presence of H2O2. A carboxylic acid is an obligatory component of the reaction mixture, while acetonitrile or acetone can be used as solvent. The reaction occurs, forming alkyl hydroperoxide, ketone, and alcohol. Substitution at the tertiary carbon atom proceeds more easily than that at the secondary carbon atom, whereas primary C-H bonds are rather inactive. Oxidation of alkanes and alcohols with peroxy acids catalyzed by complex 1 occurs with lower efficiency.
- Shul'pin,Lindsay-Smith
-
p. 2379 - 2386
(2007/10/03)
-
- Efficient stereoselective oxygenation of alkanes by peroxyacetic acid or hydrogen peroxide and acetic acid catalysed by a manganese(IV) 1,4,7- trimethyl-1,4,7-triazacyclononane complex
-
The dinuclear manganese complex [LMn(IV)(O)3Mn(IV)L](PF6)2, where L is 1,4,7-trimethyl-1,4,7-triazacyclononane. catalyses the oxygenation of alkane by peroxyacetic acid or by H2O2 in the presence of acetic acid to give alkanols, alkanones and alkyl hydroperoxides. The reactions can give large turnovers (up to 1350 after 1 h at 20°C) and can occur with a high degree of retention of stereochemistry at tertiary carbon atoms.
- Lindsay Smith, John R.,Shul'pin, Georgiy B.
-
p. 4909 - 4912
(2007/10/03)
-
- Vanadium-substituted MCM-41 zeolites as catalysts for oxidation of alkanes with peroxides
-
Alkanes are oxidized selectively to ketones using V-MCM-41 with isobutyraldehyde/dioxygen as the preferred oxidant in terms of product selectivity and catalyst stability and recycle.
- Neumann, Ronny,Khenkin, Alexander M.
-
p. 2643 - 2644
(2007/10/03)
-
- Samarium(II) triflate as a new reagent for the grignard-type carbonyl addition reaction
-
On treatment of a THF solution of Sm(OTf)3 with 1 equiv of an organolithium or organomagnesium reagent at ambient temperature, the purple or deep green solution of the divalent samarium triflate [Sm(OTf)2] was readily obtained. For this preparation, s-BuLi was the most effective as was evidenced by the reduction of 2-phenylethyl iodide in the presence of HMPA. The Sm(OTf)2 reagent mediated the Grignard-type reaction effectively in THF/HMPA; alkylation and allylation of ketones or aldehydes with alkyl, allyl, or benzyl halides proceeded via organosamarium intermediates. Diastereoselectivity of the samarium-Grignard reaction was examined using 2-methylcyclohexanone, 4-tert-butylcyclohexanone, and 2-phenylpropanal and was found to be higher in each case than that with SmI2. With 2-methylcyclohexanone, for example, Sm(OTf)2 gave the greatest ratio of axial alcohol:equatorial alcohol = 99:1, and SmI2 gave a ratio of 91:9. Halides containing an ester or a silyl group were reactive in the Reformatsky- or Peterson-type reaction, respectively, using the Sm(OTf)2 reagent.
- Fukuzawa, Shin-Ichi,Mutoh, Keisuke,Tsuchimoto, Teruhisa,Hiyama, Tamejiro
-
p. 5400 - 5405
(2007/10/03)
-
- Ligand effects on diastereoselective addition of organocerium reagents to aldehydes and cyclic ketones
-
A new class of chiral dialkoxy- and diaryloxyorganocerium reagent has been prepared and the diastereoselectivity of carbonyl addition reactions has been compared with that of conventional organocerium reagents.
- Greeves, Nicholas,Lyford, Lisa,Elizabeth Pease
-
p. 285 - 288
(2007/10/02)
-
- Samarium(II) Iodide-Induced Reductive Cyclization of Unactivated Olefinic Ketones. Sequential Radical Cyclization/Intermolecular Nucleophilic Addition and Substitution Reactions
-
Samarium(II) iodide in the presence of HMPA effectively promotes the intramolecular coupling of unactivated olefinic ketones by a reductive ketyl-olefin radical-cyclization process.The reaction is quite general for the formation of 5- and 6-membered carbocycles and even provides modest yields in less facile cyclization processes as evidenced by the generation of methylcyclooctanol via an 8-endo cyclization.Sequential radical cyclization-intermolecular nucleophilic addition/substitution processes set the SmI2 reaction apart from its radical-chain, photochemical, and electrochemical counterparts.In addition to delineating the synthetic potential of this reaction, the role played by HMPA in enhancing SmI2 reactivity has been further refined, and a model correlating the high diastereoselectivity and product distribution in SmI2-promoted reductive coupling processes with HMPA concentration has been established.
- Molander, Gary A.,McKie, Jeffrey A.
-
p. 3132 - 3139
(2007/10/02)
-
- Diastereoselective Addition of Organoytterbium Reagents to Carbonyl Substrates
-
Organoytterbium reagents, prepared by the addition of organolithiums or organomagnesiums to ytterbium(III) triflate, react in a highly diastereoselective fashion with a variety of chiral aldehydes and ketones.
- Molander, Gary A.,Burkhardt, Elizabeth R.,Weinig, Peter
-
p. 4990 - 4991
(2007/10/02)
-
- Oxidations by Methyl(trifluoromethyl)dioxirane. 2. Oxyfunctionalization of Saturated Hydrocarbons
-
The reaction of methyl(trifluoromethyl)dioxirane (1b), a novel dioxirane species, with two open-chain, four cyclic, and five polycyclic saturated hydrocarbons and two aralkyl hydrocarbons in CH2Cl2/1,1,1-trifluoropropanone has been studied; under mild conditions (-22 to 0 deg C), it gives alcohols and/or ketones (deriving from further oxidation of secondary alcohols) in high yields and within very short reaction times.Primary C-H bonds are not appreciably oxidized and high regioselectivities were determined for attack at tertiary over secondary C-H bonds, with theexception of norbornane, which showed opposite regioselectivity.The reaction is also highly stereoselective, since hydroxylations of cis- and trans-decalin and of cis- and trans-1,2-dimethylcyclohexane were found to be in each case stereospecific with retention.From kinetic data, Ea = 14.3 kcal mol-1 and log A = 9.9 were estimated for cyclohexane oxidation.Relative rates change in the order cyclohexane (0.78) octane (9.2) adamantane (146); cis-1,2-dimethylcyclohexane was observed to be 7-fold more reactive than its trans isomer, demonstrating remarkable discrimination for equatorial vs axial C-H attack (also noticed in the case of cis- and trans-decalin).The relative rate of oxidation of cumene vs ethylbenzene was found to be ca. 3.1 (after statistical correction), i.e., in sharp excess over values usually recorded in classical radical H-atom abstraction from benzylic position.Rate constants determined for the reactions of cumene and of ethylbenzene show the title dioxirane (1b) is more reactive than dimethyldioxirane (1a) by factors of ca. 600 and over 700, respectively.The whole of theobservations is better accommodated by an "oxenoid" mechanism, involving concerted O-atom insertion by dioxirane into C-H bonds of hydrocarbons.
- Mello, Rossella,Fiorentino, Michele,Fusco, Caterina,Curci, Ruggero
-
p. 6749 - 6757
(2007/10/02)
-
- Amphiphilic Reactions by Means of Exceptionally Bulky Organoaluminum Reagents. Rational Approach for Obtaining Unusual Equatorial, Anti-Cram, and 1,4 Selectivity in Carbonyl Alkylation
-
Exceptionally bulky, oxygenophilic organoaluminum reagents, methylaluminum bis(2,6-di-tert-4-alkylphenoxide) (MAD and MAT), have been successfully utilized for stereoselective activation of carbonyl moiety.Combination of MAD or MAT with carbon nucleophiles such as organolithiums or Grignard reagents generates a new amphiphilic reaction system in which the alkylation may be interpreted as the nucleophilic addition of a reactive organometallic compound to an electrophilically activated carbonyl substrate in order to account for the regio- and stereochemical consequences.In contrast to the ordinary alkylations, the amphilic alkylation disclosed herein would be categorized into the new, yet unexplored class of alkylation that exhibits high chemoselectivity to carbonyl compounds, and more significantly it allows excellent equatorial and anti-Cram selectivity in carbonyl alkylations, hitherto difficult by the existing methodologies.Further, unusual conjugate addition of organolithium reagents to α,β-unsaturated carbonyl compounds has been accomplished by using the amphiphilic reaction system.
- Maruoka, Keiji,Itoh, Takayuki,Sakurai, Minoru,Nonoshita, Katsumasa,Yamamoto, Hisashi
-
p. 3588 - 3597
(2007/10/02)
-
- A Highly Stereoselective Synthesis of Trans-1,2-Disubstituted Cycloalkanols
-
The study of the reactions of 1,4-bis(bromomagnesio)pentane, 1,1-bis(bromomagnesio)hexane, 1,4-bis(bromomagnesio)heptane, and 1,4-bis(bromomagnesio)octane with aliphatic and aromatic carboxylic acid esters in tetrahydrofuran solution was undertaken in order to ascertain which factors influence isomer distribution.The yields of this annelation processes are in the range of 68-88percent.The formation of trans OH 1,2-disubstituted cyclopentanols having the alkyl groups cis is generally observed with yields of 77-96percent.The reaction of 1,5-bis(bromomagnesio)hexane with carboxylicacid esters is less stereoselective and is more influenced by steric effects.
- Cannone, P.,Bernatchez, M.
-
p. 4025 - 4031
(2007/10/02)
-
- Stereoselective Addition of Organotitanium Reagents to Carbonyl Compounds
-
Titanation of alkyllithium or -magnesium compounds using ClTi(OR)3 results in reagents which show markedly increased diastereofacial selectivity (80 - 90percent) in reactions with α-chiral aldehydes or ketones.Titanation is also the method of choice in Grignard-type additions to substituted cyclohexanones; CH3Ti(OCHMe2)3 (6a) adds predominantly from the equatorial direction, while allyltitanium reagents 11b and 12 show axial preference.Crotyltitanium compounds react with carbonyl compounds to afford primarily adducts having anti-configuration, a process which is of particular value in case of ketones (anti/syn ratios up to 99:1).Titanation of (trimethylsilyl)allyllithium (48) with Ti(OCHMe2)4 reverses regioselectivity in reactions with aldehydes and ketones, β-hydroxy silanes 50 being the only observed products.These have anti-configuration and can be converted either into Z- or E-dienes using the Peterson elimination under basic or acidic conditions, respectively.
- Reetz, Manfred T.,Steinbach, Rainer,Westermann, Juergen,Peter, Roland,Wenderoth, Bernd
-
p. 1441 - 1454
(2007/10/02)
-
- On the Mechanism of the Ozonolysis of C-H Bonds: Influence of SiO2-Solvent, H/D Isotope Effect, Intermediates
-
Ozonation of trans-1,2-dimethylcyclohexane (1) in CFCl3 solution, on dry silica gel, and in CFCl3/SiO2 suspension under otherwise equivalent conditions leads to comparable product distributions with very similar rates.In the presence of SiO2 mainly the fraction of retention product is definitely higher.Any activation of substrate or ozone is therefore highly improbable.This is supported by the kinetic H/D isotope effects for trishomobarrelene (10b) and spirooctane (12b), which both in solution and on SiO2 are found to be equal within the error limits.Upon ozonation of trishomobarrelene (10), trishomobullvalene (15), hexahydrobullvalene (7), and norcarane (20) hydrotrioxides occur as detectable intermediates which decompose above -40 deg C to give alcohols and singlet oxygen.Especially the secondary representatives from 7 and 20 are the first of their kind ever fully characterized by 1H NMR spectroscopy.
- Zarth, Martin,Meijere, Armin de
-
p. 2429 - 2449
(2007/10/02)
-
- Mechanistic and Preparative Studies on the Regio- and Stereoselective Paraffin Hydroxylation with Peracids
-
Reactions of more than 20 hydrocarbons with p-nitro- or, e.g., 3,5-dinitroperbenzoic acid in chloroform show regioselectivities of Rst = 90 (relative rates of attack at tertiary and secondary C-H bonds, after statistical correction) to 500 and configurational retention, if applicable, of typically 97-99.7percent.Radical side reactions are recognized by concomitant formation of, e.g., nitrobenzene and are responsible for a decrease in regio- and stereoselectivity.Steric effects are observed in attack at axial tertiary C-H bonds and at bridgehead positions.Electronegative and hydrogen-bonding substituents in the alkane diminish, and alkyl groups enhance the rates; the observed Taft ρ* value of -2.2 indicates substantial positive charge accumulation in the transition state in agreement with the high regioselectivity.A Hammett reaction constant of +0.63 is obtained from substituted perbenzoic acids; activation parameters of ΔH* = 15-19 kcal mol-1 and ΔS* = -22 to -29 eu with three alkanes of different flexibility and an isotope effect of kH/kD = 2.2 with methylcyclohexane are measured.Aromatic rings are usually not attacked but lead to deactivation of the peracid even at remote alkane C-H positions; similar deactivation is found in hydrogen-bonding solvents.Androstanes yield preferentially 9α- and 5α-hydroxy products, if, e.g., a 17β-acetoxy substituent is used to steer the reaction.Diols usually are only observed as a result of a proximity effect of a peracid associated at the first formed hydroxy group.The results point to relatively late and oxenoid transition states with substantial charge separation in the substrate.Attempts to achieve selective oxidations using macrocyclic azacyclophanes with attached carboxylic functions were not successful, although the host compounds showed selective complexation of hydrocarbons.
- Schneider, Hans-Joerg,Mueller, Walter
-
p. 4609 - 4615
(2007/10/02)
-