Terminal Substituents

It is also possible to prepare FOSS species with halogen-terminated substituents using hydrosilylation in good-to-excellent yields (see Table 7). For example, Liu and Dare have prepared a wide range of bromo- and chloro-terminated compounds with long alkyl chains using H2PtCl6 or Ft/C as catalysts (Table 7, entries 2-7). Dare et al. have studied the hydrosilylation of a chlorohexyne however, two isomers were obtained which could not be separated (Table 7, entry 8). 

The efficiency of cyclization can also be affected by stereoelectronic factors. For example, there is a significant difference in the efficiency of the cyclization of the Z- and F-isomers of 3. Only the Z-isomer presents an optimal alignment for electronic stabilization.14 These effects of the terminating substituent point to considerable concerted character for the cyclizations. 

ID and 2D 13C NMR were carried out in a series of novel nematogens by Bayle and co-workers to study the effects on the conformation and order due to the addition of lateral and/or terminal substituents.249 251 For example, lateral flexible substituents are found to adopt a mean conformation more or less parallel to the mesogenic core. As a result, lateral chains are less disordered than terminal chains. Nematogens containing polyoxyethylene ether (POE) chain as a lateral crown ether and terminal chain(s) have been... 

The major side reaction associated with the use of mixed anhydrides is aminolysis at the carbonyl of the carbonate moiety (Figure 7.4, path B). The product is a urethane that resembles the desired protected peptide in properties, except that the amino-terminal substituent is not cleaved by the usual deprotecting reagents. Hence, its removal from the target product is not straightforward. The problem is serious when the residues activated are hindered (Val, lie, MeXaa), where the amounts can be as high as 10%. Other residues generate much less, but the reaction cannot be avoided completely, with the possible exception of activated proline (see Section 7.22). This is one reason why mixed anhydrides are not employed for solid-phase synthesis. 

The number of known, isolated and characterized complexes depends strongly on the length of the chain and drastically decreases with the number of carbon atoms in the chain. A great number of vinylidene complexes of many metals, with different terminal substituents R and various co-ligands have been synthesized and the reactivity has been studied extensively. At present, the solid-state structure of more than 230 vinylidene complexes has been determined by X-ray structure analyses. The number of isolated allenylidene complexes is somewhat smaller. 

The LUMO in d pentatetraenylidene complexes is predominantly localized on the odd carbon atoms and to a lesser extent on the metal. The coefficients on Cl and C3 are very similar, independent of the metal-ligand fragment and the terminal substituent. The coefficient at C5 is somewhat larger. In square-planar d rhodium and iridium complexes the coefficient at the metal is comparable to that on C5 and is larger than those on Cl and C3. Thus, a nucleophilic attack at the metal of d complexes has also to be taken into account. 

Alkylidenemalonates were found to be excellent acceptor molecules (111). Reactions of lithium ylides with dimethyl alkylidenemalonates at —78 °C in THF in the presence of f-BuOH were diastereoselective for all the substituents R except methyl, producing Michael adducts as single diastereomers (Scheme 11.24). The only exception was dimethyl ethylidenemalonate, which produces an 86 14 mixture of diastereomeric adducts, the minor diastereomer being syn-adduct. Since dimethyl alkylidenemalonates bear two geminal methoxycarbonyl moieties, one is cis to the terminal substituent R and the other trans, so of these ester substituents can participate in chelate formation in the transition state. When the terminal substituent R is small, there is a chance for the syn-adduct to be produced, which... 

The stereochemistry of 1,3-dipolar cycloadditions of azomethine ylides with alkenes is more complex. In this reaction, up to four new chiral centers can be formed and up to eight different diastereomers may be obtained (Scheme 12.4). There are three different types of diastereoselectivity to be considered, of which the two are connected. First, the relative geometry of the terminal substituents of the azomethine ylide determine whether the products have 2,5-cis or 2,5-trans conformation. Most frequently the azomethine ylide exists in one preferred configuration or it shifts between two different forms. The addition process can proceed in either an endo or an exo fashion, but the possible ( ,Z) interconversion of the azomethine ylide confuses these terms to some extent. The endo-isomers obtained from the ( , )-azomethine ylide are identical to the exo-isomers obtained from the (Z,Z)-isomer. Finally, the azomethine ylide can add to either face of the alkene, which is described as diastereofacial selectivity if one or both of the substrates are chiral or as enantioselectivity if the substrates are achiral. 

The notion that silyl groups and alkyl groups belong to different categories in the polarity sense is now established. The cyclization of an iminium salt by alkyne participation is dramatically controlled by the terminal substituent of the acetylene in... 

In complexes (16) and (26), the situation is rather different, because there is now steric hindrance from the diene terminal substituent. As a result, the electronically less favorable products are predominant. However, when steric effects at both methylene groups are balanced, as in complex (19), the hydride abstraction proceeds with electronic control to give (30 equation 13). 

Scheme 29 Diaza crown ethers 51-53 of different sizes and with different terminal substituents...

Thus, the CESD with anionic terminal substituent are able of anionic capping complex formation during the irradiation and thus to achieve photochemical control over binding of metal cations. 

A unifying hypothesis for the observed organic chemistry was advanced by Huisgen [132], who suggested that all tetramethylenes lie on a continuous scale between zwitterionic and diradical structures and may be regarded as resonance hybrids of the two extreme forms. The predominant nature of the tetramethylene intermediate is determined by the terminal substituents, and the termini can interact with each other by through-bond interaction [132, 134]. 

The tetramethylene is a resonance hybrid of 1,4-diradical (, = y in Eq. (22)) and zwitterionic (, = +,—) limiting structures. The character of the tetramethylene is determined by the nature of the terminal substituents. A very strong donor substituent at one of the terminal carbons and a very strong acceptor substituent at the other leads to zwitterionic intermediates. Otherwise, for instance, phenyl or vinyl group at the donor terminal and diester, cyano-ester or anhydride at the acceptor terminal, will favor the diradical form. 

Bicyclic a-pyrans and -pyrones. The Ni(0)-catalyzed reaction of an aldehyde with a terminally dialkyl-substituted 1,7-octadiyne results in bicyclic a-pyrans. This reaction is highly dependent on the length of the chain connecting the triple bonds and on the presence of the terminal substituents. 

Even alkene starting materials without a terminal substituent may rearrange stereospecifically, and 169 gives products 171 with 60-96% ee.122 The same transition state model 170 reliably predicts the sense of the chirality transfer. 

Asymmetrical diynes, including diynes possessing one sterically demanding substituent, did undergo clean conversion to pyrones [22], As shown in Table 2, when the steric difference between the two terminal substituents on the diyne is small (e.g., Me vs. Et), a nearly equal mixture of two py-... 

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