Secondaiy bonds

The indimerization of cyclopentadiene again two products are formed, the major being formed by the endo addition. Here also there are secondaiy bonding interactions. 

Few, mostly thallium-oxygen, compounds are mentioned here, because most organothallium derivatives self-assemble via weaker bonds which can be properly regarded as secondaiy bonds in some borderline cases differentiation is difficult. 

Where helical secondaiy structures are represented by the cylinder model, the /i-strand. structures are visualized by the ribbon model (see the ribbons in Figure 2-124c). The broader side of these ribbons is oriented parallel to the peptide bond. Other representations replace the flat ribbons with flat arrows to visualize the sequence of the primary structure. 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

Piimaiy alcohols do not dehydrate as readily as secondaiy or tertiaiy alcohols, and their dehydration does not involve a primaiy caibocation. A proton is lost from the (3 caibon in the same step in which caibon-oxygen bond cleavage occurs. The mechanism is E2. 

Knowledge of the stracture and bonding of molecnles to snrfaces has been obtained from such techniques as LEED, electron energy-loss spectroscopy (EELS), secondaiy-ion mass spectrometry (SIMS), infrared spectroscopy (IRS), Raman spectroscopy, and NMR spectrometiy. The scope of snch studies needs to be greatly expanded to include the effects of coadsorbates, promoters, and poisons. Greater emphasis should be given to developing new photon spectroscopies that would permit observation of adsorbed species in the presence of a gas... 

Abstraction of the secondaiy hydrogen atom is more exothermic than abstraction of the primary hydrogen atom, for the related reasons that (1) secondary C-H bonds are weaker than primary ones and (2) secondary radicals are more stable than primaiy ones. So, we get more 2-chloropropane than l-chloropropane. But in this case, that isn t the only factor involved remember that there are six primaiy hydrogen atoms and only two secondary ones, so the relative reactivity of the primaiy and secondaiy positions is even more different than the simple ratio of products from the reaction suggests. This statistical factor is more evident in the second example we gave above, the chlorination of iso butane. Now the choice is between formation of a tertiaiy radical and formation of a primary one. 

Ammonium cerium(IV) nitrate or cerium(IV) sulfate will catalyze the selective oxidation of secondaiy alcohols with sodium bromate as cooxidant, in this case remote C—C double bonds interfere, but 1,2-diols are not cleaved. It has been found that sodium bromite in aqueous acetic acid will act as a selective oxidant for secondary mary diols without the need for other catalysts (Scheme 21). ... 

Table 2 Relative Reactivity of Teitiaiy, Secondaiy and Piimaiy C—H Bonds in 2-Methylbutane towards Nitrene...

To develop a new substrate satisfying all these requirements, workers in those labs have investigated secondaiy resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development, and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. The work referred to here appeared in 2005. It serves as living proof and indication that LIGA methods are in a constant process of development-improvement as well as in the widest practical use. 

In the presence of sodium methoxide, trifprimaTy alkyl)boranes wacX smomfaly to yield 3 mol equiv. of iodoalkam. In the presence of sodium hydroxide, two C—bcmds of tii(primary aIkyl)boranes are cleaved to the corresponding iodides.In the same conditions tri(secondaiy alkyl)boranes react significantly more slowly and only one C—B bond is broken.For anti-Maikovnikov addition of HI to terminal alkenes the inocess shown in equation (60) is therefore licable. ... 

The first step is protonation of the double bond. If the proton adds to the secondaiy carbon, the product will be different from the one formed if the proton adds to the tertiary carbon. 

An enamine results from the reaction of a ketone or aldehyde with a secondary amine. Recall that a ketone or aldehyde reacts with a primary amine (Section 18-16) to form a carbinolamine, which dehydrates to give the C=N double bond of an imine. But a carbinolamine from a secondaiy amine does not form a C=N double bond because there is no proton on nitrogen to eliminate. A proton is lost from the a carbon, forming the C—C double bond of an enamine. 

Although only a part of the peptide sequence appears to be essential, it should not be supposed that the secondary structure of this protein plays no role in its activity. The fact that papain activity can be destroyed by acid, alkali, or heat, under conditions w hich do not appear to break peptide bonds, indicates that some d ree of secondaiy organization is involved in the catalytic specificity of the enzyme. 

Unfortunately, catalytic asymmetric additions of organic alanes to imines are almost unknown, and admittedly, zinc reagents are better applicable for this purpose. But the characteristics of aluminum organyls reveal their potential the high Lewis acidity should enable activation of the rather imreactive CNl-double bonds and the low Br0nsted basicity should preclude deprotonation and formatiOTi of the respective azaenolates. Because chiral a-tertiary and a-secondaiy amines are ubiquitous structural motifs in natural products and synthetic bioactive compounds, exploration of alane additions is highly relevant. 

In 1996 it was found that condensing carboi lic acids and amines under azeotropic conditions in the presence of substituted benzeneboronic acid derivatives yielded the desired amides. Under these conditions, water was removed and non polar solvents disfavoured salt formation, resulting in amide formation. It was reported that these commercially available catalysts presented a more water and air stable method of catalysing amide bond formation reactions. Secondaiy amines and aromatic substrates also showed reactivity in the presence of catalytic trifluorobenzeneboronic acid. However, less nucleophilic amines such as aniline required an increase in temperature with a consequent change in reaction solvent to mesitylene. 

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