Pattern of reactivity

Acyl-pyrroles, -furans and -thiophenes in general have a similar pattern of reactivity to benzenoid ketones. Acyl groups in 2,5-disubstituted derivatives are sometimes displaced during the course of electrophilic substitution reactions. iV-Alkyl-2-acylpyrroles are converted by strong anhydrous acid to A-alkyl-3-acylpyrroles. Similar treatment of N-unsubstituted 2- or 3-acyIpyrroles yields an equilibrium mixture of 2- and 3-acylpyrroles pyrrolecarbaldehydes also afford isomeric mixtures 81JOC839). The probable mechanism of these rearrangements is shown in Scheme 65. A similar mechanism has been proposed for the isomerization of acetylindoles. 

The pattern of reactivity is similar to that discussed for the azolinones in Sections 4.02.1.1.4 and 4.02.3.7.1. A difference is the greater nucleophilicity of sulfur, and thus more reaction of the ambident anion with electrophiles occurs at sulfur. 

The rates of addition to the unsubstituted terminus of monosubstituted and 1,1-disubstiluted olefins (this includes most polymerizable monomers) are thought to be determined largely by polar Factors.2 16 Polymer chemists were amongst the first to realize that polar factors were an important influence in determining the rate of addition. Such factors can account for the well-known tendency for monomer alternation in many radical copolymerizations and provide the basis for the Q-e, the Patterns of Reactivity, and many other schemes for estimating monomer reactivity ratios (Section 7.3.4). 

Many researchers have applied similar approaches to develop or apply linear free energy relationships, when the substituent is directly attached to the double bond, with some success. Two of the more notable examples can be found in the Patterns of Reactivity Scheme (Section 7.3.4) and the works of Giese and coworkers.16 19... 

Various empirical schemes have also been proposed as predictive tools with respect to the outcome of radical addition reactions.9193 Two-parameter schemes, including the Q-e scheme (Section 7.3.4.1), Patterns of Reactivity (Section 7.3.4.2)... 

The basic Hammett scheme often does not offer a perfect correlation and a number of variants on this scheme have been proposed to better explain reactivities in radical reactions.-0 However, none of these has achieved widespread acceptance. It should also be noted that linear free energy relationships are the basis of the Q-e and Patterns of Reactivity schemes for understanding reactivities of propagating species in chain transfer and copolymerization. 

Chain transfer is kinetically equivalent to copolymerization. The Q-e and Patterns of Reactivity schemes used to predict reactivity ratios in copolymerization (Section 7.3.4) can also be used to predict reactivities (chain transfer constants) in chain transfer and the same limitations apply. Tabulations of the appropriate parameters can be found in the Polymer Handbook 3 ... 

The most popular methods are the Q-e (Section 7.3.4.1) and Patterns of Reactivity schemes (Section 7.3.4.2). Both methods may also be used to predict transfer constants (Section 6.2.1). For furtherdiscussionontheapplicationofthe.se and other methods to predict rate constants in radical reactions, see Section 2.3.7. 

Patterns. of Reactivity scheme 365 6 Q-c scheme 363-5 radical-radical term inaiirm... 

PAM sec polyacrylamide PAM see polyacrylonitrile palladium complexes, as catalysts for ATRP 492 Patterns of Reactivity scheme 11,26, 31 for prediction of reactivity ratios 365-6 lor prediction of iransfer constants 287 PB see polybutadieue PE see polyethylene... 

If there is a series of related reactants A each of which competes with A2 for B, the rate constant for each member can be evaluated analogously. If k2 is known, all of the kj s can be determined. If k2 is unknown, then the experiments yield only the ratios kjk2. Assigning (relative) = 1, the full pattern of reactivity of the A s toward B can be determined quantitatively. 

Because the breadth of chemical behavior can be bewildering in its complexity, chemists search for general ways to organize chemical reactivity patterns. Two familiar patterns are Br< )nsted acid-base (proton transfer) and oxidation-reduction (electron transfer) reactions. A related pattern of reactivity can be viewed as the donation of a pair of electrons to form a new bond. One example is the reaction between gaseous ammonia and trimethyl boron, in which the ammonia molecule uses its nonbonding pair of electrons to form a bond between nitrogen and boron ... 

A similar pattern of reactivity has been demonstrated for the anions formed by deprotonation of S-alkyl-A-p-tohienesulfoximines (see Entry 14 in Scheme 2.21).287... 

Hydrogenolysis of halides and benzylic groups presumably involves intermediates formed by oxidative addition to the active metal catalyst to generate intermediates similar to those involved in hydrogenation. The hydrogenolysis is completed by reductive elimination.58 Many other examples of this pattern of reactivity are discussed in Chapter 8. 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

Predicting the outcome of the reactions between LE chalcogen-donor molecules and di- and inter-halogens is still a challenge, and attempts have been made in this direction by judging whether all the different products could be formed from a common intermediate species following a general pattern of reactivity. In particular, S. Husebye proposed that this species was the [LE-X]4... 

While palladium, ruthenium, and rhodium are the most common metal catalysts used to facilitate Alder-ene cyclization, a few successful examples of catalysis using different metals have been published. Both of the references reviewed in this section demonstrate chemistry that is novel and complimentary to the patterns of reactivity exhibited by late transition metals in the Alder-ene cyclization. 

Lucas, J.S., et al., Kiwi fruit is a significant allergen and is associated with differing patterns of reactivity in children and adults, Clin. Exp. Allergy, 34, 1115, 2004. 

In order to recognize the pattern of reactivity of alcohols on modified nickel surfaces, it is essential to know the reaction pathways exhibited by less reactive surfaces. Initially the dehydrogenation of CH3OH was studied on copper (4 ) and silver (5 ) single crystal surfaces. On Cu(110), following the preadsorption of submonolayer quantities of atomic oxygen, methanol reacted via the following sequence (4,6) ... 

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