A variety of substituted schiff base complexes of the composition (“salen”)ZrCl2(thf) ( 1 – 21 ) were synthesized, with methylalumoxane (“MAO”) activated and used for a systematic study of their catalytic activity towards hexa‐1,5‐diene (“salen”: substituted salicylidene–ethylene‐iminato ligands). Main product of the catalytic cycle is methylenecyclopentane. Dimers are only formed in minor amounts. The catalytic activity and selectivity of the Ziegler–Natta systems strongly depend on the nature and the position of the peripheric substituents in the Schiff base ligands. Electron‐withdrawing substituents in para‐position to the phenolato oxygen (5‐position) decrease the catalytic activity. Improved activity and selectivity were obtained with electron‐donating substituents in 5‐position. Altering the ethylene bridge causes a lowering of the activity or inactivation. According to the x‐ray analysis the metal center in the related complex (L)ZrCl2 ( 22 ) (L: N′,N′‐bis(ethylene)‐N′‐methyl‐N,N′′‐bis(benzoylacetonato‐imine) has a pentagonal‐bipyramidal environment. The pentadentate schiff base ligand lies in the plane, and both chloro groups occupy the axial positions. In contrast to the catalytically active salene complexes 22 can not rearrange to form a species in which the both chlorides are cis to each other. Consequently 22 is catalytically inactive. 相似文献
A new and efficient catalytic asymmetric synthesis of the potent cannabinoid receptor agonist (−)‐CP‐55940 has been developed by using ruthenium‐catalyzed asymmetric hydrogenation of racemic α‐aryl ketones via dynamic kinetic resolution (DKR) as a key step. With RuCl2‐SDPs/diamine [SDPs=7,7′‐bis(diarylphophino)‐1,1′‐spirobiindane] catalysts the asymmetric hydrogenation of racemic α‐arylcyclohexanones via DKR provided the corresponding cis‐β‐arylcyclohexanols in high yields with up to 99.3% ee and >99:1 cis‐selectivities. Both ethylene ketal group at the cyclohexane ring and ortho‐methoxy group at the phenyl ring of the substrates 6 have little effect on the selectivity and reactivity of the hydrogenations. Based on this highly efficient asymmetric ketone hydrogenation, (−)‐CP‐55940 was synthesized in 13 steps (the longest linear steps) in 14.6% overall yield starting from commercially available 3‐methoxybenzaldehyde and 1,4‐cyclohexenedione monoethylene acetal. 相似文献
Both cis‐ and trans‐but‐2‐ene‐1,4‐diamines have been prepared and efficiently applied as sacrificial cosubstrates in enzymatic transamination reactions. The best results were obtained with the cis‐diamine. The thermodynamic equilibrium of the stereoselective transamination process is shifted to the amine formation due to tautomerization of 5H‐pyrrole into 1H‐pyrrole, achieving high conversions (78–99%) and enantiomeric excess (up to >99%) by using a small excess of the amine donor. Furthermore, when the reaction proceeded, a strong coloration was observed due to polymerization of 1H‐pyrrole. A structurally related compound, cis‐but‐2‐ene‐1,4‐diol, has been utilized as cosubstrate in different alcohol dehydrogenase (ADH)‐mediated bioreductions. In this case, high conversions (91–99%) were observed due to a lactonization process. Both strategies are convenient from both synthetic and atom economy points of view in the production of valuable optically active products.
A family of iron complexes with general formula [Fe(II)(R,Y,XPyTACN)(CF3SO3)2], where R,Y,XPyTACN=1‐[2′‐(4‐Y‐6‐X‐pyridyl)methyl]‐4,7‐dialkyl‐1,4,7‐triazacyclononane, X and Y refer to the groups at positions 4 and 6 of the pyridine, respectively, and R refers to the alkyl substitution at N‐4 and N‐7 of the triazacyclononane ring, are shown to be catalysts for efficient and selective alkene oxidation (epoxidation and cis‐dihydroxylation) employing hydrogen peroxide as oxidant. Complex [Fe(II)(Me,Me,HPyTACN)(CF3SO3)2] ( 7 ), was identified as the most efficient and selective cis‐dihydroxylation catalyst among the family. The high activity of 7 allows the oxidation of alkenes to proceed rapidly (30 min) at room temperature and under conditions where the olefin is not used in large amounts but instead is the limiting reagent. In the presence of 3 mol% of 7 , 2 equiv. of H2O2 as oxidant and 15 equiv. of water, in acetonitrile solution, alkenes are cis‐dihydroxylated reaching yields that might be interesting for synthetic purposes. Competition experiments show that 7 exhibits preferential selectivity towards the oxidation of cis olefins over the trans analogues, and also affords better yields and high [syn‐diol]/[epoxide] ratios when cis olefins are oxidized. For aliphatic substrates, reaction yields attained with the present system compare favourably with state of the art Fe‐catalyzed cis‐dihydroxylation systems, and it can be regarded as an attractive complement to the iron and manganese systems described recently and which show optimum activity against electron‐deficient and aromatic olefins. 相似文献
The 1:1 cocrystallization of 1,4‐diaryl‐1,4‐bisimines (Ar–CHN–CH2‐)2 4 – 11 and substituted meso‐1,2‐diaryl‐1,2‐ethanediols 1 – 3 leads to supramolecular structures in which the diol is hydrogen bonded by one of its hydroxy groups to an imine nitrogen atom of a 1,4‐bisimine. The second functionality in each molecule leads to the generation of ladderlike polymeric structures where each molecule of the diol is linked to two molecules of the 1,4‐bisimine and vice versa. If the diol carries electron donor groups in the aromatic residue and the 1,4‐bisimine correspondingly acceptor groups, then charge transfer interactions are observed. The excited CT complex which corresponds to a radical ion pair is stabilized by migration of a proton of a hydroxy group to the nitrogen atom of an imino group. This is supported by the appearance of a N–H vibration in the IR spectra. The reorganization is also accompanied by changes in the UV/Vis spectra and by the generation of paramagnetism in the crystalline material. The results represent a type of photochromism which has its origin in a light‐induced cooperative electron–proton transfer. The photochromism is thermally reversible. 相似文献
Catalytic asymmetric conjugate arylation of racemic 6‐substituted cyclohexenones with arylboronic acids was catalyzed by 3 mol % of chiral amidophosphane‐[RhCl(C2H4)]2 in a 10:1 mixture of 1,4‐dioxane and water at 70 °C to afford a nearly 1:1 mixture of trans‐ and cis‐5‐aryl‐2‐substituted cyclohexanones in high enantioselectivity, which was subsequently epimerized with sodium ethoxide in ethanol to give thermodynamically stable trans‐5‐aryl‐2‐substituted cyclohexanones with 99–97 % ee in high two‐step yields. 相似文献
The ternary Ziegler‐Natta‐type catalyst system based on neodymium versatate (NdV), diisobutylaluminium hydride (DIBAH) and ethylaluminium sesquichloride (EASC) was used for the in situ preparation of a compatibilized blend consisting of poly(cis‐1,4‐butadiene) (BR = butadiene rubber) and poly(ε‐caprolactone) (PCL). Poly(cis‐1,4‐butadiene)‐block‐poly(ε‐caprolactone) which acts as compatibilizer for the two immiscible polymers BR and PCL was obtained by a two step sequential polymerization with the preparation of a living cis‐1,4‐BR building block in the first stage and the subsequent polymerization of CL during the second stage. This preparation method resulted in a polymer blend comprising the homopolymers BR and PCL as well as the block copolymer BR‐block‐PCL. For detailed characterization the block copolymer was separated from the respective homopolymers BR and PCL by means of fractionation with the binary solvent mixture dimethylformamide/methylcyclohexane (DMF/MCH) which mixes well at elevated temperature and exhibits phase separation at ambient temperature. 1H NMR, IR, SEC and TEM were used for characterization of the block copolymer.
1,4‐Dimethyl‐5‐aminotetrazolium 5‐nitrotetrazolate ( 2 ) was synthesized in high yield from 1,4‐dimethyl‐5‐aminotetrazolium iodide ( 1 ) and silver 5‐nitrotetrazolate. Both new compounds ( 1, 2 ) were characterized using vibrational (IR and Raman) and multinuclear NMR spectroscopy (1H, 13C, 14N, 15N), elemental analysis and single crystal X‐ray diffraction. 1,4‐Dimethyl‐5‐aminotetrazolium 5‐nitrotetrazolate ( 2 ) represents the first example of an energetic material which contains both a tetrazole based cation and anion. Compound 2 is hydrolytically stable with a high melting point of 190 °C (decomposition). The impact sensitivity of compound 2 is very low (30 J), it is not sensitive towards friction (>360 N). The molecular structure of 1,4‐dimethyl‐5‐aminotetrazolium iodide ( 1 ) in the crystalline state was determined by X‐ray crystallography: orthorhombic, Fddd, a=1.3718(1) nm, b=1.4486(1) nm, c=1.6281(1) nm, V=3.2354(5) nm3, Z=16, ρ=1.979 g cm−1, R1=0.0169 (F>4σ(F)), wR2 (all data)=0.0352. 相似文献