- Titanocenes as Photoredox Catalysts Using Green-Light Irradiation
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Irradiation of Cp2TiCl2 with green light leads to electronically excited [Cp2TiCl2]*. This complex constitutes an efficient photoredox catalyst for the reduction of epoxides and for 5-exo cyclizations of suitably unsaturated epoxides. To the best of our knowledge, our system is the first example of a molecular titanium photoredox catalyst.
- Flowers, Robert A.,Gans?uer, Andreas,Hilche, Tobias,Oloyede, Ugochinyere N.,Rietdijk, Niels R.,Slak, Daniel,Zhang, Zhenhua
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supporting information
p. 9355 - 9359
(2020/04/30)
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- Amide-Substituted Titanocenes in Hydrogen-Atom Transfer Catalysis
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Two new catalytic systems for hydrogen-atom transfer (HAT) catalysis involving the N-H bonds of titanocene(III) complexes with pendant amide ligands are reported. In a monometallic system, a bifunctional catalyst for radical generation and reduction through HAT catalysis depending on the coordination of the amide ligand is employed. The pendant amide ligand is used to activate Crabtree's catalyst to yield an efficient bimetallic system for radical generation and HAT catalysis.
- Zhang, Yong-Qiang,Jakoby, Verena,Stainer, Katharina,Schmer, Alexander,Klare, Sven,Bauer, Mirko,Grimme, Stefan,Cuerva, Juan Manuel,Gans?uer, Andreas
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supporting information
p. 1523 - 1526
(2016/02/12)
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- Directed metal (oxo) aliphatic C-H hydroxylations: Overriding substrate bias
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The first general strategy for a directing effect on metal (oxo)-promoted C-H hydroxylations is described. Carboxylic acid moieties on the substrate overcome unfavorable electronic, steric, and stereoelectronic biases in C-H hydroxylations catalyzed by the non-heme iron complex Fe(PDP). In a demonstration of the power of this directing effect, C-H oxidation is diverted away from an electronically favored C-1 H abstraction/rearrangement pathway in the paclitaxel framework to enable installation of C-2 oxidation in the naturally occurring oxidation state and stereoconfiguration.
- Bigi, Marinus A.,Reed, Sean A.,White, M. Christina
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supporting information; experimental part
p. 9721 - 9726
(2012/07/14)
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- Non-Cp titanium alkoxide-based homolytic ring-opening of epoxides by an intramolecular hydrogen abstraction in β-titanoxy radical intermediates
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A low-valent titanium species derived in situ from Ti(O-i-Pr)4, Me3SiCl and Mg powder in tetrahydrofuran reacted with epoxides to selectively provide less hindered alcohols via a homolytic ring-opening of epoxides, in which the intermediate β-titanoxy radical intramolecularly abstracted a hydrogen atom from an alkoxy moiety in the titanium complexes.
- Kawaji, Tsuyoshi,Shohji, Noriaki,Miyashita, Kenji,Okamoto, Sentaro
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supporting information; experimental part
p. 7857 - 7859
(2011/09/15)
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- Radical reduction of epoxides using a titanocene(III)/water system: Synthesis of β-deuterated alcohols and their use as internal standards in food analysis
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We describe a comprehensive study into the Cp2TiCl-mediated reductive epoxide ring opening using either water as a hydrogen source or deuterium oxide as a deuterium source. The remarkable chemical profile of this reaction allows access to alcohols with anti-Markovnikov regiochemistry from different epoxides. The use of D2O as a deuterium source leads to an efficient synthesis of β-deuterated alcohols, including a deuterated sample of tyrosol, a bioactive compound contained in the leaves of the olive, which was successfully applied as an internal standard in food analysis.
- Jimenez, Tania,Campana, Araceli G.,Bazdi, Btissam,Paradas, Miguel,Arraez-Roman, David,Segura-Carretero, Antonio,Fernandez-Gutierrez, Alberto,Oltra, J. Enrique,Robles, Rafael,Justicia, Jose,Cuerva, Juan M.
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experimental part
p. 4288 - 4295
(2010/10/21)
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- Titanocene-catalyzed reductive epoxide opening: The quest for novel hydrogen atom donors
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Novel hydrogen atom donors for the reductive titanocene-catalyzed epoxide opening are presented. While the potentially attractive cyclopentadienes gave only moderate yields of the desired alcohols, substituted, nontoxic, and commercially available 1,4-cyclohexadienes, e.g. γ-terpinene, in combination with more elaborate catalysts gave better or similar results than the much more expensive and carcinogenic 1,4-cyclohexadiene. In the practically important reactions of Sharpless epoxides and their derivatives excellent levels of regioselectivity for the epoxide opening could be obtained. The toxic and unpleasant to handle tert-butyl thiol could be replaced while increasing the yields of the desired products.
- Gansaeuer, Andreas,Barchuk, Andriy,Fielenbach, Doris
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p. 2567 - 2573
(2007/10/03)
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- Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals. An experimental and theoretical study
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(Chemical Equation Presented) Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals gave distinct EPR spectra thanks to surprisingly large β-hydrogen atom hyperfine splittings that enabled them to be characterized and monitored. DFT computations indicated that the axial species (X = CH2) had a higher barrier to rotation about the (O)Cα-Cα bond. The computed difference ΔH° for the axial and equatorial radicals (R = H, X = CH2) was 0.8 kcal mol -1.
- DiLabio, Gino A.,Ingold, Keith U.,Roydhouse, Mark D.,Walton, John C.
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p. 4319 - 4322
(2007/10/03)
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- Photochemical synthesis of highly functionalized cyclopropyl ketones
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A series of di- and trisubstituted cyclopropyl ketones 11 were prepared by irradiation of ketones 3 and 5, which bear a leaving group adjacent to the carbonyl C-atom. The required ketones 3 could be easily synthesized either by functionalization of ketones 1 with a hypervalent iodine reagent, 2, or by O-sulfonylation of α-hydroxy ketones 7. The nitrates 5 were obtained by treatment of the corresponding α-bromo ketones with AgNO3. The irradiation of 3 and 5 must be performed in the presence of an acid scavenger (1-methyl-1H-imidazole) to obtain the cyclopropanes 11 in good yields. The synthetic efficiency of the method was, among other things, demonstrated by the preparation of a highly strained bicyclo[2.1.0]pentane 11i in good yield. The mechanism of the photochemical cyclization was investigated by means of photokinetic measurements, as well as by quantum-chemical calculations. It was shown that the presence of the leaving group substantially influences all steps of the photochemical reaction cascade. The X-ray crystal structures of 11j and exo-11k were also determined.
- Wessig, Pablo,Muehling, Olaf
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p. 865 - 893
(2007/10/03)
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- Regioselective reduction of epoxides and conjugated carbonyl compounds using zeolite supported zinc borohydride
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Zeolite supported zinc borohydride is a versatile catalyst for the regeoselective reduction of epoxides, conjugated ketones, and aldehydes to the corresponding alcohols in good yields.
- Sreekumar,Padmakumar,Rugmini
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p. 5151 - 5154
(2007/10/03)
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- Stereoselective Metal Catalysed Hydroboration of 4-Substituted 1-Methylidenecyclohexanes
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Rhodium catalysed hydroboration of 4-substituted 1-methylidenecyclohexanes gives cis-hydroboration products with high stereoselectivity.The effects influencing the stereochemistry are discussed.
- Hou, Xue-Long,Xie, Quan-Cheng,Dai, Li-Xin
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p. 2665 - 2678
(2007/10/03)
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- A mild, convenient, Non-Acidic conversion of enol ethers into alcohols using Hg(OAc)2 - NaBH4
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Alkyl enol ethers can be converted into the corresponding alcohols in good to excellent yields by treatment with aqueous Hg(OAc)2 - NaBH4 in one reaction flask. This method is sufficiently mild to allow the survival of acid-sensitive groups such as silyl ethers, THP-protected alcohols and N-Boc-protected amines.
- Crouch, R. David,Mitten, Jeffrey V.,Span, Amelia R.,Dai, H. George
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p. 791 - 794
(2007/10/03)
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- Lipase-mediated kinetic separation of a diastereomeric mixture of 4-tert-butylcyclohexanemethanol
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Diastereomerically pure trans- and cis-4-tert-butylcyclohexanemethanols have been obtained by kinetic acylation of the diastereomeric alcohol in an organic medium and by kinetic deacylation of the diastereomeric acetate in an aqueous medium both in the presence of the same lipase (lipase PS, Pseudomonas sp., Amano). The reactions take place preferentially with the trans-isomers both in organic and aqueous media to give the trans-acetate with recovery of the cis-alcohol on acylation in an organic medium,and the trans-alcohol with recovery of the cis-acetate on deacylation in an aqueous medium.
- Hiroya,Hasegawa,Watanabe,Ogasawara
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p. 379 - 381
(2007/10/02)
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- Silica Gel-supported Zinc Borohydride. Part 3. Regioselective Reductive Cleavage of Methylenecycloalkane Oxides to the Less-substituted Alcohols
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Highly selective reductive cleavage of methylenecycloalkane oxides to less-substituted alcohols has been achieved by a simple procedure using silica gel supported zinc borohydride in tetrahydrofuran.
- Ranu, Brindaban C.,Das, Asish R.
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p. 1881 - 1882
(2007/10/02)
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- CCK ANALOGS WITH APPETITE REGULATING ACTIVITY
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The invention is directed to novel CCK analogs wherein Tryptophan and/or Phenylalamine are substituted with a radical which provides enhanced appetite suppressant activity to the peptide.
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- Alkane Functionalization on a Preparative Scale by Mercury-Photosensitized Cross-Dehydrodimerization
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Alkanes can be functionalized with high conversions and in high chemical and quantum yields on a multigram scale by mercury-photosensitized reaction between an alkane and alcohols, ethers, or silanes to give homodimers and cross-dehydrodimers.The separation of the product mixtures is often particulary easy because of a great difference in polarity of the homodimers and cross-dimers.It is also possible to bias the product composition when the ratio of the components in the vapor phase is adjusted by altering the liquid composition.This is useful either to maximize chemical yield or to ease separation by favoring the formation of the most easily separated pair of compounds.The mechanistic basis of the reaction is discussed and a number of specific types of syntheses, for example of 2,2-disubstituted carbinols, are described in detail.The selectivity of cross-dimerization is shown to exceed that for homodimerization and reasons are discussed.Relative reactivities of different compounds and classes of compound are MeOHp-dioxanecyclohexane1,3,5-trioxacyclohexaneethanolisobutaneTHFEt3SiH.The observed selectivities generally parallel those for homodimerization, reported in the preceding paper, but certain differences are noted, and reasons for the differences are proposed.The bond-dissociation energy of Et3SiH is estimated from the reactivity data to be 90 kcal/mol.Eleven new carbinols are synthesized.
- Brown, Stephen H.,Crabtree, Robert H.
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p. 2946 - 2953
(2007/10/02)
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- Cycloalkylmethyl Radicals. Part 3. Dynamic Stereochemistry of Axial and Equatorial Cyclohexylmethyl and 4-Alkylcyclohexylmethyl Radicals
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For cyclohexylmethyl and 4-alkylcyclohexylmethyl radicals the conformer in which the CH2. group adopts the axial position and that in which the CH2. group adopts the equatorial position can both be observed by e.s.r. spectroscopy.At 140 K the axial conformers have a(Hβ) ca. 42-43 G; the equatorial conformers have a(Hβ) ca. 30-31 G.For cis-4-methylcyclohexylmethyl radicals the ratio of the concentrations of the two conformers was studied as a function of temperature and shown to depend on the rate of radical ring inversion vs. the radical lifetime; the rate constant for ring inversion was obtained.As a check on the e.s.r. results the conformational equilibrium of cis-4-methylcyclohexylmethyl bromide was studied by 1H n.m.r. spectroscopy, which gave -ΔG0300(CH2Br) = 1.91 kcal mol-1.The relative conformer concentrations were also measured as a function of temperature for cyclohexylmethyl radicals and the conformational free energy difference of the CH2. group (-ΔG0300) was found to be 0.71 kcal mol-1.The preponderance of the conformer of the cis-4-methylcyclohexylmethyl radical with the CH3 group axial at T . group cannot because of its planarity.The barriers to rotation about the Cα-Cβ bonds in the axial radicals were found to be ca. 1.0 kcal mol-1 greater than those of the equatorial radicals; this is responsible for the greater a(Hβ) values of the axial radicals.The axial and equatorial conformers of cyclohexylmethyl radicals were investigated by semi-empirical SCF MO-methods.
- Ingold, Keith U.,Walton, John C.
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p. 1337 - 1344
(2007/10/02)
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- Deoxy-nitrosugars. Stereoelectronic Control in the Reductive Denitration of Tertiary Nitro Ethers. A Synthesis of 'C-Glycosides'
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The separate, radical denitration with Bu3SnH of the pyranose derivatives 3, 4, 9, and 10 gave in good yields exclusively the 'C-glycosides' 5 and 11, respectively (Scheme 1).Similar reduction of the cylohexyl derivatives 15, 16, 19 and 20 gave 4:1 mixtures of 17, 18, 21 and 22, respectively, always with predominant formation of an axial C,H-bond.In the furanose series a divergent behaviour was observed for the D-mannose-derived nitro ethers 25 and 27 and the D-ribose-derived nitro ethers 30 and 31, respectively, in that the former two gave isomerically homogeneous reduction products (26 and 28, respectively; Scheme 3) and the latter a 1:1 mixture of the diastereoisomers 32 and 33 (Scheme 4).The stereochemical results were explained on the basis of the stereoelectronic effect of the ring O-atom, the preferred conformation of the intermediate, pyramidal alkoxyalkyl radicals and steric effects in the trioxabicyclooctane ring system.
- Baumberger, Franz,Vasella, Andrea
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p. 2210 - 2222
(2007/10/02)
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- 127. Conformationally Controlled Odor Perception in 'Steroid-type' Scent Molecules
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A series of compounds possessing a 'steroid-type' scent and related to 4-(4'-t-butylcyclohexyl)-4-methyl-2-pentanones (1 and 2) have been synthesized.The odor of these compounds has been found to be dependent on their conformation; only when the molecule can assume a steroid-like shape there is an interaction with the odor chemoreceptor.
- Ohloff, Guenther,Giersch, Wolfgang,Thommen, Walter,Willhalm, Bruno
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p. 1343 - 1354
(2007/10/02)
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- Hydrogenolyse en phase liquide des epoxydes du t-butyl-4 methylene-cyclohexane sur divers catalyseurs metalliques supportes
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Epoxides of 4-t-butylmethylenecyclohexane have been hydrogenolised on various supported metal catalysts: Pd, Pt, Rh, Ni, at ordinary temperature and 1 atm pressure in various solvents.All these catalysts present a good regioselectivity, leading to preferential cleavage of the more substituted C-O bond.In the isomerisation process the order of reactivity is Ni Pt Pd = Rh with a yield in aldehydes of 60 percent for Pd and 80 percent for Rh and an important percentage of configuration inversion.The reaction could be used as a good method for preparing cis or trans 4-t-butylcyclohexanecarboxaldehyde.On Pt and Ni, hydrogenolysis yields primary alcohols with cleavage of the oxiran ring by trans or cis addition of hydrogen.
- Accrombessi, Georges,Geneste, Patrick,Olive, Jean-Louis,Pavia, Andre Armand
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- Alumina: Catalyst and support. XL (1) ring expansion during the dehydration of alcohols over alumina catalysts (2)
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An investigation of ring expansion during the alumina-catalyzed dehydration of alcohols was made by the micro-pulse technique. The dehydration at 380 °C over fresh alumina of cyclobutane-, cyclopentane-, cyclohexane-, and cycloheptane-methanol yielded 99, 55, 7, and 6% of ring-expanded product, respectively, whereas over alumina which has been deactivated by passing large quantities of alcohol over it, 100, 74, 19, and 18% of the ring-expanded products were found. The stereochemical aspects of this catalytic reaction were investigated by the dehydration of cis- and trans-4-t-butylcyclohexanemethanol. More ring expansion and a higher conversion to olefins occurred in the dehydration of the cis alcohol than of the trans alcohol. In the dehydration of 1-methyl-1-cyclohexanemethanol at 310 °C, 60% of product formation involved methyl migration whereas 40% involved ring expansion. The percentage of ring expansion increased to 55 when deactivated catalyst was used. More skeletal rearrangement was found in dehydrations over alumina which had been deactivated by passing either an unsaturated or a saturated hydrocarbon over it than when fresh catalyst was used. Although the presence of sodium ions or pyridine on the catalyst lowered the amount of double bond isomerization which occurred, it did not affect the amount of skeletal rearrangement or the deactivation of the catalyst. The dehydration of 2-methylpropanol over alumina catalyst was reinvestigated and it was found that the extent of skeletal isomerization to produce n-butenes increases with the deactivation of the aluminas.
- Pines, Herman,Brown, Stanley M.
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