On the structure of carotenoid iodine complexes

Previous work on carotenoid-iodine complexes is briefly reviewed. The formation of iodine complexes of beta,beta-carotene and of (3R,3' R )-beta,beta-carotene-3,3'-diol (zeaxanthin) has been studied by modern methods including UV/VIS/NIR, IR MS, EPR, ENDOR and NMR (1H, 1H-1H COSY, TOCSY, 2D ROESY, 1H-13C HSQC and 1H-13C HMBC) spectroscopy, and chemical reactions monitored by HPLC, TLC and spectral analysis (VIS, MS, 1H NMR). beta,beta-Carotene formed a solid complex C40H56 x 4I with iodine in hexane and a solvent complex with lambdamax 1010 nm in chlorinated solvents. Iodine was not covalently bound to the carotene. Spectroscopic and chemical evidence is consistent with the representation of the beta,beta-carotene-iodine complex containing iodine in a pi complex with cationic/radical cationic properties. Extensive E/Z isomerisation was noted for all quenching products obtained in acetone, with thiosulfate, by dilution, or by reaction with nucleophile (MeOH). Key products obtained from the beta,beta-carotene-iodine complex were 4',5'-didehydro-4,5'-retro-beta,beta-carotene (isocarotene) and 4-methoxy-beta,beta-carotene. The zeaxanthin-iodine complex was not suitable for a practical synthesis of (3S,3'S)-4',5'-didehydro-4,5'-retro-beta,beta-carotene-3,3'-diol (eschscholtzxanthin).     

The charge delocalised beta,beta-carotene dication--preparation, structure elucidation by NMR and reactions with nucleophiles

The reaction between beta,beta-carotene and BF3-etherates has been investigated, leading to structural elucidation of the blue product, formed in appropriate organic solvents, as a symmetrical charge delocalised dication (lambda(max) 985 nm at room temperature in CHCl3) with considerable stability. The reaction, monitored by EPR studies at -25 degrees C, occurred via free radical intermediates. A C40H56BF3 intermediate was captured by EIMS. The detailed structure of the dication was established by COSY, HSQC, HMBC and 1D and 2D ROESY NMR techniques (600 MHz, CDCl3, -20 degrees C) leading to complete assignments of 1H and 13C chemical shifts and 3J(H,H) coupling constants. The effects of the two delocalised charges on chemical shift (charge distribution) and bond distance (3J(H,H)) were considered. The results are consistent with charge delocalisation mainly in the C-5-C-9 and C-5'-C-9' regions and with bond inversion to retro shifted double bonds in the central C-13-C-13' region. A convention for denoting the charge delocalisation and bond types is presented. The experimental results are discussed relative to previous theoretical calculations of the beta,beta-carotene dication structure. (All-E) and (15-Z)-beta,beta-carotene provided the same dication. The NIR spectra and stability of dications prepared in the same manner from the related carotenes 20,20'-dinor-beta,beta-carotene, heptapreno-beta,beta-carotene and nonapreno-beta,beta-carotene were examined for comparison. Reactions of the beta,beta-carotene dication with selected nucleophiles provided products including isocryptoxanthin, isocarotene and mutatochrome with H2O as nucleophile, and isocryptoxanthin methyl ether, 8-methoxy-7,8-dihydro-beta,beta-carotene and isocarotene with CH3ONa as nucleophile. The formation of these products is rationalised from the structure assigned to the dication.     

Nucleophilic reactions of charge delocalised carotenoid mono- and dications

In the present study insight was gained on the larger complexity of cationic mixtures of diaryl (phi,phi-carotene, isorenieratene) and aliphatic (psi,psi-carotene, lycopene) carotenes, prepared by reaction with BF3-etherate, compared with beta,beta-carotene. Chemical reactions of the mono- and dications prepared in situ from the allylic carotenols beta,beta-caroten-4-ol (isocryptoxanthin) and beta,beta-carotene-4,4'-diol (isozeaxanthin), and from isorenieratene and lycopene were investigated using selected O, N and S nucleophiles; water, methanol, azide and thioacetate. In total 22, including 18 new, neutral carotenoid products were isolated and identified by VIS, MS and NMR (in part) spectroscopy. Their structures were compatible with the structures of the cationic intermediates. The formal addition of hydride to the various dications, required to rationalise minor reaction products, is discussed in terms of more likely hydrogen radical or proton transfer in cationic reactions. Extensive E/Z isomerisation was observed for all quenching products. The potential use of carotenoid cations for the synthesis of 4,(4')-substituted beta,beta-carotenes and 7-oxabicyclo[2,2,1]heptane derivatives is discussed.     

Kinetics of the radical scavenging activity of beta-carotene-related compounds

To clarify the non-enzymatic radical-scavenging activity of beta-carotene-related compounds and other polyenes, we used differential scanning calorimetry to study the kinetics of radical polymerization of methyl methacrylate (MMA) by 2,2'-azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO) in the absence or presence of polyenes under nearly anaerobic conditions at 70 degrees C, and analyzed the results with an SAR approach. The polyenes studied were all-trans retinol, retinol palmitate, calciferol, beta-carotene and lycopene. Polyenes produced a small induction period. The stoichiometric factor (n) (i.e. the number of radicals trapped by each inhibitor molecule) of polyenes was close to 0. Tetraterpenes (beta-carotene, lycopene) suppressed significantly more of the initial rate of polymerization (R(inh)) than did diterpenes (retinol, retinol palmitate). The inhibition rate constants (k(inh)) for the reaction of beta-carotene with AIBN- or BPO-derived radicals were determined to be 1.2-1.6x10(5) l/mol s, similar to published values. A linear relationship between (k(inh)) and the kinetic chain length (KCL) for polyenes was observed; as (k(inh)) increased, KCL decreased. KCL also decreased significantly as the number of conjugated double bonds in the polyenes increased. Polyenes, particularly beta-carotene and lycopene, acted as interceptors of growing poly-MMA radicals.     

Cross metathesis approach to retinoids and other β-apocarotenoids

Cross metathesis (CM) reactions between polyenes, such as β-carotene, canthaxanthin or retinyl acetate, and various alkenes or dienes in the presence of second generation Hoveyda-Grubbs (H II) or Grubbs (G II) catalysts were investigated. Depending on the cross partner different apocarotenoids were obtained. Cross metathesis reactions of retinyl acetate proved to be fully regioselective. Carotenoid CM reactions afforded mixtures of two products due to competing cleavage of the C11-C12 and C15-C15′ double bonds. However, regioselectivity can be controlled by choice of appropriate reaction conditions. The reactions of polyenes with dienes worked better in respect of yields and diastereoselectivities than those with monounsaturated cross partners.     

Cyclization of dienols, polyenes, and dienyne mediated by mercuric salts: course of cyclizations controlled by stability of cationic intermediates

Course of cyclization of dienols, polyenes, and dienyne mediated by mercuric salts were controlled by stability of cationic intermediates and source of mercuric salts. Reaction of (E)-5,9-dimethyl-4,8-decadiene-1-ol with mercuric acetate gave the intramolecular oxymercuration product, whereas one with mercuric triflate produced the olefin cyclization products.     

Cis-trans isomerizations of beta-carotene and lycopene: a theoretical study

The all-trans to mono-cis isomerizations of polyenes and two C40H56 carotenes, beta-carotene and lycopene, at the ground singlet (S0) and triplet (T1) states are studied by means of quantum chemistry computations. At the S0 state of polyenes containing n acetylene units (Pn), we find that the energy barrier of the central C=C rotation decreases with n. In contrast, however, at the T 1 state, the rotational barrier increases with n. For the C40H56 carotenes, the rotational barriers of lycopene are lower than those of their beta-carotene counterparts. This difference renders the rotational rates of lycopene to be 1-2 orders of magnitude higher than those of beta-carotene at room temperature. For both these carotenes, the barrier is lowest for the rotation toward the 13-cis isomer. The relative abundances are in the following order: all-trans > 9-cis > 13-cis > 15-cis. Although the 5-cis isomer of lycopene has the lowest energy among the cis isomers, its formation from the all-trans form is restricted, owing to a very large rotational barrier. The possible physiological implications of this study are discussed.     

OFF沸石中锂离子在骨架外的分布及可接近性

对OFF沸石进行离子交换制备出NH4-OFF沸石,然后通过Li2CO3/NH4-OFF体系的固态反应制备了不同锂离子交换度的Li-OFF沸石。对吸附氮气和氧气的Li-OFF样品进行了7Li MAS NMR表征。研究发现,所有吸附氧气样品的部分7Li MAS NMR共振峰均发生了顺磁位移。通过对实验谱的拟合,得知Li-OFF沸石中的锂离子可分布在三类阳离子位置（A、B和C）上,对应于三个不同位移的谱峰。Li+离子并非等比例地进入三个可交换的位置,而是优先占据主孔道中的A位置,随着交换度的升高,位能较高的C和B位置上的Li离子占有率逐渐增加。在100％固态交换度样品中,Li+离子在不同离子位的占有率分别为17％(位置B)、29％(位置C)和54％(位置A)。其中,位置B是O2分子不可接近的,所以Li-OFF沸石中有83％的阳离子是可以接近的。     

Generation of 4,5-Diazacyclopentane-1,3-diyl Radical Cations by Chemical Electron Transfer (CET) Oxidation of Urazole-Bridged Bicyclic Housanes (Bicyclo[2.1.0]pentanes) and Their Chemical Transformations

4,5-Diazacyclopentane-1,3-diyl radical cations 3(*)(+)() were generated from urazole-bridged bicyclic housanes 3 through chemical oxidation by using tris(4-bromophenyl)aminium hexachloroantimonate as oxidant to afford the two olefinic products 4 and 5. Product studies establish that the bisolefins 5 are the result of double oxidation of the housanes 3, whereas the monoolefins 4 are formed by acid-catalyzed rearrangement, which can be suppressed by excess of base (2,6-di-tert-butylpyridine). In the case of dibenzyl substitution (3c), disproportionation of two monoradical species 5(H)c(*) serves as an alternative pathway to the corresponding olefins 4 and 5 because higher amounts of double oxidation product were isolated in the absence of base than expected if only a stoichiometric reaction were operating. Semiempirical MO calculations suggest that ionization takes place from one of the nitrogen lone pairs rather than from the strained central C-C bond as implied by the significantly lower (by ca. 0.5 eV) ionization potential. Furthermore, in the initially puckered radical cation, the positive charge is mainly located at the two nitrogen atoms, while after relaxation to the planar geometry, the charge shifts essentially entirely to the radical cation carbon atoms. The trapping reaction with methanol leads to the hemiaminal-type products 6 and 7, which establish the involvement of the cationic intermediates 3(H)(+)() and 5(H)(+)(). In addition, (13)C NMR spectroscopy confirmed these cationic intermediates [3(H)(+)() and 5(H)(+)()] by detection of the characteristic signals below delta 250 for carbenium ions. Unquestionably, the urazole ring significantly influences the radical cation reactivity of the housanes 3. Thus, in contrast to the corresponding homocyclic tricyclooctane derivatives, stoichiometric instead of catalytic amounts of CET oxidant are needed, the two nitrogen atoms of the hydrazino bridge stabilize the radical cation 3(*)(+)() by conjugation, and the carbonyl groups of the urazole moiety assist the deprotonation to the exocyclic double bonds to prevent 1,2 alkyl migration.     

Comparative studies on radical cation formation from carotenoids and retinoids

A comparative study of radical cation formation from selected polyenes, namely carotenoids (C₄₀) and retinoids (C₂₀), has been carried out by treatment with the Lewis acids BF₃ as its etherate or SbCl₃. The reaction in chloroform was monitored by vis/NIR and EPR spectroscopy at variable temperature. β,β-Carotene, β,β-caroten-4-ol, retinol (vitamin A), retinyl acetate and anhydroretinol were used as substrates. It is concluded that whereas BF₃-diethyl etherate or SbCl₃ is capable of effecting one-electron transfers to produce radical cations from the longer polyenes (carotenoids), no radical cations were obtained from the retinoids. The results of SbCl₃ treatment of the retinoids have a bearing on the current studies on the mechanism of the Carr-Price blue colour reaction previously used for quantitative analysis of vitamin A.     

Zn粉作为催化剂前体催化对甲苯磺酰胺/二溴海因与烯烃的区域选择性和立体选择性氨溴加成反应

Zn粉作为催化剂前体, 在反应中可被原位氧化成高效Zn(Ⅱ)-催化剂. 在该催化剂催化下, 以对甲苯磺酰胺和1,3-二溴-5,5-二甲基乙内酰脲(二溴海因)为氮源/卤素源, 二氯甲烷作溶剂, 建立了烯键上的高度区域选择和立体选择性氨溴加成反应新体系. 该方法在室温下可高产率地制得邻位氨基溴的加成产物, 最高收率可达99%. 实验结果表明, 当与双键直接相连的苯环对位有强给电子基团(CH₃O)时, 反应收率高, 并可得到唯一的氨溴加成产物(α-溴-β-氨基); 当与双键直接相连的苯环对位有强拉电子基团(NO₂)或弱拉电子基团(如Br和F)时, 该反应的产率相对较低, 但也能制得另一个唯一的氨溴加成产物异构体(α-氨基-β-溴型). 本文考察了20种不同结构底物的氨溴加成反应情况, 产物结构经过¹H NMR、¹³C NMR和元素分析确证, 并探讨了该反应的机理.     

Unexpected solids from enantiopure cationic palladium complexes and racemic anions: A structural study of chiral non-discrimination

Enantiomerically pure cationic complexes were obtained via cyclopalladation of primary amines and subsequent addition of a chelating ethylendiamine ligand. No diastereomeric resolution was observed upon combining these cations with racemic mandelate or hydratropate anions, but four less popular products, namely three double salts and a solid solution, were obtained and structurally characterized. For one of the double salts, the alternative ionic compounds based on different stereoisomers of the same residues were synthesized independently: The conventional racemic solid and both diastereomeric salts formed by enantiopure cations and anions were studied by single crystal X-ray diffraction. Lattice energy calculations confirm that the diastereomeric salts differ significantly; formation of the partially racemic double salt, however, is energetically favourable and precludes resolution.     

Formation and decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y investigated by in situ stopped-flow MAS NMR spectroscopy

Methylation of aniline by methanol on zeolite H-Y has been investigated by in situ (13)C MAS NMR spectroscopy under flow conditions. The in situ (13)C continuous-flow (CF) MAS NMR experiments were performed at reaction temperatures between 473 and 523 K, molar methanol-to-aniline ratios of 1:1 to 4:1, and modified residence times of (13)CH(3)OH between 20 and 100 (g x h)/mol. The methylation reaction was shown to start at 473 K. N,N,N-Trimethylanilinium cations causing a (13)C NMR signal at 58 ppm constitute the major product on the catalyst surface. Small amounts of protonated N-methylaniline ([PhNH(2)CH(3)](+)) and N,N-dimethylaniline ([PhNH(CH(3))(2)](+)) were also observed at ca. 39 and 48 ppm, respectively. After increase of the temperature to 523 K, the contents of N,N-dimethylanilinium cations and ring-alkylated reaction products strongly increased, accompanied by a decrease of the amount of N,N,N-trimethylanilinium cations. With application of the in situ stopped-flow (SF) MAS NMR technique, the decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y to N,N-dimethylanilinium and N-methylanilinium cations was investigated to gain a deeper insight into the reaction mechanism. The results obtained allow the proposal of a mechanism consisting of three steps: (i) the conversion of methanol to surface methoxy groups and dimethyl ether (DME); (ii) the alkylation of aniline with methanol, methoxy groups, or DME leading to an equilibrium mixture of N,N,N-trimethylanilinium, N,N-dimethylanilinium, and N-methylanilinium cations attached to the zeolite surface; (iii) the deprotonation of N,N-dimethylanilinium and N-methylanilinium cations causing the formation of N,N-dimethylaniline (NNDMA) and N-methylaniline (NMA) in the gas phase, respectively. The chemical equilibrium between the anilinium cations carrying different numbers of methyl groups is suggested to play a key role for the products distribution in the gas phase.     

Degradation of epoxidized polyenes in the presence of phthalic acid

Viscometry was used to follow the epoxidation of polybutadiene and styrene–butadiene triblock copolymers (SBS) with monoperoxyphthalic acid (MPPA). Long reaction periods lead to an abnormal increase and then drop in reduced viscosity. The increasing viscosity is due to repulsion between oxirane group and its ring-opening product with phthalic acid, while the viscosity drop can be correlated with a decrease of molecular weight. Long-term storage of the epoxidized polyenes in the reaction solution containing phthalic acid, which is the derived product from MPPA, also leads to the dramatic decrease in molecular weights. The occurrence of degradation was confirmed by GPC analysis of molecular weight and molecular weight distribution. Chloroform accelerated the degradation reaction, possibly through the generation of hydrochloric acid. The degraded products were isolated and analyzed by GPC and NMR. A free radical mechanism was proposed for the oxidative degradation of epoxidized polyenes after excessive reaction.     

中间体C60(CH3)-和产物C60(CH3)2的结构及产物

用INDO系列方法对C₆₀^2-与CH₃反应的中间体C₆₀(CH₃)^-进行理论研究,得到具有Cs对称性的构型。结果表明,CH₃加成到C₁₅上,将使与其相邻的双键碳(C₃₀)的电荷密度和自旋密度达极大值,故加成反应部位在C₃₀处;另外,C₁₅的对位C₁₂(或C₂₇)也较其它部位易于反应,且有两个反应场所,因而产物C₆₀(CH₃)₂可能为六元环上的1,2-加成和1,4-加成两种异构体的混合物。同时对两种加成产物的结构和电子光谱进行了理论研究,指认其电子跃迁,并讨论了其光谱红移的原因。     

Thiophene-thiophene versus phenyl-phenyl coupling in 2-(Diphenylamino)-Thiophenes: an ESR-UV/Vis/NIR spectroelectrochemical study.

The oxidation of several 2-diphenylamino-substituted thiophenes and N,N'-bis(2-diphenylamino-5-thienyl)-substituted phenylene-1,4-diamines with different substitution patterns in the 5-position of their thiophene moieties was studied by cyclovoltammetric and spectroelectrochemical measurements (ESR, UV/Vis/NIR). These measurements revealed both the structure of the oxidation products obtained and that of their cationic intermediates, as well as the pathway of their formation and follow-up reactions. Thus, the formation of the radical cations in the first electrochemical oxidation step of the target molecules was demonstrated. Depending on the substitution pattern, these radical cations give rise to several consecutive processes to a different extent. Thus radical dimers, dehydrodimers with a 2,2-bithiophene, 3,3'-bithiophene, or benzidine structure, as well as radical monocationic and dicationic products can be formed. For N,N'-bis(2-diphenylamino-5-thienyl)-substituted phenylene-1,4-diamines rather stable radical cations and dications were primarily formed and unambiguously identified. These species were transformed into 2,2'-bithiophene dimers and oligomers when the 5-position of the thiophene moiety was unsubstituted. By in situ spectroelectrochemistry the influence of the substituents on the type of coupling reactions can be demonstrated and followed in detail.     

Structure elucidation of polyene systems with extensive charge delocalization-carbocations from allylic carotenols

[structure: see text] The structures of two diastereomeric cations, readily prepared from beta, beta-caroten-4-ol (1) by treatment with trifluoroacetic acid, have been determined by NIR and NMR spectroscopy, resulting in the complete structure elucidation of the most extensively delocalized carbocations so far described. Higher partial charge was observed toward the center of the polyene chain (larger filled red circles). Bond reversion occurs in the central region of the molecule.     

Cyclopropyl alkynes as mechanistic probes to distinguish between vinyl radical and ionic intermediates

[reaction: see text] The reactions of (trans-2-phenylcyclopropyl)ethyne, 1a, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, 1b, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1c, with either aqueous sulfuric acid or tris(trimethylsilyl)silane (or tributyltin hydride) and AIBN have been investigated. Protonation and addition of the silyl (or stannyl) radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively. Under both reaction conditions, 1a yielded products derived from ring opening toward the phenyl substituent. Alkynes 1b and 1c, however, gave different products depending on whether radical or cationic conditions were used. When radical conditions were employed, products derived from regioselective ring opening toward the phenyl substituent were obtained. In contrast, when cationic conditions were employed, products derived from selective ring opening toward the methoxy substituent were isolated. The corresponding alpha-cyclopropyl-substituted vinyllithium derivatives were also synthesized and were found to be stable toward rearrangement. An estimate of the rate constants for ring opening of the alpha-cyclopropylvinyl cations was also made: values of 10(10)-10(12) s(-1) were found for the vinyl cations derived from protonation of the terminal carbon of alkynes 1a-c. Based on these results, cyclopropyl alkynes 1a-c can be classified as hypersensitive mechanistic probes for the detection of vinyl radical or cationic intermediates generated adjacent to the cyclopropyl ring and, in the case of 1b and 1c, the distinction between a radical or cationic intermediate is possible.     

Cationic polymerization of γ-methyl- and α-methylphenylallene

The cationic polymerizations of γ-methylphenylallene ( 1 ) and α-methylphenylallene ( 2 ) were carried out with some Lewis acids at 25 and 0°C in dichloromethane to obtain the corresponding polymers through allyl cations, respectively. Tin (IV) chloride was found to be an effective catalyst for the cationic polymerization of both allenes 1 and 2 compared with other Lewis acids. Thus, in the polymerization of 1 , methanol-insoluble polymer was only obtained using Tin (IV) chloride, and M?_n of methanol-insoluble polymer obtained by Tin (IV) chloride was the highest in the polymerization of 2 . From the analysis of ¹H- and ¹³C-NMR spectra of the obtained polymers, the polymer from 1 consisted of two kinds of units polymerized by each double bonds of allene 1 , whereas the polymer from 2 consisted of only one unit polymerized by terminal double bond of allene 2 . Moreover, effect of solvent on the cationic polymerizations of 1 and 2 were discussed.     

Organometallic compounds in organic synthesis—part 17: Reactions of tricarbonylcyclohexadienyliron salts with o-silylated enolates,allyl silanes and aspects of their synthetic equivalents

Efficient CC bond formation results during reactions of O-silylated enolates and allytrialkysilanes with a range of tricarbonylcyclohexadienyliron salts to give tricarbonylcyclohexadieneiron complexes in good to excellent yields. This represents a new and efficient type of conversion of aldehydes, ketones, esters and lactones, through enol TMS ethers, into synthetically useful products. An advantage of the allylsilane process is that CC formation occurs at the end of the double bond remote from the silane group. The cations employed can be defined as synthetic equivalents of the 5-cation of substituted cyclohexadienes, or of the 4-cation of cyclohex-2-enones, or alternatively of specifically substituted aryl cations, dependent on their structures and the subsequent treatment of the reaction product.