Intermolecular reactions Isomerism

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

When HCl salts of arylalkylamines are heated at 200-300°C, migration occurs. This is called the Hofmann-Martius reaction. It is an intermolecular reaction, since crossing is found. For example, methylanilinium bromide gave not only the normal products o- and p-toluidine but also aniline and di- and trimethylanilines." As would be expected for an intermolecular process, there is isomerization when R is primary. 

At the beginning of the new millennium, Hashmi et al. presented a broad research study on both intramolecular and intermolecular nucleophilic addition to alkynes and olefins [18]. One of the areas covered by these authors was the isomerization of co-alkynylfuran to phenols [19]. After that, Echavarren and coworkers identified the involvement of gold-carbene species in this type of process, thus opening a new branch in gold chemistry [20]. And subsequently, Yang and He demonstrated the initial activation of aryl —H bonds in the intermolecular reaction of electron-rich arenes with O-nucleophiles [21, 22]. 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

The additive-modified catalyst mixture developed for intramolecular olefin coupling is sufficiently active to catalyze the analogous intermolecular reaction. A variety of five-membered N-heterocydes can be catalytically alkylated at the C-2 position (Table 2) [8], Additional functional groups on the heterocyde[15] (Table 2, Entry 1) and on the olefin (Table 3) are well tolerated. Products corresponding to linear addition are usually obtained exclusively, even when the olefin is rapidly isomerized under the reaction conditions. 

The transfer of hydrogen to peroxyl radicals may proceed intra- or inter-molecularly. Intramolecular transfer reaction (isomerization) of peroxyl macroradicals of polypropylene occurs during the oxidation of the polymer in a solution of inactive solvent [75] while the intermolecular transfer is preferred during the oxidation in reactive solvent or in the crystalline state [76]. 

Over HBEA zeolites, acetylation of 2-methoxynaphthalene with acetic anhydride leads mainly to l-acetyl-2-methoxynaphthalene. However, the desired product, i.e. 2-acetyl-6-methoxynaphthalene, precursor of Naproxen is obtained at long reaction time by an intermolecular irreversible isomerization process. A very selective production of II (83%) can be obtained by acetylation of 2-methoxynaphthalene over a commercial HBEA zeolite (Si/Al = 15) at 170°C, with nitrobenzene as a solvent. With dealuminated HBEA samples (framework Si/Al ratio between 20 and 40), better results could be expected. Furthermore, preliminary experiments showed that this selective synthesis of 2-methoxynaphthalene can be carried out in a flow reactor system. 

Modifying the aldehyde function in the pyrroles 1162 into 1,3-dipoles 1163 (nitrone) and 1167 (nitrile oxide) furnished tricyclic heterocycles 1164 and 1168 via intramolecular 1,3-dipolar cycloaddition reactions (Scheme 226) <2000T3013>. None of the isomeric bridged product 1165 is produced despite the preference for that regiochem-istry in the intermolecular reaction. Generated in situ (from the oximes 1166) nitrile oxides 1167 cyclized spontaneously to the dihydroisooxazole 1168 in quantitative yield at room temperature. 

The structural isomerization of cyclopropane to propene does not involve radical chain processes if a trimethylene diradical intermediate is involved, it must be of such a short lifetime that it may not be easily trapped through gas-phase intermolecular reactions Yet the trimethylene diradical hypothesis does account for the thermal interconversion of cis and trans 1,2-d2-cyclopropanes in a most plausible manner. Homolytic cleavage of one carbon-carbon bond would form a 1,3-diradical intermediate rotations of terminal methylene groups in this trimethylene diradical followed by reformation of the cyclopropane ring would rationalize the isomerization According to this model, the net outcome of a stereomutation process would be dictated by the relative magnitudes of rate... 

There is evidence strongly supporting the two-step mechanism. For example, the zwitterionic intermediate has been trapped, and, furthermore, cis- and tranj -isomeric electrophilic olefins give the same product The reaction has been applied to many different conjugated systems and its regio- and stereoselectivity has been extensively studied . Some examples of this heterocycloaddition include the preparation of 1,4-dihydropyran derivative 331, 332 and 333 by intermolecular reaction. In another example 336 was obtained by intramolecular enamine-enal cycloaddition from... 

The isomerization is presumed to proceed through intermolecular reaction on the basic sites of the solid as illustrated in Scheme 1[2]. 

The extent and the sense of diastereoselection largely depends on the steric and electronic nature of the cycl oaddends, and is commonly rationalized in terms of frontier molecular orbital (FMO) theory and steric considerations1-2,6-26 34. Although endo transition states generally seem favored in intermolecular reactions, stereoselection is often unpredictable. In some cases, isomerization of the commonly obtained Z-nitrone to the more reactive -form is invoked to explain anomalous results. However, as in the case of cyclic nitrones where isomerization... 

Pressure effects on the diastereoselectivity can also be observed for intramolecular cycloadditions such as the hetero-Diels-Alder reaction of 90, even though the AAV are smaller compared to intermolecular reactions (Scheme 8.23). The kinetics were again measured by on-line FT-IR spectroscopy and the stereoselectivity by HPLC. At atmospheric pressure in toluene under reflux the reaction of 90 led to a 5.2 1 ratio of the diastereomeric cycloadducts 91 and 92 in 93 % yield. Increasing pressure favors the formation of the cis-adduct 91, which is probably formed via an endo-E-syn transition structure. Interestingly, in the ground state of 90 the Z-configuration is more stable and it is therefore assumed that isomerization of the Z- to the B-double bond occurs prior to the cycloaddition [51]. From the slope of the plot of ln(91/92) versus pressure, AAV is calculated to be -(1.6 + 0.2) cm mol with the individual values for AV = -(19.4 + 0.2) and AV = —(17.9 0.6) cm mol at 343 K. Although the AV values are relatively low in comparison with data reported for other intramolecular Diels-Alder reactions e.g. 

Cumene conversion under excess of benzene was studied over H-ZSM-11 in the adsorbed phase at 473 K by in situ C MASNMR. To follow the fate of different carbon atoms during the reaction, cumenes labelled with C-isotopes either on a-or on p-positions of the alkyl chain or in the aromatic ring have been synthesized. The primary product of cumene conversion over H-ZSM-11 was found to be n-propylbenzene. It is formed via intermolecular reaction of cumene and benzene. At long reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of both cumene and n-propylbenzene alkyl chain carbon atoms and formation of toluene, ethylbenzene and butylbenzene. The rate of isomerization is higher than the rate of scrambling and fragmentation. 

---