Rearrangements pathways

Both reactions proceed via triplet excited species and, to some extent, are controlled by whether the ti-tt (path A) or n-rr states are involved. The di-rr-methane rearrangement pathway is restricted to 4-aryl- or 4-vinylcyclohexenones. At the most basic level of... 

Williams and McClymont have observed that acylation reactions of the dianion of 2-(5-oxazolyl)-l,3-dithiane (15) lead to formation of 4,5-disubstituted oxazole products through a Comforth rearrangement pathway under base-induced, low-temperature conditions. For example, deprotonation of 15 with LiHMDS (3.0 equivalents) at -78°C, followed by addition of benzoyl chloride or p-chlorobenzoyl chloride and warming to 0°C, provided 16 in 74% and 47% yield, respectively. 

In addition to the formation of the pyridine framework by de novo approaches (see section 8.1) or by the cycloaddition/cycloreversion sequence (see section 8.2), one can employ reactions that proceed through a rearrangement pathway. The Boekelheide reaction (see section 8.3.1) involves the rearrangement of an existing pyridine skeleton to a more functionalized scaffold, while the Ciamician-Dennstedt reaction (section 8.3.2) generates the pyridine nucleus by rearrangement of an alternative heterocycle. 

The authors conclude from these results that these (Z)/(E) rearrangements proceed through the ionization-rearrangement pathway. They assume that the ratios quoted arise solely from the difference in polarity of methanol and ethanol. For an intramolecular rearrangement (kY) one would not expect to find significantly different rates for these two solvents. 

Then, contrary to what was reported previously, the olefin dissociates from the zirconium metal complex. This conclusion was further supported by other experimental observations. However, it cannot be completely excluded that competition between dissociative and direct rearrangement pathways could occur with the different isomerization processes studied up to now. Note that with cationic zirconocene complexes [Cp2Zr-alkyl], DFT studies suggest that Zr-alkyl isomerizations occur by the classical reaction route, i.e. 3-H transfer, olefin rotation, and reinsertion into the Zr-H bond the olefin ligand appears to remain coordinated to the Zr metal center [89]. 

Analogous olefin trapping chemistry employing 2,7-cyclooctadienone has been investigated51. With this substrate, the oxyallyl zwitterion 112 could be intercepted intermolecularly with simple alkenes in a (presumably stepwise) 3 + 2 manner (Scheme 27). Importantly, intermediate 112 differs from 2,5-cyclohexadienone-derived zwitterions in terms of its lack of a facile rearrangement pathway. An interesting side-issue... 

Clearly, rearrangements do occur in the excited states of diazirine and diazo carbene precursors. Kinetic studies of carbenic rearrangements need to consider the possible intervention of RIES when absolute rate constants are partitioned between competitive rearrangement pathways on the basis of product distributions.28... 

The significant inverse correlation of sulfinamide formation with thiol concentration18,22 24 25 28-30 33 35 36 38 has already been discussed in Section II.C.2. Accordingly, the rearrangement pathway from the semimercaptal to the sulfinamide is favored at low thiol concentrations at the expense of N-hydroxyarylamine formation (see equation 2). In the case of bulky thiols as t-butylthiol38 or Hb-SH40 the sulfinamide is the main product since reduction by a second thiol is sterically hindered. 

The presence of one or more spirotetrahydrofuran rings in a molecule can provide for operation of unprecedented rearrangement pathways. A case in point is the remarkable ease with which 6 can be reversibly transformed into 7 under acidic con-... 

Fig. 6.5. Competitive reactions of rearrangement (Pathway a) and hydrolysis (Pathway b) in amino acid prodrugs of prazosin [21]... 

These unexpected results suggest a modification of the conclusions reached in our earlier studies. Specifically, the observations indicate that in order to detect ODPM photoreactivity in p,7-unsaturated aldehydes, substiments should be present to stabilize intermediate biradicals in the rearrangement pathway, but they should not enhance alternative reactions, such as allylic homolytic cleavage. Further studies will be necessary to confirm this hypothesis and to determine the scope of these new reactions. 

The steric course at the Li-bearing center has been investigated nsing an enantiomer-ically enriched ethynylvinylmethanol-derived system. A reaction of ether 97 (78% ee) with n-BuLi gave [l,4]-prodnct ketone 98 only in the racemic form (equation 55 f. This lack of stereospecificity is partly due to racemization of the a-oxyafkyllithium and partly due to the [1,4]-rearrangement pathway. 

It was again observed that rearrangement pathways comprise a substantial portion of the oxidation routes for alkylated aromatics.Since this phenomenon is mainly due to peroxy radical reactivity rather than to identity of the parent compound, it is clear that comparable rearrangements would be factors for PAHs, as well as for nitrogen-, oxygen-, and sulfur-containing heteroaromatic rings and their alkylated derivatives. 

ROS and other radical intermediates dictate the oxidative decomposition of fuels. We have noted that peroxy radical intermediates provide an enormous amount of flexibility in the combustion of a given compound, specifically in the unimolecular steps available to that compound. In an instructive display of the interaction of experimental and theoretical techniques, rearrangement pathways of the peroxy radicals have been modeled computationally and provide justification for several unexpected products. 

Scheme 7 Possible thermal rearrangement pathways of 2,5-dimethylthiophene endoperoxide.

An important example of a 1,2-C migration (C—C insertion) is the ring expansion of chlorocyclopropylcarbene (54) to 1-chlorocyclobutene (Scheme 7.20). The 1,2-C shift takes precedence over 1,2-H shift. Chloromethylenecyclopropane (55), the putative product of a 1,2-H shift, is not formed. Interaction of the electron-rich bent cyclopropane C—C bond(s) with the vacant p orbital of carbene 54 leads to a more favorable rearrangement pathway than the alternative Oc-H/p interaction leading to 55. 

Extensive theoretical studies have been carried out in order to rationalize the rearrangement pathways in the cationic system92. The characterization of the potential surface of C9H9+ cations93 at MP2-, MP3- and MP4(SDQ)/6-31G levels showed that the open 9-barbaralyl cation 83a is more stable than the completely charge delocalized (nonclassical) structure of Z)1h symmetry, and the bicyclo[3.2.2]nona-3,6,8-trien-2-yl cation 83b by 6.9... 

When one attempts E2 reactions with a-halo ketones using strong bases such as alkoxides, an interesting rearrangement pathway may occur called the Favorskii rearrangement. In this reaction, the a-halo ketone is converted to an ester. For example, 2-chlorocyclohexanone is converted to the methyl ester of cyclopentanecarboxylic acid by treatment with sodium methoxide in ether ... 

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