Urea-adduction method

Norma/andBranc/jedAlip/jatic Hydrocarbons. The urea-adduction method for separating normal and branched aHphatic hydrocarbons can be carried out in sulfolane (38,39). The process obviates the necessity of handling and washing the soHd urea—normal paraffin adduct formed when a solution of urea in sulfolane is contacted with the hydrocarbon mixture. OveraH recovery by this process is typicaHy 85% normal paraffin purity is 98%. 

Currently, there is continuing work on an iadustry standard method for the direct determination of monomer, dimer, and trimer acids. Urea adduction (of the methyl esters) has been suggested as a means of determining monomer ia distilled dimer (74). The method is tedious and the nonadductiag branched-chain monomer is recovered with the polymeric fraction. A micro sublimation procedure was developed as an improvement on urea adduction for estimation of the polymer fraction. Incomplete removal of monomer esters or loss of dimer duriag distillation can lead to error (75). 

A second method of achieving latency is by using a curative precursor, which is chemically inactive at room temperature but then converts to an active curative at the cure temperature. Examples of this type include Monuron and di-urea adduct of toluene diisocyanate and dimethylamine. Full cure in about 1 h at 125°C is achievable with these materials. 

Amino acid ester isocyanates are produced cleanly by this method and can often be used without purification. If desired, volatile amino acid ester isocyanates, such as the title compound, can be purified to analytical purity by Kugelrohr distillation. The amino acid ester isocyanates generated by this method are formed without detectable racemization (>99.5% ee) the er antiomeric purity of the isocyanates can be checked by trapping with (S)-l-phenylethylamine, followed by 1H NMR analysis of the resulting urea adducts.2 If this method is used to generate isocyanates of peptides, then efficient stirring is necessary to prevent epimerization of the peptide isocyanates.3 4... 

Other separating techniques may be used to separate total hydrocarbons into different classes. Thus, the normal paraffins are selectively removed by 5 A molecular sieve (Mortimer and Luke, 1967) or by urea adduction, although it is less specific than the former method. Unsaturated hydrocarbons are separated from the saturated fraction by thin layer or column chromatography on silicic acid/AgNOj. 

The classification of literature was carried out with the controlled key-word phrases denoting broader groupings of the host-species concerned such as hexa-host, Hofmann-type, quinol, urea adduct, Werner-type, etc. Crystal structure analysis, spectroscopic technique, and other physico-chemical methods applied were also included in the keyword phrases. In the original literature various terms have been used to express the group of inclusion complexes, and the selection appears to depend on the author s preference. Such confusion or controversy causes severe difficulties to retrieve important information from vast amount of chemical literatures. Therefore, we applied rather loosely controlled groupings as cited above. The citation number in the Chemical Abstracts (Chem. Abstr.) was added to ease the reference of the original items written in various unfamiliar languages. 

Room temperature latency of the nitrogen-containing curatives is typically achieved in one of two ways. One method is to employ a curative which is insoluble, or only marginally soluble, in the adhesive mixture at room temperature, but which becomes soluble at the cure temperature. The classic example of this is dicyandiamide (10). The second method is to employ a curative precursor which is inactive at room temperature, but which converts to the active curative at the cure temperature. Examples include Monuron (11) and the di-urea adduct (12) of toluene diisocyanate and dimethylamine. Full cure in one hour at about 250 F is achieved. These solid curatives are incorporated into the epoxy under high-shear conditions. Efforts to replace the water-soluble dicyandiamide with adipic dihydrazide and solid adducts of ethylenediamine such as HY-940, sold by Ciba-Geigy, have been successful. Most latent curatives typically have some activity at room temperature, which often requires the storage of the uncured adhesive at subambient temperatures. 

A substitution on the acyl chain prevents adduct formation. Thus, it is possible to separate branched or oxidized fatty acids or their methyl esters from the corresponding straightchain compounds on the basis of the formation of urea adducts. This principle is used as a method for preparative-scale enrichment and separation of branched or oxidized acids from a mixture of fatty acids. 

Sulfamation is the formation (245) of a nitrogen sulfur(VI) bond by the reaction of an amine and sulfur trioxide, or one of the many adduct forms of SO. Heating an amine with sulfamic acid is an alternative method. A practical example of sulfamation is the artificial sweetener sodium cyclohexylsulfamate [139-05-9] produced from the reaction of cyclohexylamine and sulfur trioxide (246,247) (see Sweeteners). Sulfamic acid is prepared from urea and oleum (248). Whereas sulfamation is not gready used commercially, sulfamic acid has various appHcations (see SuLFAMiC ACID AND SULFAMATES) (249—253). 

In addition to modification of the catalyst, several variants of the Biginelli reaction have emerged as viable alternatives however, each method requires pre-formation of intermediates that are normally formed in the one-pot Biginelli reaction. First, Atwal and coworkers reported the reaction between aldol adducts 39 with urea 40a or thiourea 40b in the presence of sodium bicarbonate in dimethylformamide at 70°C to give 1,4-dihydropyrimidines 41. DHPM 42 was then produced by deprotection of 41. 

For the preparation of [l]benzothieno[3,2- ][l]benzofuran-10,10-dioxide 64, hydrogen peroxide in acetic acid, 3-chloroperoxybenzoic acid, and also the urea-hydrogen peroxide adduct with phthalic anhydride were employed. All three methods provided 64 in excellent yields (88-94%). Monitoring the course of the reaction showed the... 

The method of adduct formation with urea is now being investigated (5, 68, 90)... 

In addition, it is occasionally possible to use special methods, such as that of separating large rt-paraffins through the formation of adducts with urea (65). 

One type of chemical approach to the analysis of liquid and solid hydrocarbons that will probably see considerable development is that involving reaction or complex formation to yield precipitates that can be separated from the unreacted mass and subsequently be treated to regenerate the hydrocarbons or class of hydrocarbons so precipitated. This field is certainly not extensively developed. In fact very few examples come to mind but among these are Gair s (21) determination of naphthalene by precipitation with picric acid determination of benzene by Pritzker and Jungkunz (52) by an aqueous solution of specially prepared nickel ammonium cyanide Bond s (8) nitrous acid method for styrene and more recently the determination of normal alkanes in hydrocarbons of more than 15 carbon atoms by adduct formation with urea as described by Zimmerschied et al. (71). 

Polyacrylonitrile, as another example, displays well separated nitrile and methine carbon signals for isotactic, syndiotactic and atactic sequences [523]. 13C NMR analysis of polyacrylonitrile samples obtained by different methods of polymerization clearly indicates the isotactic sequence to predominate in the material obtained by y irradiation of the acrylonitrile-urea channel adduct [523],... 

Many noticeable examples of chiral Lewis base catalyzed allylation of carbonyl compounds have also appeared. Iseki and coworkers published a full paper on enantioselective addition of allyl- and crotyltrichlorosilanes to aliphatic aldehydes catalyzed by a chiral formamide 28 in the presence of HMPA as an additive [41]. This method was further applied to asymmetric allenylation of aliphatic aldehydes with propargyltrichlorosilane [40]. Nakajima and Hashi-moto have demonstrated the effectiveness of (S)-3,3 -dimethyl-2,2 -biquinoline N,AT-dioxide (29) as a chiral Lewis base catalyst for the allylation of aldehydes [42]. In the reaction of (fs)-enriched crotyltrichlorosilane (54 , E Z=97 3) with benzaldehyde (48), y-allylated anfi-homoallylic alcohol 55 was obtained exclusively with high ee while the corresponding syn-adduct was formed from its Z isomer 54Z (fs Z= 1 99) (Scheme 6). Catalytic amounts of chiral urea 30 also promote the asymmetric reaction in the presence of a silver(I) salt, although the enantioselectivity is low [43]. 

The method in Figure 17 often gives a product which contains traces of the corresponding (E,E)-diene. This isomer can be selectively removed from the (E., Z)-isomer, as described above, by formation of its Diels-Alder adduct with excess tetracyano-ethylene in tetrahydrofuran followed by chromatography on silica gel (cf. 17,18). Alternatively, the (E.,E)-isomer can be removed in many cases by the selective formation of its crystalline urea inclusion complex in methanol (cf. 13). 

The best known and most widely used diazoalkane is diazomethane (95 equation 39). Preparative methods for diazomethane involve, in general, the nitrosation of a methylamine derivative (93), followed by cleavage under alkaline conditions. Methylamine derivatives used have included the urethanes, ureas,carboxamides, sulfonamides, guanidines and even the methylamine adducts of unsaturated ketones and sulfones. N-nitroso-N-methyl p-toluenesulfonamide (Diazald, Aldrich) is currently the most commonly used diazomethane precursor. Diazomethane is both toxic and explosive. Although in the past it has been purified by codistillation with ether, it is now usually generated, stored and used as an ether solution without distillation. 

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