Isomerization reactivity

The same cyclopropanone gives a cycloadduct with furans—this must surely be a reaction of the oxyallyl cation and we can conclude that the three isomeric reactive intermediates (allene oxide, cyclopropanone, and oxyallyl cation) are all in equilibrium and give whichever product is appropriate for the conditions. 

As described above, high-resolution reaction time control enables switching product selectivity at will. A product derived from a reactive species that has yet to be isomerized can be obtained by setting a shorter reaction time, or a product derived from an isomerized reactive species can be obtained by setting a longer reaction time. 

Figure Bl.7.10. Tliree mass spectra showing the results of reactive collisions between a projectile ion C H. NH, isomeric butenes. (Taken from Usypchiik L L, Harrison A G and Wang J 1992 Reactive...

Usypchuk L L, Flarrison A G and Wang J 1992 Reactive collisions in quadrupole cells. Part I. Reaction of [CFl3NFl2] with the isomeric butenes and pentenes Org. Mass Spectrom. 27 777-82... 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

The suggestion outlined above about the way in which through-conjugation influences the nitration of p-chloronitrobenzene is relevant to the observed reactivities (ortho > meta > para) of the isomeric chloronitrobenzenes. Application of the additivity principle to the... 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

Anilines react with ct-haloacetophenones to give 2-arylindoles. In a typical procedure an W-phenacylaniline is heated with a tw o-fold excess of the aniline hydrobromide to 200-250°C[1]. The mechanism of the reaction was the subject of considerable investigation in the 1940s[2]. A crucial aspect of the reaction seems to be the formation of an imine of the acetophenone which can isomerize to an aldimine intermediate. This intermediate apparently undergoes cyclization more rapidly (path bl -> b2) than its precursor (Scheme 7.3). Only with very reactive rings, e.g, 3,5-dimethoxyaniline, has the alternative cydiz-ation (path al a2) to a 3-arylindole been observed and then only under modified reaction conditions[3],... 

Imino-4-thiazolines are far more basic than their isomeric 2-aminothiazoles (see Table VI-1). They react with most electrophDic centers through the exocyclic nitrogen and are easily acylated (37, 477, 706) and sulfonated (652). The reaction of 2-imino-3-methyi-4-thiazoline (378) with a-chloracetic anhydride yields 379 (Scheme 217) (707). This exclusive reactivity of the exocyclic nitrogen precludes the direct synthesis of endocyclic quaternary salts of 2-imino-4-thiazolines. although this class of compounds was prepared recently according to Scheme 218 (493). 

Nucleophilic reactivity of the sulfur atom has received most attention. When neutral or very acidic medium is used, the nucleophilic reactivity occurs through the exocyclic sulfur atom. Kinetic studies (110) measure this nucleophilicity- towards methyl iodide for various 3-methyl-A-4-thiazoline-2-thiones. Rate constants are 200 times greater for these compounds than for the isomeric 2-(methylthio)thiazole. Thus 3-(2-pyridyl)-A-4-thiazoline-2-thione reacts at sulfur with methyl iodide (111). Methyl substitution on the ring doubles the rate constant. This high reactivity at sulfur means that, even when an amino (112, 113) or imino group (114) occupies the 5-position of the ring, alkylation takes place on sulfiu. For the same reason, 2-acetonyi derivatives are sometimes observed as by-products in the heterocyclization reaction of dithiocarba-mates with a-haloketones (115, 116). 

A more quantitative approach to the influence of the thiazole ring on the reactivity of a lateral functional chain was made in a recent study by Noyce and Fike (383), already discussed in Section 10.4. The first-order rates of solvolysis for three isomeric 1-thiazolylethyl chlorides were determined in 80% ethanol. The order of relative reactivity observed. 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

Extraction of C-8 Aromatics. The Japan Gas Chemical Co. developed an extraction process for the separation of -xylene [106-42-3] from its isomers using HF—BF as an extraction solvent and isomerization catalyst (235). The highly reactive solvent imposes its own restrictions but this approach is claimed to be economically superior to mote conventional separation processes (see Xylenes and ethylbenzene). 

Thiothionyl Fluoride and Difluorodisulfane. Thiothionyl fluoride [1686-09-9] S=SF2, and difluorodisulfane [13709-35-8] FSSF, are isomeric compounds which may be prepared as a mixture by the action of various metal fluorides on sulfur vapor or S2CI2 vapor. Chemically, the two isomers are very similar and extremely reactive. However, in the absence of catalytic agents and other reactive species, FSSF is stable for days at ordinary temperatures and S=SF2 may be heated to 250°C without significant decomposition (127). Physical properties of the two isomers are given in Table 6. The microwave spectmm of S=SF2 has been reported (130). 

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

Bromination of isoprene using Br2 at —5 ° C in chloroform yields only /n j -l,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with one-third excess of isoprene at —15 ° C to form the 1,2-addition product, 2-chloro-2-methyl-3-butene (60). When an equimolar amount of HCl is used, the principal product is the 1,4-addition product, l-chloro-3-methyl-2-butene (61). The mechanism of addition is essentially all 1,2 with a subsequent isomerization step which is catalyzed by HCl and is responsible for the formation of the 1,4-product (60). The 3,4-product, 3-bromo-2-methyl-1-butene, is obtained by the reaction of isoprene with 50% HBr in the presence of cuprous bromide (59). Isoprene reacts with the reactive halogen of 3-chlorocyclopentene (62). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Bisphosphites such as (7) combine excellent reactivity, straight-chain selectivity, and high resistance to the typical phosphite degradation reactions (29). Further, the corresponding 0x0 catalysts are excellent olefin isomerization catalysts so that high normal-to-branched isomer ratios are obtained even from internal olefins, enabling, in certain instances, the use of inexpensive mixed isomer olefin feedstocks. 

Polyester resins can also be rapidly formed by the reaction of propylene oxide (5) with phthaUc and maleic anhydride. The reaction is initiated with a small fraction of glycol initiator containing a basic catalyst such as lithium carbonate. Molecular weight development is controlled by the concentration of initiator, and the highly exothermic reaction proceeds without the evolution of any condensate water. Although this technique provides many process benefits, the low extent of maleate isomerization achieved during the rapid formation of the polymer limits the reactivity and ultimate performance of these resins. 

Isomerization is faciUtated by esterification at temperatures above 200°C or by using catalysts, such as piperidine and morpholine (6), that are effective in raising isomerization of fumarate to 95% completion. Resins made by using fumaric acid are exclusively fumarate polymers, demonstrate higher reactivity rates with styrene, and lead to a complete cross-linking reaction. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

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