Group 2, dimers

Fig. 25. Stereoscopic packing illustration of the 1 bromobenzene (1 1) clathrate 82). The guest molecules are shown by enlarged atomic radii (arbitrary values). H-bond rings coming from carboxylic group dimerization are indicated by hatching...

Azolealdehydes with an a-NH group dimerize (Section 3.4.3.4.4). Cyclic N+—N- links can be formed using nitrene intermediates (Section 3.4.3.4.2). 

Some olefins conjugated with electron-withdrawing, unsaturated groups dimerize to cyclobutanes as photoproducts, apparently through their triplet states. For example, when acrylonitrile in acetone is irradiated under con-... 

Simple silenes readily couple to yield the head-to-tail dimers, 1,3-disilacyclobuta-nes1,2,15,157. The dimerization is extremely facile and silenes bearing only small alkyl groups dimerize in an argon matrix even at 40 K, i.e., the dimerization proceeds at a diffusion controlled rate. Bulky substituents slow down the dimerization rate and allow the isolation of stable silenes. The head-to-tail dimerization (equation 98) is the predominant dimerization path for silenes, including those of the Auner-Jones 79,80 and Wiberg type72,73,78. 

Dimerization of functionalized 1,3-dienes, cyclooctadienes. Butadienes carrying an electron-donating group dimerize in the presence of either Ni(COD)2 or the Ni(0) obtained by reduction of Ni(acac>2 with (C2H,) AlOCcH to 1,2-rrani-disubstituted 3,7-cyclooctadienes in 70-90% yield. 

Slightly different behavior was found for reduction of 2- and 4-cyanodiphenyl ether (94b and 94a) in DMF [267]. On CV and DPSC time scales, the radical anions of 94a dimerize irreversibly to the 4,4 - and 2,4 - (with respect to the CN group) dimer dianions in the ratio 1 3. 

A dimer (58) of a-lithiated 2,6-dimethylpyridine crystallizes with TMEDA solvation. This dimer is completely unlike the polymeric benzyllithium (53) in that no T) -intramolecular bonding is observed. The centrd core of the dimer (58) consists of an eight-membeied ring formed from two intermolecular chelated Li atoms and nearly ideal perpendicular conformations of the a-CHaLi groups. Dimer (58) is a relatively rare example of a lithiated ir-system where Li exhibits only one carbon contact. 

From the spectra, it is clear that the dimer with m/z 216 is a norbornene dimer terminated with a vinyl group (dimer-4, 113.36 and 141.91 ppm). The compound with m/z 244 is a norbornene dimer with an ethyl and a vinyl group at the head (14.11 and 22.18 ppm) and the tail (113.72 and 142.87 ppm) of the molecule (di-mer-5), respectively. Assignments have been made based on analysis of a C 2D-INADEQUATE spectrum and are presented in Fig. 4.18. Both dimers are enchained in the 2,3-fashion consistent with the proposal based on the NMR analysis of the norbornene polymer made with naked nickel . 

C-6 The Ag(I) ion of complex [(oetplAgt>< S( 1 3 P (117) i lies in the cavity of oetp, coordinated with two O atoms and two C=C groups. Dimerization is achieved by bridging Ag ions with triflate anions209. 

Ammonia and carbon dioxide result from decarboxylation and deamination following hydrolysis of the amide group. Dimers of cyclopentanone and the cyclic monomer of nylon-6,6, namely,... 

Mention has been made earlier of the dimerization of (289) to an unstable spiro compound. The stability could be increased by the introduction of an ester group. Dimerization of (292) (R = H) (prepared from 2,3-bis-(chloromethyl)thiophene-5-carboxylic ester by treatment with Nal in DMF) gave the spiro compound (293) (54%) (Scheme 54). The spiro ring could be opened by treatment with acid to form the alcohol (294). Reaction of either (293) or (294) with cone. H2SO4 gave the cycloheptene (295), thus proving the structure of the spiro compound <9UOC6948>. 

The lone-pair electrons in a carboxylate group are designated syn or antiy as shown in Fig. 18. The proton on another molecule can approach in either of these directions. In the syn conformation (Z-form) the proton is on the same side of the C-0 bond as the other C-0 bond this is the conformation found when carboxyl groups dimerize by forming two hydrogen bonds. On the other hand, in the anti conformation (E-form) the proton is on the opposite side of the C-0 bond from the other C-0 bond. Ab initio quantum chemical studies of formic acid indicate that the syn (Z) conformation is more stable than the anti (E) conformation by about 4.5 kcal mol" implying that the syn lone pairs are more basic (and therefore bind metal ions more readily) than do the anti lone pairs [55]. Carboxylates in active sites of enzymes generally seem to employ the more basic syn lone pairs for metal chelation [56], and it has been estimated that syn protonation is 10 -fold more favorable than anti protonation (since 1.4 kcal mol corresponds approximately to a tenfold increase in rate). The carboxylate ion is therefore a weaker base when constrained to accept a proton in the anti (E) direction. 

For the NCO excess group consumption many more reactions have been suggested. Also evidenced was the spontaneous isocyanate group dimerization at room temperature [322, 323]. In the hot cure reaction, the principal process, admitted by A. Awater, was the appearance of the allophanate and/or biuret linkages [335]. K.A. 

Lithium methoxide forms a sheet polymer [LiOMe] in which the lithiums are four-coordinated by yu,4-OMe groups but with more sterically hindered groups a hexanuclear species [LiOCMe2Ph]6 (Fig. 4.9) is formed involving 3-coordinated Li with /i3-alkoxo groups. " With excessively bulky alkoxo groups, dimeric molecules [Li(/i-OCBu3)]2 are... 

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