Dimeric-Lewis

Spherical, pentagonal dodecahedrane is the thermodynamically most stable CjoHjo-polycycloalkane. It is the so-called CjjHjo stabilomer . It should therefore be available by thermod5mamically controlled, e.g. acid-catalyzed, isomerization of less stable C24H20-isomers. Experiments along this line, e.g. treatment of the basketene photo-dimer with Lewis... 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Hydrocarbon resins based on CPD are used heavily in the adhesive and road marking industries derivatives of these resins are used in the production of printing inks. These resins may be produced catalyticaHy using typical carbocationic polymerization techniques, but the large majority of these resins are synthesized under thermal polymerization conditions. The rate constants for the Diels-Alder based dimerization of CPD to DCPD are weU known (49). The abiHty to polymerize without Lewis acid catalysis reduces the amount of aluminous water or other catalyst effluents/emissions that must be addressed from an environmental standpoint. Both thermal and catalyticaHy polymerized DCPD/CPD-based resins contain a high degree of unsaturation. Therefore, many of these resins are hydrogenated for certain appHcations. 

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

Dimerization and polymerization can be initiated by electrophilic attack on an oxirane by a protonated or Lewis acid coordinated oxirane (Section 5.04.3.2.2). 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

Figure 8.7 The N-terminal domains of lambda repressor form dimers, in spite of the absence of the C-terminal domains that are mainly responsible for dimer formation in the intact repressor. The dimers are formed by interactions between a helix 5 from each subunit. The different subunits are colored green and brown, except the helix-turn-hellx motif, which is colored blue and red as in Figure 8.4. (Adapted from C. Pabo and M. Lewis, Nature 298 443-447, 1982.)...

The lac repressor monomer, a chain of 360 amino acids, associates into a functionally active homotetramer. It is the classic member of a large family of bacterial repressors with homologous amino acid sequences. PurR, which functions as the master regulator of purine biosynthesis, is another member of this family. In contrast to the lac repressor, the functional state of PurR is a dimer. The crystal structures of these two members of the Lac I family, in their complexes with DNA fragments, are known. The structure of the tetrameric lac repressor-DNA complex was determined by the group of Mitchell Lewis, University of Pennsylvania, Philadelphia, and the dimeric PurR-DNA complex by the group of Richard Brennan, Oregon Health Sciences University, Portland. 

Figure 8.22 The lac repressor molecule is a V-shaped tetramer in which each arm is a dimer containing a DNA-hinding site. The helix-tum-helix motifs (red) of each dimer bind in two successive major grooves and the hinge helices (purple) bind adjacent to each other in the minor groove between the two major groove binding sites. The four subunits of the tetramer are held together by the four C-terminal helices (yellow) which form a four helix bundle. The bound DNA fragments are bent. (Adapted from M. Lewis et al., Science 271 1247-1254, 1996.)...

Another difference between the data and the calculations shows up at extended reaction times. Freeman and Lewis show a maximum concentration of trimethy-lolphenol at about 50 h reaction time. After this point, the concentration falls dramatically. This is apparently due to formation of condensation products which reach noticeable, but low, levels at about 50-60 h. After the reaction has been allowed to proceed for 1000 h, the level of the condensation product reported is equivalent to that of trimethylolphenol, which has fallen to about one third of its maximum concentration. This accounts reasonably well for the loss in trihydroxymethyl phenol, since two moles are consumed to make one mole of dimer. As the calculation contains no information about the condensation reaction, it predicts that the level of trimethylolphenol plateaus at about 98% yield on phenol. 

In an effort toward indolo[3,2-h]carbazoles based on Lewis acid-assisted dimerization of benzotriazole derivatives, the necessary starting materials 197 were prepared in good yields from 1-propargylbenzotriazole and 2-iodoaniline followed by alkylation. In the ensuing dimerization step, compounds 197 were treated with zinc chloride in refluxing dichlorometane to afford the indolo[3,2-h]carbazoles 199 or the dihydro derivatives 198 (Scheme 27). Under similar conditions 197, where R = = H, did not undergo dimerization, which suggests that the alkyl... 

The next step in the calculations involves consideration of the allylic alcohol-carbe-noid complexes (Fig. 3.28). The simple alkoxide is represented by RT3. Coordination of this zinc alkoxide with any number of other molecules can be envisioned. The complexation of ZnCl2 to the oxygen of the alkoxide yields RT4. Due to the Lewis acidic nature of the zinc atom, dimerization of the zinc alkoxide cannot be ruled out. Hence, a simplified dimeric structure is represented in RTS. The remaining structures, RT6 and RT7 (Fig. 3.29), represent alternative zinc chloride complexes of RT3 differing from RT4. Analysis of the energetics of the cyclopropanation from each of these encounter complexes should yield information regarding the structure of the methylene transfer transition state. 

Free borane (2) exists as gaseous dimer—the diborane BaHg. In addition Lewis acid/Lewis base-complexes, as for example formed in an ethereal solvent, e.g. 4, are commercially available ... 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

The Lewis structures of the oxides of nitrogen are shown in Figure 21.6. Two of these species, NO and NO are paramagnetic, with one impaired electron. When nitrogen dioxide is cooled, it dimerizes the unpaired electrons combine to form a single bond between the two nitrogen atoms ... 

Diene 265, substituted by a bulky silyl ether to prevent cycloaddition before the metathesis process, produced in the presence of catalyst C the undesired furanophane 266 with a (Z) double bond as the sole reaction product in high yield. The same compound was obtained with Schrock s molybdenum catalyst B, while first-generation catalyst A led even under very high dilution only to an isomeric mixture of dimerized products. The (Z)-configured furanophane 266 after desilylation did not, in accordance with earlier observations, produce any TADA product. On the other hand, dienone 267 furnished the desired macrocycle (E)-268, though as minor component in a 2 1 isomeric mixture with (Z)-268. Alcohol 269 derived from E-268 then underwent the projected TADA reaction selectively to produce cycloadduct 270 (70% conversion) in a reversible process after 3 days. The final Lewis acid-mediated conversion to 272 however did not occur, delivering anhydrochatancin 271 instead. 

In the gas phase, BeCl2 forms a dimer by forming chlorine-atom bridges like those in the AICI dimer. Draw the Lewis structure for the BeCl2 dimer and assign formal charges. 

Model experiments with 2,4,4-trimethyl-l-pentene (C8H16, TMP) and H20 / AlBr3/MeBr at —80 °C, ia, with a conventional Lewis acid system which would give AEjjv = —6.6 kcal/mole in isobutylene polymerization, gave exclusively a dimer (C16H32) by proton elimination, ia, by a mechanism which mimics transfer in polymerization ... 

---