Rearrangement reactions intermolecular

In general the para isomer is the main product but about 5 % ortho isomer was detected6 when the solvent was aqueous acetic acid. All the evidence suggests that the rearrangement is intermolecular involving the reversible formation of free chlorine and the anilide, reaction (2), followed by C-chlorination of the anilide, reaction (3)... 

These reactions have very low activation energies when the intermediate is a free carbene. Intermolecular insertion reactions are inherently nonselective. The course of intramolecular reactions is frequently controlled by the proximity of the reacting groups.113 Carbene intermediates can also be involved in rearrangement reactions. In the sections that follow we also consider a number of rearrangement reactions that probably do not involve carbene intermediates, but lead to transformations that correspond to those of carbenes. 

The most common rearrangement reaction of alkyl carbenes is the shift of hydrogen, generating an alkene. This mode of stabilization predominates to the exclusion of most intermolecular reactions of aliphatic carbenes and often competes with intramolecular insertion reactions. For example, the carbene generated by decomposition of the tosylhydrazone of 2-methylcyclohexanone gives mainly 1- and 3-methylcyclohexene rather than the intramolecular insertion product. 

In intermolecular rearrangement reactions (also referred to as apparent rearrangement reactions ), the migrating group has been completely separated from the parent species and is then attached at a new position in a subsequent step. In intramolecular rearrangements the migrating group is never separated from the parent compound. 

A quantitative solid-state ene addition between two ligands of the platinum complex 397 gives the rearranged platinum complex 398 upon extensive heating to 140 °C [123]. This rearrangement reaction (cf. Sect. 24) is treated here, as it is the only known quantitative solid-state ene addition to date. Further quantitative solid-state [4-1-4] additions and higher vinylogs, as well as ene additions also of the intermolecular type, await detection both as thermal and photochemical reactions. 

Aryl hydroxylamines treated with acids rearrange to aminophenols.255 Although this reaction (known as the Bamberger rearrangement) is similar in appearance to 1-32 to 1-36, the attack on the ring is not electrophilic but nucleophilic. The rearrangement is intermolecular, with the following mechanism ... 

The same authors have also reported the synthesis of 2-amino quinolines46, Scheme 5.28. Secondary amines were reacted with aldehydes to form enamines subsequent addition of 2-azidobenzophenones initially forms triazoline intermediates, which undergo a thermal rearrangement and intermolecular base-catalysed cyclocondensation to produce 2-amino quinolines. The reactions were run at 180°C for 10 min (the time includes 3 min required for enamine formation) to give the products in 57-100% yields. 

Detailed mechanisms of intramolecular rearrangement reactions have been difficult to determine. Classical rate measurements seldom lead to unambiguous mechanistic predictions. Generally only after extensive examination of concentration, solvent, and substituent effects on the reaction rate can a general mechanistic class be proposed for example, intra vs intermolecular or bond rupture of a bidentate chelate vs non-bond rupture twist pathways. Indeed, only two examples of slow complexes are known where detailed rate comparisons for geometrical and optical iso-merizations were made and used to eliminate several mechanisms however, a single most probable pathway was not demonstrated in either case.12,13 Only with DNMR can detailed environmental site interchanges be directly observed and with this in-... 

On the other hand, fluorine substitution at both ortho positions (21a) raises the CASPT2/6-31G barrier from 8.6 to 13.0 kcal mol The increase in the barrier height inhibits the rearrangement reaction and allows 21a to participate in intermolecular chemistry. Subsequent experiments determined that its activation barrier is 7.3-8.0 kcal mol (depending on the solvent). Calculations predict a... 

It seems likely that PVAc containing CH3-C = O groups, with more acidic protons, should be more effective in McLafferty-type rearrangement reactions and intermolecular proton-transfer to PMMA chains, even if the distances between PVAc-PMMA and PMMA-PMMA are comparable, i.e., PMMA and PVAc chains in the coalesced blend are very intimately mixed. Making use of the above proposal permits the approximate reaction scheme shown in Scheme 2 to be written. 

Coupling of a Fischer carbene complex with an alkene can generate a vinylcarbene intermediate 12 via an insertion-rearrangement reaction, which can then further react with a double bond. For intramolecular reactions of tethered enynes 10, the products formed are bicyclic cyclopropanes 14 intermolecular reactions lead to cycloalkenylcyclopropanes. 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to intermolecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester p-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

The carbenes 1 can be trapped by intermolecular [1 + 2] cycloaddition to cyclopentadiene to give 2,357 and furan to give 3 and 5,356-357 but intramolecular reactions always compete. As expected, the furocyclopropanes undergo facile thermal rearrangement reactions, e.g. 3 —> 4 and 5 -> 6. 

The rearrangement reaction of a variety of alkyl phenyl ethers over a dealumi-nated HY zeolite has been shown to involve both intramolecular and intermolecular processes to afford phenol, (alkoxyalkyl)benzenes and alkylphenols as the main products. o-Benzylphenol has been obtained as the exclusive product in the rearrangement of benzyl phenyl ether in the presence of montmorillonite. The mechanism for a novel zeolite /3-catalysed rearrangement of alkoxybenzyl allyl ethers to aldehydes and ketones has been investigated by the use of cross-over reactions and deuterium labelling. The reaction was found to be mainly intramolecular and has been described as a nucleophilic attack of the double bond on the electrophilic benzylic carbon of the ether-Lewis acid complex, followed by a... 

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