Lateral attack

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. 

Machining damages the surface, destroys coatings and gelcoats if they exist and, to avoid the risks of later attack, it is sometimes necessary to (gel)coat after machining. 

Aqueous cleaning, in which spraying of aqueous acidic or alkaline detergent solutions is followed by normal and deionized water rinses. Strong detergents dissolve greases, oils and possibly oxides. It is necessary to rinse thoroughly to avoid later attacks. 

This sensitivity toward lateral attacks explains the four times shorter duration of action of sila-meprobamate compared to its carbon isostere on a model of tranquiUizing activity in mice (rotarod test, potentiation of hexobarbital-induced sleep, and intraperitoneal injection). " On the other hand, when given orally, sila-meprobamate is practically inactive. One of the first metabolites formed has been characterized as being a di-siloxane (Figure 15.67). For the two phenyl-trimethylsilyl-derived AChE inhibitors, the rather positively charged trimethyl-silyl group mimics the trimethyl-ammonium function present in acetylcholine. Eor these compound metabolic oxidation does not take place on the silicon, but on one of methyl groups (S1-CH3 Si—CH2—OH). ... 

FIGURE 15.69 Due to the presence of a vacant d orbital, a lateral attack can substitute for dorsal attacks in organo-silicon derivatives. ... 

It is envisioned that the defensive systems of plants include preformed physical and chemical barriers as well as inducible defenses that also can be either physical or chemical. An example of an inducible defense is the synthesis of specific antimicrobial compounds such as phytoalexins or phytoanticipins. However, in other instances plants react to pests by developing long-lasting, broad-spectrum systemic resistance to later attacks by pathogens. This type of resistance is referred to as systemic acquired resistance, and was discovered in tobacco by Ross (1961). This form of resistance is almost universally... 

A second possible mechanism of sensitization involves the induced decomposition of hydroperoxides caused by the direct photolysis of groups such as peroxides and ketones to give radicals which later attack hydroperoxide groups to produce peroxy radicals. 

PCya do not react at all and the smaller but less basic P(OMe)3 reacts at about the same rate as the larger and much more basic PMc3. Lateral attack by the nucleophile at one metal atom, coupled with 77 — 77 slippage, is proposed. 

Because the attack atYpres was for all concerned a new form of battle, even experimental in nature, the Germans were not prepared. Their reserves of manpower were low, and they therefore were not prepared to take advantage of their success. Furthermore, by not pressing its hand at this first attack, Germany lost the element of surprise. In later attacks, both sides took primitive— but somewhat effective— protection measures, making chlorine less and less effective throughout the remaining years ofWorld War I. 

In later attacks, modifications were made to the tanks in order to solve some of these problems, and chlorine was mixed with other irritants in the hope that enemy soldiers could be forced to take off their gas masks. Eventually, chlorine, as an armament, quickly fell out of favor as a weapon with both sides in the conflict. But it had done its damage. 

FIGURE 15. The hatched areas between Yc,min Yc,max proportional to the reaction cross section at the indicated position of E witB respect to v. (a) lateral attacks lead to products (b) frontal attacks lead to products. 

Figure 33. Schematic representation of iron atoms superficially blocked by adsorbed anions removable by lateral attack (left) or by undermining effect (right).

By comparing the rate of anion removal by electrochemical lateral attack, V3 = taken from Ref. 50 as equal to - a) x... 

Figure 39. Anodic polarization curves for metal dissolution in one step concomitantly with the lateral attack of laterally vulnerable, superficial blocked atoms, calculated for particular values of rate constants, fitting the oscillatory behavior of the Tafel slope for iron dissolution in chloride electrolytes. (From Ref. 180.)...

M ARCI ANUS, perhaps a Catholic priest at Urga, near Hippo Regius. As a Donatist priest he converted to the Catholic church prior to the Edict of Unity in 405. He was later attacked by Donatist clergy and by Circumcellions some time prior to June 410 (cf. Letter 105.3). 

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