Intramolecular positional isomerization

Intramolecular positional isomerization (Berry pseudorotation) resulting in an exchange between the apical and the equatorial ligands is important in explaining the reaction mechanisms of hypervalent compounds. 

As mentioned, positional isomerization can also take place in the a complexes through intramolecular 1,2 shifts prior to deprotonation 126... 

Intramolecular ring closure by valence isomerization to a carbon in the 3-position is not common, but known l,l-di-t-butyl-2-nitrosoethylene, stable at room temperature, cyclizes at 220 °C to the stable 4,4-di-f-butyl(4iT)oxazete (75AG(E)70). 

As a substrate, 4-amino-5-acetylenylpyrazole 71 was chosen in this compound the position of interacting groups is the most favorable for intramolecular cyclization. Indeed, 4-amino-l,3-dimethyl-5-phenylethynylpyrazole 71 isomerized into l,3-dimethyl-5-phenylpyrrolo[3,2-c]pyrazole 70 in 65% yield under heating in DMF for 4 h in the presence of Cul (83IZV688). The authors have noticed that the cyclization of 71 is accelerated by the addition of CuC=CPh. 

It is also well known that alkyl groups can be tran.sferred intramolecularly from one position to another on the same ring and intermoiccularly from one aromatic ring to another through dealkylation reactions catalyzed by Lewis acid. The intramolecular alkyl-transfer is called reorientation or isomerization and the intermolecular alkyl transfer is referred to as disproportionation. Reorientation processes arc normally faster than disproportionation. 

Early experiments [1] using (+)-apopinene and deuterium showed, however, that in the isomerized molecules the deuterium content was very low and the isomerization was much faster than deuterium incorporation into the allylic position. Therefore it seemed probable that isomerization takes place through an intramolecular hydrogen shift. A sigmatropic 1,3-hydrogen shift was suggested, in which the allylic endo-H shifted top shift) [2]. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

The first spirophosphorane with an azaphosphetidine ring 63 was prepared from the intramolecular cyclization and dehydration of phosphane oxides with DEAD/PPh3 <1996AGE1096, 1996PS489>. Dissolution of 63 in [r/8]-toluene led to some isomerization to the pseudorotamer 64, where the electronegative nitrogen occupies the equatorial position (Scheme 32). Such rotamers were previously unknown for the analogous oxaphosphetanes. 

The rate of this intramolecular isomerization depends on the chain length, with the maximum in the case of a six-atomic transition state, i.e., when the tertiary C—H bond is in the (3-position with respect to the peroxyl group [13]. For the values of rate constants of intramolecular attack on the tertiary and secondary C—H bond, see Table 2.9. The parameters of peroxyl radical reactivity in reactions of intra- and intermolecular hydrogen atom abstraction are compared and discussed in Chapter 6. 

The peculiarities of the oxidation of PP, whose molecules have alternating tertiary C—H bonds in the (3-position, are of special interest. Such branched alkanes are oxidized with the formation of polyatomic hydroperoxides produced by the intramolecular isomerization of the peroxyl radical [88],... 

During PP oxidation, hydroxyl groups are formed by the intramolecular isomerization of alkyl radicals. Since PP oxidizes through an intense intramolecular chain transfer, many of the alkyl radicals containing hydroperoxy groups in the 0-position to an available bond can undergo this reaction. An isomerization reaction has also been demonstrated for the liquid-phase oxidation of 2,4-dimethylpentane [89], Oxidation products contain, in addition to hydroperoxides, oxide or diol. 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

As a further illustration of the phenomenon of H-bond resonance coupling let us consider the intramolecular H-bond of (3-hydroxyacrolein (0=CHCH=CH0H), a prototypical enolone (2-en-3-ol-l-one, or enol isomer of (3-diketone).55 This molecule may be envisioned as existing in two distinct isomeric forms, according to the position of the proton in the O- H—O hydrogen bond ... 

Intramolecular hydrosilylation.1 Hydrosilylation of internal double bonds requires drastic conditions and results in concomitant isomerization to the terminal position. However, an intramolecular hydrosilylation is possible with allylic or homoallylic alcohols under mild conditions by reaction with 1 at 25° to give a hydrosilyl ether (a), which then forms a cyclic ether (2) in the presence of H2PtCl6-6H20 at 60°. Oxidative cleavage of the C—Si bond results in a 1,3-diol (3). 

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

A regioselective [3 + 2]-cycloaddition approach to substituted 5-membered carbo-cycles was made available by the use of allenylsilanes [188]. The reaction involves regioselective attack of an unsaturated ketone by (trimethylsilyl)allene at the 3-position. The resulting vinyl cation undergoes a 1,2-silyl migration. The isomeric vinyl cation is intercepted intramolecularly by the titanium enolate to produce a highly substituted (trimethylsilyl)cyclopentene derivative. 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

The isomerization of terminal epoxyalkynes into furans catalyzed by RuCl(Tp)(PPh3) (MeCN) inthe presence of Et3N as abase at 80 °C in 1,2-dichloroethaneis explained by a related intramolecular nucleophilic addition of the oxygen atom of the epoxide to the a-carbon atom of a ruthenium vinylidene intermediate, as shovm by deuteration in the 3-position of the furan (Scheme 10.10) [45]. This reaction is specific for terminal alkynes and tolerates a variety of functional groups (ether, ester, acetal, tosylamide, nitrile). 

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