Solvent cyclic ureas

Kim and Varma have described the preparation of a range of cyclic ureas from diamines and urea [366]. In the example highlighted in Scheme 6.203, ethylenedi-amine and urea were condensed in the presence of 7.3 mol% of zinc(II) oxide in N,N-dimethylformamide as solvent at 120 °C to furnish imidazolidin-2-one in 95% isolated yield. Key to the success of this method is that the reaction needs to be performed under reduced pressure in order to remove the ammonia formed from the reaction mixture. This method was extended to a variety of diamines and amino alcohols [366]. 

Recently, several ILs, in the presence of alkali metal compounds as promoters and ethanol as solvent, have been investigated as catalysts [70c]. The best catalytic activity was exhibited by the system B M ImBr/K2C03. However, under the working conditions (423 K 6h C02 = 6-10MPa IL = 19mol%), the carbamate yields and selectivities were modest because of the side-formation of cyclic ureas, and oligomeric byproducts. 

Dipolar aprotic solvent. This cyclic urea can serve as a substitute for the carcinogenic hexamethylphosphoric triamide (HMPT) in reactions of highly nucleophilic and basic reagents. It mimics the effect of HMPT in Wittig olefination und in selective generation of various enolates. It forms homogeneous solutions with I IIF even at -78°. ... 

In contrast, dipolar aprotic solvents possess large relative permittivities (sr > 15), sizeable dipole moments p > 8.3 10 ° Cm = 2.5 D), and average C.f values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C—H bonds are not sufficiently polarized. However, they are usually good EPD solvents and hence cation sol-vators due to the presence of lone electron pairs. Among the most important dipolar aprotic solvents are acetone, acetonitrile [75], benzonitrile, A,A-dimethylacetamide [76, 77], A,A-dimethylformamide [76-78], dimethylsulfone [79], dimethyl sulfoxide [80-84], hex-amethylphosphoric triamide [85], 1-methylpyrrolidin-2-one [86], nitrobenzene, nitro-methane [87], cyclic carbonates such as propylene carbonate (4-methyl-l,3-dioxol-2-one) [88], sulfolane (tetrahydrothiophene-1,1-dioxide) [89, 90, 90a], 1,1,3,3-tetramethylurea [91, 91a] and tetrasubstituted cyclic ureas such as 3,4,5,6-tetrahydro-l,3-dimethyl-pyr-imidin-2-(l//)-one (dimethyl propylene urea, DMPU) [133]. The latter is a suitable substitute for the carcinogenic hexamethylphosphoric triamide cf. Table A-14) [134]. 

The required novel bifunctional monomers are synthesized by reacting cyclic ureas with aliphatic or aromatic dicarboxylic acid chlorides in the presence of triethylamine as hydrogen chloride scavenger. This reaction is best conducted in an inert organic solvent using an excess of the cyclic urea to prevent polymer formation (Scheme I). 

The utility of the bis cyclic ureas for curing of coatings was demonstrated by dispersing them in a functional acrylic polymer formulated for powder coating application, or by using them as additive in solvent based coatings or in aqueous polymer emulsions for electrocoating. 

To achieve good compatibility with functionalized acrylic and epoxy resins a bis cyclic urea with n=3> and R=-(CH2)7-(see Scheme I) was synthesized. Acrylic- and epoxypolymer solutions were prepared using 15% by weight of the bis cyclic urea (based on the dry polymer) and methyl ethyl ketone as solvent. Films cast from these solutions on steel sheets were clear, and had a thickness of 0.4 mil. 

Further work is in progress to optimize solvent free one component systems based on macroglycols and the novel difunctional cyclic urea monomers. 

Replacement of CCI4 and Other Toxic Organic Solvents Substitution of toxic solvents with safer ones is, of course, a desirable step, but impediments to its implementation sometimes include the reaction requirements themselves. Several potential solvent substitutes exist, however, which are compatible with minimal side-product formation and good product yield. These include supercritical CO2, 1,3-dioxolane, and DMPU, a cyclic urea. 

The cyclic urea known as DMPU (l,3-Dimethyl-3,4,5,6-tetrahydro-2(H)-pyrimidone or N,N -dimethylpropyleneurea), has been demonstrated by Seebach and colleagues in Switzerland to closely reproduce the solvent capabilities of HMPA. Moreover, with a structure quite different from HMPA, DMPU was found to be non-mutagenic and non-carcinogenic when tested to determine if it might cause similar genetic and chronic health effects and, therefore, was proposed as a safe replacement. Based on successful substitution of DMPU for HMPA, as shown in the following reaction schemes, DMPU can be recommended as a possible replacement for HMPA wherever possible in large-scale commercial processes. 

The intramolecular cyclization of ureas derived from 3-hydroxy-4-pentenylamines and 4-hydroxy-5-hexenylamines was also investigated (Table 5)3 From these (chiral) substrates two different products can arise because the intermediate Pd-acyl species can either react with the hydroxy group to form a lactone or with the nitrogen function to form a cyclic urea derivative. The selectivity was found to depend both on the substrate and on the acidity of the solvent (MeOH or AcOH). 

Finally, Seebach has used the cyclic urea (69), DMPU, as a co-solvent in double lithiations, oxirane ring-opening, Wittig reactions, Michael additions of lithiated dithianes to cycloalkenones, and the selective generation of enolates." The interesting point here is that DMPU exhibits the same solvent effect as the carcinogen HMPA and might therefore be a safe substitute. 

If the latter reaction proceeds through a closed transition state (e.g., 5 in Scheme 7.2), good diastereocontrol can be expected in the case of trans- and cis-CrotylSiCl3 (2b/2c) [14, 15]. Here, the anh-diastereoisomer 3b should be obtained from trans-crotyl derivative 2b, whereas the syn-isomer 3c should result from the reaction of the cis-isomer 2c (Scheme 7.2). Furthermore, this mechanism creates an opportunity for transferring the chiral information if the Lewis base employed is chiral. Provided that the Lewis base dissociates from the silicon in the intermediate 6 at a sufficient rate, it can act as a catalyst (rather than as a stoichiometric reagent). Typical Lewis bases that promote the allylation reaction are the common dipolar aprotic solvents, such as dimethylformamide (DMF) [8,12], dimethyl sulfoxide (DMSO) [8, 9], and hexamethylphosphoramide (HMPA) [9, 16], in addition to other substances that possess a strongly Lewis basic oxygen, such as various formamides [17] (in a solution or on a solid support [7, 8, 18]), urea derivatives [19], and catecholates [10] (and their chiral modifications [5c], [20]). It should be noted that, upon coordination to a Lewis base, the silicon atom becomes more Lewis acidic (vide infra), which facilitates its coordination to the carbonyl in the cyclic transition state 5. 

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