Electronic reasons

The reference electrode (RE) is connected to the inverting input of an operational amplifier (for example Texas Instruments TL 074), and the setpoint is applied between ground and the noninverting input of the operational amplifier. For electronic reasons Equation 6.2-1 applies. 

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions for both steric and electronic reasons. Sterically, the presence of only one large substituent bonded to the C=0 carbon in an aldehyde versus two large substituents in a ketone means that a nucleophile is able to approach an aldehyde more readily. Thus, the transition state leadingto the tetrahedral intermediate is less crowded and lower in energy for an aldehyde than for a ketone (Figure 19.3). 

Acylsilanes are most useful synthetic intermediates (1), providing, inter alia, controlled routes to silyl enol ethers. They are relatively unreactive towards nucleophilic reagents for both steric and electronic reasons. 

R=Me the connectivity is TiCp2(/U-S3)(/U-S4)NMe. This different behavior between the SyNH and SyNMe systems cannot be attributed to either recognizable steric or electronic reasons. By contrast, the reaction with RN(/t-S2)2NR (R=Me, n-Oct) in hexane at 20 °C gave not the expected seven-membered ring compound but a six-membered metallacycle, TiCpyl/f-SylNR [36] (Scheme 5). 

The common feature of compounds [5]-[15] is that the electrophoric units are linked by saturated spacers, thus establishing only weak electronic (through-bond or through-space) interaction of the Tt-systems. In contrast, the binaphthyl [16], the biperylenyl [17] and the bianthryl [18] as well as the structurally related homologues [19], [20] and [21] allow for a direct 7r,7r-interaction of the subunits it will be shown, however, that for both steric and electronic reasons the inter-ring conjugation can be weak and thus lead to electronically independent redox groups in a similar fashion as in [5]-[15]. 

Coordinative unsaturation arises from the fact that because of steric and electronic reasons, only a limited number of ligands or nearest neighbors can be within bonding distance of a metal atom or ion. In most transition metal oxides, the oxygen anions in the bulk form closed-packed layers and the metal cations occupy holes among the anions as schematically depicted in Fig. 2.1. In this picture, the oxide ion ligands appear to have saturated the coordination sphere of the bulk cation. 

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

V,/V-Disubstituted thioformamides, R1R2NCH=S, are obtained from primary or secondary amines and dimethylthioformamide at 110°C. Aromatic amines do not react for electronic reasons nor does A-methylcyclohexylamine because of steric hindrance323. Decomposition of carbon disulphide in a high-voltage discharge gives CS, which reacts... 

The trend towards higher linearities breaks down at two instances one involves a rather bulky ligand with a 0-value of 190°, the other one involves hexafluoroisopropyl phosphite that has a very high %-value. Both give rise to unstable rhodium carbonyl complexes either for steric reasons (vide infra) or for electronic reasons. The %-value of hexafluoroisopropyl phosphite is very high indeed and it is thought that this value of 51 means that electronically it is very similar to CO, i.e. a strong electron acceptor. Hence, the propensity of its... 

Aldehydes are generally more reactive than ketones in nucleophiUc addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophUicity of the carbonyl carbon more effectively than in former. 

List and coworkers reasoned that BINOL phosphates (specific Brpnsted acid catalysis) could be suitable catalysts for an asymmetric direct Pictet-Spengler reaction [26], Preliminary experiments revealed that unsubstituted tryptamines do not undergo the desired cyclization. Introduction of two geminal ester groups rendered the substrates more reactive which might be explained by electronic reasons and a Thorpe-Ingold effect. Tryptamines 39 reacted with aldehydes 40 in the presence of phosphoric acid (5)-3o (20 moI%, R = bearing 2,4,6-triisopropyI-... 

Free Radicals The inductive effect of fluorine atoms destabilizes radicals. For electronic reasons, fluorination has an important impact on... 

For the synthesis of bidentate ligands, supramolecular approaches have led to a renaissance in homogeneous catalyst discovery (Chapters 2, 4, 8, 9,10), and in a few cases even monodentate ligands have been modified in a supramolecular fashion (Chapter 8, Section 8.2). Combinations of monodentate ligands can be left to chance and in several instances this has led to successful, new catalysts [96]. Such heterocombinations can form spontaneously for steric or electronic reasons or the reactivity of the combinations can be different such that on certain occasions highly enantioselective catalysts are obtained. There are many ways to synthesize the desired heterocombinations selectively and the ionic modification outlined in Section 10.4 is only one of them since nitration (followed by reduction to amines) and sulfonation are robust methods, the ionic route may prove useful. Hydrogen bonding between different donor-acceptors (Chapters 2 and 8), Lewis add-base interactions (Chapter... 

The six axial bonds are directed upward or downward from the plane of the ring, while the other six equatorial bonds are more within the plane. Conversion of one chair form into another converts all axial bonds into equatorial bonds and vice versa. In monosubstituted cyclohexanes, for electronic reasons, the more stable form is usually the one with the substituent in the equatorial position. If there is more than one substituent, the situation is more complicated since we have to consider more combinations of substituents which may interact. Often the more stable form is the one with more substituents in the equatorial positions. For example, in ct-1,2,3,4,5,6-hexachlorocyclohexane (see above) four chlorines are equatorial (aaeeee), and in the /Tisomer all substituents are equatorial. The structural arrangement of the /3-isomer also greatly inhibits degradation reactions [the steric arrangement of the chlorine atoms is unfavorable for dehydrochlorination (see Chapter 13) or reductive dechlorination see Bachmann et al. 1988]. 

As discussed earlier, if bonding occurs through Al-O-Mo linkages, the Mo sites in that vicinity will most likely be inactive as they are more difficult to reduce to a low-valent state. The sites on the Type II crystal, illustrated in Fig. 21, are much more accessible. Even about 75% of all of the edge sites can be approached by a 4,6-DMDBT molecule in a perpendicular alignment with the alumina surface. Thus, for either geometric or electronic reasons, Type I sites should be expected to have lower activity than Type II sites. 

The relative energies of the two forms of each ring are given in Table VII. The electronic reason for these preferences is clear. The amido N-electron density is tetrahedral-like and interactions with two lithium centers in the same plane as H2N are much less effective (see Section III,A and Fig. 26a). Contast this with the directionality of the imide N (sp2) electron density, which dictates the formation of ring systems with substituent atoms in the same plane. For example, planar (H2C= NLi)2 is preferred to the perpendicular form by 17.0 kcal mol-1 [6-31G (72, 74)] or by 6.8 kcal mol1 [MNDO (108)] (Section II,D Fig. 20). 

Finally it should be mentioned that the uncharged azoles could also coordinate principally via the N-l atoms (bearing a hydrogen or a substituent). This is highly unlikely for steric and electronic reasons, and although it has been proposed by a few investigators,48 this mode has never been proved. 

The choice of silicon as the bridging atom results primarily from the ease with which these compounds can be prepared (vide infra). Nevertheless, certain silicon-containing bridging units have been chosen for electronic reasons for example, it is well known that a disilanyl group qualitatively behaves like an ethylene unit. 

The simple alkylation of an amine with an alkyl halide can occasionally be used if the product is less nucleophilic than the starting material. This may be for electronic reasons glycine 6 can be made by alkylation of ammonia with 5 as it exists mostly as the zwitterion 7 which no longer has a nucleophilic nitrogen. It may be for steric reasons the alkylation of the a-bromoketone 8, mentioned at the end of chapter 7, with the sterically hindered amine 9 gives a good yield of the even more sterically hindered amine 10 and no quaternary salt is formed. If the reaction is a cyclisation (chapter 7) it may also work well. 

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