Mixed dimers

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

Methylindole is, however, incorporated into mixed dimers and trimers. It thus reacts with indole and with skatole to form mixed dimers, and 2 moles react with 1 mole of indole for form a mixed trimcr. Likewise indole forms a dimer with 1,2-dimethylindole, 2-phenylindole, and even with 2,5-dimethylpyrrole, ... 

Irradiation of 5-acetyldibenz[i>,/]azepine (19) in the presence of. V-methylmaleimide results in a mixture of the homo dimer 20 and the mixed dimer 21. No [2 + 2] cycloadducts are obtained, however, with other alkenes such as styrene, acetylenedicarboxylic acid, dimethyl fumarate or acrylonitrile.32 Later studies have demonstrated that the dimerization occurs via the excited triplet state.196... 

The bis(PPh3) adduct has a long Rh-Rh bond of 2.936 A, whereas in the mixed dimer Rh2(OCOMe)2(DMG)2(PPh3)2 where only two acetates bridge, Rh-Rh is 2.618 A (Figure 2.41). 

Phenols are important antioxidants, with vitamin E being the most important endogenous phenolic membrane-bound antioxidant. Membrane levels of vitamin E are maintained through recycling of the vitamin E radical with ascorbate and thiol reductants. Vitamin E is a mixture of four lipid-soluble tocopherols, a-tocopherol being the most efiective radical quencher. The reaction of a-tocopherol with alkyl and alkylperoxyl radicals of methyl linoleate was recently reported. These are facile reactions that result in mixed dimer adducts (Yamauchi etal., 1993). 

Entries 10 and 11 were obtained by the reaction of the bridged tetrachlorides Cl2RSn(CH2)4SnRCl2 with the polymeric bridged oxides [R1(0)Sn(CH2)4Sn(0)R1]B. In the crystal, the neopentyl groups occupy the outer four corners of the tetrameric block in solution, the tetramers are in equilibrium with their corresponding mixed dimers. 

Microwave spectroscopy is probably the ultimate tool to study small alcohol clusters in vacuum isolation. With the help of isotope substitution and auxiliary quantum chemical calculations, it provides structural insights and quantitative bond parameters for alcohol clusters [117, 143], The methyl rotors that are omnipresent in organic alcohols complicate the analysis, so that not many alcohol clusters have been studied with this technique and its higher-frequency variants. The studied systems include methanol dimer [143], ethanol dimer [91], butan-2-ol dimer [117], and mixed dimers such as propylene oxide with ethanol [144]. The study of alcohol monomers with intramolecular hydrogen-bond-like interactions [102, 110, 129, 145 147] must be mentioned in this context. In a broader sense, this also applies to isolated ra-alkanols, where a weak Cy H O hydrogen bond stabilizes certain conformations [69,102]. Microwave techniques can also be used to unravel the information contained in the IR spectrum of clusters with high sensitivity [148], Furthermore, high-resolution UV spectroscopy can provide accurate structural information in suitable systems [149, 150] and thus complement microwave spectroscopy. 

C. Emmeluth, V. Dyczmons, and M. A. Suhm, Tuning the hydrogen bond donor/acceptor isomerism in jet cooled mixed dimers of aliphatic alcohols. J. Phys. Chem. A 110, 2906 2915... 

A study of a series of C-nitroso compounds, including monomers as well as dimers, by field desorption has demonstrated the superiority of this technique to this class of compounds118. All the compounds display intense molecular ions118. The method has a significant potential for studies of the equilibrium between mixed and pure C-nitroso compounds, since the amount of pure and mixed dimers present in a solution apparently can be visualized by the relative abundances of the respective molecular ions see Scheme 43. Determination of the concentrations versus time may resolve the kinetics of the dimer formation118. 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

Photodimers 4a and 4b are achiral, thus precluding the possibility of absolute asymmetric induction. The mixed dimer 5, however, is chiral, and in a... 

The triplet EPR spectra of the mixed dimers [ZnTPPS/TTAP] and [TPPS/ZnTTAP] also displayed in Figure 5 have the same characteristics as the free base dimer spectra (cf. Figures 3 and 5). A summary of the values of triplet parameters derived from the spectra is given in Table II. Included are literature values on face-to-face covalently-linked diporphyrins (8). 

The two Li NMR peaks coalesce into a single peak at room temperature (T. = -8 °C), indicating that the interior and exterior cations are able to exchange places without dissociation of the dimer The Li NMR spectra of both 1-i-Pr and 1-r-Bu corannulene "/4Li exhibit quite analogous behavior. Moreover, tetraanion solutions generated from mixtures of corannulene and r-Bu-corannulene show the presence of mixed dimers in the H NMR spectrum. ... 

The only stem materials examined were from the Setaria anceps and Digitaria decumbens grasses. The major substituted cyclobutane dimers in these walls appeared, from the mass spectral data, to be mixed dimers of ferulic acid and coniferyl alcohol (FA-ConAlc type). Further examination of the aromatics of stems is required but if such dimers are present then they may well be involved in the biosynthesis of lignin (41,42). 

The ratio of the constitutional isomers 14-16 as determined by HPLC was 16 3 1. The fact that mixed dimers are also easily accessible reflects another aspect of the great diversity within heterofullerene chemistry. As with the aza[60]fullerenes, alkoxy substituted monomers 17 and 18 are formed together with the dimers 14-16 (Scheme 12.4) [3]. These two monomers are formed in a ratio of 7 1. Also, for these higher heterospheres NMR spectroscopy reveals a closed structure. For example, the signal of the sp C-atom of 17 appears at 5 = 96.42. 

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