Steric compilations

Silyl-derived protective groups are also used to mask the thiol function. A complete compilation is not given here, since silyl derivatives are described in the section on alcohol protection. The formation and cleavage of silyl thioethers proceed analogously to those of simple alcohols. The Si-S bond is weaker than the Si-O bond, and therefore, sulfur derivatives are more susceptible to hydrolysis. For the most part, silyl ethers are rarely used to protect the thiol function, because of their instability. Silyl ethers have been used for in situ protection of the-SH group during amide formation. The use of the sterically demanding and thus more stable triisopropylsilyl thioether may prove worthwhile. ... 

An immense number of phosphine complexes of rhodium have been synthesized and detailed compilations of information are available [3d]. In this section, some case studies are presented to illustrate the variations that arise as a result of changes in the steric requirements of the phosphine. 

In this chapter, the definitions used by Perrin in his book on pA a prediction (which also includes a very convenient compilation of o values) will be used. One must be alert to the importance of the number of hydrogens directly attached to the carbonyl carbon several groups have pointed out that aldehydes and ketones give separate but parallel lines, with formaldehyde displaced by the same amount again. What this means is that given one equilibrium constant for an aldehyde (or ketone) one may estimate the equilibrium constant for other aldehydes (or ketones) from this value and p for the addition using a value from experiment, if available, or estimated if necessary. This assumes that there is no large difference in steric effects between the reference compound and the unknown of interest. 

To illustrate the application of the imf and steric parameters we consider cohesive energies of MeX at 298.15 K taken from the compilation of Majer and Svoboda73. The data set (Table 9) contains no compounds capable of hydrogen bonding. We have therefore used the IMF equation in the form ... 

Similar investigations were carried out using 2-propanol as the organic modifier instead of 1-propanol. The parameters of linear regression analyses are compiled in Table 3.5. It has been concluded from the results that the retention on both stationary phases is of mixed character hydrophilic, electrostatic and steric molecular parameters are equally involved in the retention [83],... 

A single oxo bridge may subtend an angle between 140° and 180°, this angle being determined by steric or electronic factors (Table 3).95 103 Almost all these examples refer to the solid state, but there are also several homo- and hetero-nuclear M—O—M and M—O—M—O—M species known in solution. Often these are intermediates in, or products of, electron transfer reactions with oxide-bridging inner-sphere mechanisms. Examples include V—O—V in V(aq)2+ reduction of VO(aq)2+, and Act—O—Cr in Cr(aq)2+ reduction of UOj+ or PuOj+ a useful and extensive list of such species has been compiled. Tlie most recent examples are another V—O—V unit, this time from VO(aq)2+ and VOJ,105 and an all-actinide species containing neptunium(VI) and uranium-(VI).106 An example of a trinuclear anion of this type, with the metal in two oxidation states, is provided by (31).107... 

Taft s o has nearly the same meaning as Hammett s c (with different numerical values), Es is the steric parameter for substituent X, and 8 is a measure of the sensitivity of the reaction to changes in steric properties near the reaction center (similar to p). Extensive compilations of values of o, o and Es appear in Perrin et al. (1981) and McPhee et al. (1978), respectively. 

To characterize the substituents (the X block) a set common substituent parameters were compiled from the literature. These parameters were as (Taft inductive parameter), trp (Hammett parameter for para substituents), F and R (Swain-Lupton dual substituent parameters), Es and Esc (Taft steric parameters), van der Waals radius, L, Bv B2, B3, (Verloop sterimol parameters), MR (molar refractivity), and n (Hansch lipophilicity parameter). Data are given in Refs. [1,19] and are not reproduced here. 

Table XII.5 is a compilation of some of the data on reactions that involve the attack of atoms on molecules or radicals. In most of the cases the data have been taken directly from the original authors and transformed into the form of the collision equation, when not already in that form. In some cases the rate constant for a reverse process has been calculated from that of the forward reaction and thermodynamic data for the equilibrium. Where that has been done the footnotes will so indicate The steric factors shown in the last columns have been calculated on the basis of an arbitrary collision diameter o- which is indicated in parentheses. It is quite evident that the steric factors so calculated cannot be literally interpreted, since variations within a factor of 3 could equally well have been obtained by using different but equally justifiable cross sections.

Enols sterically protected at the -carbon have also been investigated. In general, the cyclic voltammetric response exhibits only the already mentioned ECE process, while the reversible benzofuran oxidation is not detected. The pertinent electrode potentials are compiled in Table 2. In the case of the enolato species, the ECE process probably proceeds through separated one-electron oxidations as those of Table 1. 

Many other examples of steric effects are compiled in recent equilibrium data (17) j and many of the observed effects are apparently due to enthalpies and entropies of formation of the free ligand in solution. For this reason, more reliable data on heats and entropies of chelate formation and on heats and entropies of formation of metal ions and ligands in solution should be accumulated for developing further understanding of the chelate effect. 

Studies have concluded that the rate of abstraction of the hydrogen atom next to the cyano group will determine the potency of the nitrile (15). Armed with just this information, students can be asked to identify the structural features that influence the rate of hydrogen abstraction. The factors that influence the rate of cyanide release are the same as those familiar to organic chemistry students, namely electronic and steric effects. A compilation of these structural features can be assembled for other classes of molecules and be apllied by the students to other molecules. 

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