Adductive crystallization, described

Several possible commercial apphcations of adductive crystallization have been reported. For example, the system m-cresol-p-cresol forms two eutectics over the complete range of composition, and the separation of the pure components cannot be made by conventional crystallization. Savitt and Othmer (1952) described the separation of mixtures of these two components by the use of benzidine to form a sohd addition compound with p-cresol. Actually both m-cresol and p-cresol form addition compounds with benzidine, but the meta-compound melts at a lower temperature than the para-. If the process is carried out at an elevated temperature the formation of the meta- complex can be avoided. The method consists of adding benzidine to the meta-para- mixture at 110 °C. The /7-cresol benzidine crystallizes out, leaving the m-cresol in solution. The crystals are filtered off, washed with benzene to remove w-cresol, and the washed cake is distilled under reduced pressure to yield a 98 per cent pure i-cresol. 

As described above, adductive crystallization is one method for separating a binary eutectic mixture into its component parts. Another possibility is to alter the solid-liquid phase relationships by introducing a third component, usually a liquid called the solvent. This process is known by the name extractive crystallization . 

Compound VIII crystallizes in a monoclinic system the space group is C2/c with eight molecules per unit (Fig. 6.4). The distance between Ti(l) and Ct(l) measures 2.0340 A, which is considerably longer than the Ti-Ct distance in the analogue cyclopentadienyl titanium complex CpTiCl2(THF) (2.014 A) (Table 6.2), which, to our best knowledge, is the only other example of a monomeric Cp-type titanium(III) chloride compound that could be isolated [32,33]. Bis THF adducts, as described for CpTiCl2(THF)2, are not found in the case of VIII [32],... 

Chloral forms well-crystallized adducts (126) with diaziridines containing at least one NH group (B-67MI50800). Carbonyl addition products to formaldehyde or cyclohexanone were also described. Mixtures of aldehydes and ammonia react with unsubstituted diaziridines with formation of a triazolidine ring (128). Fused diaziridines like (128) are always obtained in ring synthesis of diaziridines (127) from aldehyde, ammonia and chloramine. The existence of three stereoisomers of compounds (128) was demonstrated (76JOC3221). Diaziridines form Mannich bases with morpholine and formaldehyde (64JMC626), e.g. (129). 

Both theoretical and experimental data (in the solid, liquid, and gas phases) prove that the tendency of halocarbons to work as XB donors decreases in the order I > Br > Cl [66-68]. Clearly, polarizability and not electronegativity plays a key role. 3-Halo-cyanoacetylene works as self-complementary module and the N X distance is beautifully consistent with the scale reported above, being 2.932, 2.978 and 2.984 A in the iodo, bromo and chloro derivatives, respectively [69,70]. The same trend is observed when a phenyl, rather than a triple bond, spaces the donor and acceptor sites. The N Br distance in 4-bromobenzonitrile is longer than in the 4-iodo derivative [71,72] and no XB is present in the chloro and fluoro analogues, wherein molecules are pinned by N H and X- H short contacts [73]. PFCs have a very poor tendency, if any, to work as XB donors [74-77] and no crystal engineering can be based on such tectons. However, F2 is a quite strong XB donor and several adducts have been described in the gas phase [11,18] (see also the chapter by Legon in this volume). 

A stannaketenimine has been synthesized by the low-temperature reaction of a diarylstannylene with mesityl isocyanide.87 The stannaketenimine is thermally stable, crystals being obtained by sublimation at 40°C/0.01 mmHg. In this case, the stannylene acts as the Lewis acid for formation of the adduct and, along with the stannenes described above, covers the full range of bonding modes available to the stannenes. 

The mechanism of carcinogenesis by PAHs is believed to involve alkylation of an informational macromolecule in a critical, but at present unknown, manner. Such an interaction with a protein has been modelled by alkylation of a peptide this showed a conformational change occurred on alkylation. It has not yet been possible to study the structure of DNA alkylated by an activated carcinogen this is because DNA is a fiber and the structural order in it is not sufficient for a crystal structure determination. However the crystal structures of some alkylated portions of nucleic acids are described, particularly some nucleosides alkylated by chloromethyl derivatives of DMBA. In crystals of these alkylation products the PAH portion of the adduct shows a tendency to lie between the bases of other nucleoside... 

In 1955, cyclophane derivatives of type (261) (with n = 3 or 4) were described (Stetter Roos, 1955). After recrystallization from benzene or dioxan, these species crystallized as the 1 1 adducts of these solvents. Once such adducts are formed, it is difficult to remove the solvent and it was concluded that the solvent molecules are housed inside the cavities of the respective guests. In accord with this, the smaller ring system (261 ... 

The B-N distances in as-described borane-amine adducts were found to be shorter in the solid state (156.4 161.6pm) than in the gas phase (167.2 163.7pm). Moreover, it was found that the B-N distance of such adducts in solution is strongly affected by the polarity of the solvent the more polar the solvent the shorter the B-N bond. Both findings indicate the strong influence of a dipolar field, either within a crystal or in polar solvents, on dative B-N bond distances. 

A case of a thermally induced phase change involving ring inversion was recently described by Kaftory (31). He found that a crystal of the exo isomer of the adduct, 7a, of ll-cyano-l,6-methano[10]annulene with 4-methyl-1,2,4-triazoline-3,5-dione is transformed to the endo isomer 7b on heating to 175°C. The process involves nucleation and growth of the product phase, but maintains... 

In their pioneering work. Just and co-workers have described many interesting transformations of the Diels-Alder adducts of furan to methyl nitroacrylate (77 + 77 ) and to dimethyl acetylenedicarboxylate (53). The mixture of racemic adducts 77 + 77 was hydroxylated into the exo-cis-diols 125 + 125 , separable by crystallization. Treatment of the isopropylidene acetal obtained from 125 with diazabicyclo[5.4.0]undec-5-ene (DBU) gave a high yield of alkene 126. Ozonolysis followed by a reductive work-up with dimethylsulfide, then with NaBH4, gave a mixture of epimeric triols 127. Cleavage with sodium periodate afforded 2,5-anhydro-3,4-0-isopropylidene-DL-allose (128) in 15 % yield, based on methyl 2-nitroacrylate used. TTie same allose derivative was obtained from adduct 53. ... 

Since CSj is by far the best solvent for S rings, a special interaction between the two kinds of molecules has long been suspected. From the temperature dependence of the CS2 solubility in liquid sulfur at 94-158 °C an interaction enthalpy of 27 kJ/mol has been calculated s. S,2 is the only sulfur molecule which crystallizes from CSj as an adduct its crystal structure shows, however, that there is no stronger than van-der-Waals interaction between the two components. The solvent molecules just occupy the empty spaces of the Sj2 layer lattice The conformation of sulfur rings can be described in terms of the so-called motif which is the order of the signs (-1- or —) of the torsional angles around the ring. If 0 stands for t = 0°, then the motifs are ... 

Bis(indenyl)magnesium has a polymeric strucmre in the solid state, the details of which have been discussed in a previous section. When it is recrystaUized from THF, a discrete monomeric bis-THF adduct, indenyl2Mg(THF)2 (128), is obtained . An X-ray crystal-structure determination shows that the magnesium atom has one relatively short bond [2.256(3) A] with C(l) of each of the indenyl groups, but also interactions with the two adjacent carbon atoms at a much longer distance [Mg-C 2.723(3) and 2.738(3)]. This bonding mode was described as intermediate between r and rf. 

The cyclopentadienyl /3-diketiminate 249 and its 4-terf-butylpyridine adduct 250 have been prepared and structurally characterized (equation 19). An X-ray crystal-structure determination of 249 showed that the cyclopentadienyl group is /j -bonded to magnesium. On the basis of the observed bonding parameters of magnesium with the -diketiminate skeleton [Mg-N 2.006(2) and 2.021(2) A, Mg-C 2.729(3) and 2.826(3) A and Mg-C 2.689(3) A] this bonding is described in terms of a jr-interaction. However, in 250 the /3-diketiminate is iV,iV -chelate bonded to magnesium. 

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