Terminal Substitution Effects

Perhaps the terminal functions which have been of most interest are the alkyl CH3(CH2) and alkoxy CH3(CH2) 0 chains. These flexible chains will introduce the disorder that is necessary for the formation of the nematic phase. It is generally found that  

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It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Mechanistically related to Mn, is the use of Fe as an epoxidation catalyst. Recently, iron complexes with a tetradentate amine core were reported, that were capable of activating H202 without the involvement of hydroxyl radicals [72]. For a variety of substituted as well as terminal alkenes, effective epoxidation... 

From a synthetic point of view, the most important observation is that dibutylstannylene acetals and tributylstannyl ethers can be used to effect terminal substitution of diols in excellent yields, often better than can be obtained by direct reaction at low temperatures, even for benzoylation or p-toluenesulfonylation, where direct reaction is reasonably effective. Terminal O-alkylation, which cannot be performed directly, is routine through choice of the appropriate conditions, as outlined in the sections following. These types of reactions are considered first, followed by reactions where the nonterminal oxygen atom is favored. 

Alkylations. The effect of subjoined Lewis acid (e.g., trimethyl borate) on the catalytic ally lation of aldehydes with allylstannanes promoted by a BINOL-Ti complex has been examined. With allenyltributylstannane the products are homopropargyl alcohols.. Allylation in a Sn(II)-mediated Barbier reaction exhibits much lower ee, although jllenylation with (terminally substituted) propargyltributylstannanes shows good results. 4-Trimethylsilylbut-2-ynyl)tributylstannane undergoes destannylative addition to... 

It is also interesting to mention that the compound (3.11) is the first published example that has lateral but no terminal substitutions and yet forms a liquid crystal phase. By and large, the studies on substitution effect have shown that while the terminal substitutions are favorable to the thermal stability and the formation of a liquid crystal phase, the lateral substitutions are not. The larger the lateral substitution, the more harm it can do to the thermal stability of the liquid crystalline phase. A typical and very convincing example for this idea is given by homologues of the well known para-substituted azoxyanisole (3.12) ... 

Table 5 Effect of E1ec N-terminal substitutions on activity and ability to form E1ec-E2ec subcomplex...

Short alkyl branches on the hydrophobic group can have some effect on adsorption efficiency. Carbon atoms on short branches of an alkyl hydrophobe, those located between two hydrophilic groups, or those on the shorter portion of an alkyl chain with the hydrophile not substituted in the terminal position all seem to contribute an effect equal to half that of the same number of carbons in a normal-chain hydrophobe and terminally substituted hydrophile. 

The reaction time between 4-iodopyrazoles and 1-alkynes varies from 5 to 25 h and the yield of products is 55-95%. It is noteworthy that the nature of the terminal acetylene has a greater effect on the rate of halogen atom substitution for low-reactive 4-iodopyrazoles. Thus, the reaction time for ethynylarenes is 5-6 h, and for less acidic aliphatic 1-alkynes is 10-25 h (Table XTT). 

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