Intramolecular pathway

The stereochemical outcome of the reaction is determined by the geometry of the transition state for the Claisen rearrangement a chairlike conformation is preferred,and it proceeds strictly by an intramolecular pathway. It is therefore possible to predict the stereochemical course of the reaction, and thus the configuration of the stereogenic centers to be generated. This potential can be used for the planning of stereoselective syntheses e.g the synthesis of natural products. 

The reaction mechanism " is not rigorously known. Evidence for an intramolecular pathway as well as an intermolecular pathway has been found ... 

Cycloaddition of nitrone (508) to allyl alcohol at ambient temperature gave a mixture of four cycloadducts in a 23 5 4 1 ratio (Scheme 2.244). All of the adducts (509) are derived from the regiochemical approach opposite to the intramolecular pathway (Fig. 2.35). Formation of the cycloadduct in the intramolecular cycloaddition reaction is ascribed to a high preference for an endo-syn transition state, due to the constraint imposed by the short, three atom connecting chain (116). The major product in the intermolecular cycloaddition reaction was the exo-anti -(509) adduct (Scheme 2.244 and Fig. 2.35). 

Radical cydizations onto allenes are feasible via the dig (addition to C ) or the exotrig mode of ring dosure (addition to QJ. The significance of the a-addition is more pronounced in the case of the intramolecular pathway than in its intermolecular counterpart owing to (i) the chain length between the reacting entities and (ii) the stereoelectronic prerequisites for a radical attack on a Jt-bond [4],... 

Further bisalkynylphosphine oxide (53) cyclizations have been applied to heterocyclic synthesis.45 The phosphine oxide (54) has been prepared as shown, and found to undergo Diels-Alder addition by an intramolecular pathway, to give (55).48... 

A different mechanism probably operates for the reaction of TV-hydroxy-TV-phenylamides in the presence of (m-Bu)3P, CCI4 and MeCN, with the 2-isomer in the product53 suggestive of some intramolecular pathway as outlined in Scheme 8. 

If intramolecular pathways dominate the nuclear spin relaxation mechanism, a separation between translational and rotational contributions is... 

Phenylperoxy radical, originally assumed to be a factor in low-temperature combustion only, has actually been shown to play a substantial role in dictating the overall combustion trends of benzene. Just as the isomerizations and eliminations of the alkylperoxy radicals significantly affected their overall combustion pathways, rearrangements and other intramolecular pathways available to phenylperoxy radical similarly impact the overall progress of benzene combustion. This knowledge can be extrapolated to more complex aromatic species. 

This qualitative interpretation of structural and electronic similarity has also been employed to rationalize the fact that the quantum yield for the dioxetane derivative 6, in which the phenoxy substituent is directly linked to the peroxidic ring, is two orders of magnitude higher than for the dioxetane 7, in which the trigger function is separated by a methylene bridge. Furthermore, the different quantum yields were rationalized in terms of a competition between the intramolecular (pathway A) and intermolecular back-electron transfer (pathway B) in the decomposition of 7, whereas the intramolecular back-electron transfer was believed to occur exclusively in the decomposition of 6, due to the higher stability of the radical anion of the benzaldehyde derivative, as compared with the radical anion of acetone (Scheme 14). 

The transformations of o-iodoalkenylbenzenes in the presence of CO and palladium catalysts may involve either carbonylative cylization, or intramolecular Heck reaction, as well as a number of intramolecular pathways leading to oligomeric byproducts. Non-carbonylative pathways can be reasonably suppressed by applying elevated pressures of GO. Under such conditions, the products are formed in good yields, and the predominant termination stage is Pd hydride / -elimination to form exo-cycWc double bond (Equation (27)). 

Schemes 24 and 34. Not all specific examples follow these generalized pathways. Although 78h does decompose predominately into 99 (Scheme 44) at neutral pH, evidence suggests that it may not do so via an addition-elimination mechanism. This intermediate is detectable in reaction mixtures, but it has never been isolated due to its high reactivity. The N-benzoyl analogue 78h has been prepared by anodic oxidation of N-benzoyl-2-aminofluorene and has been shown by labeling experiments to generate 99 by an intramolecular pathway presumably involving the intermediate 100. ...

Crich D, Huang X, Newcomb M (2000) Inter- and intramolecular pathways for the formation of tet-rahyrofurans from (5-(phosphatoxy)alkyl radicals. Evidence for a dissociative mechanism. J Org Chem 65 523-529... 

Cytochromes c, small blue-copper proteins, or an internal heme c group can function as natural electron acceptors for the dehydrogenases. Since these are soluble proteins and the genes have been cloned in most cases, they provide excellent possibilities to study electron transfer pathways in vitro and intermolecular as well as intramolecular pathways between a quinone and Cu or heme c in particular. 

TI) (46) by pathway a. However, when protons are made available intra- or inter-molecularly, loss of urea from the TI (46) via pathway b also occurs to give a formami-dine (47) (Scheme 14). The proposed mechanism for the intramolecular pathway b is illustrated by the reaction between the r-butyl compound (48) and an ty-hydroxyamine (such as 5-aminopentanol) in Scheme 15.44... 

X 109 and 8. 9 x 109 s 1 for Ndm and Ybm, respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eum and Tbm (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Ybm its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the 3Pi, 3Po, and 1D2 levels) and in the NIR at 850 nm ( Do -> 3F2 transition) and 1100 nm ( D2 3F4 transition). The excited... 

Figure 9. Important intramolecular pathways in the analysis of catalytic cracking of gas oil feedstocks.

Most reactions of this category involve the base-induced generation of alkylidene-carbenes (R2C = C ) which undergo an intramolecular 1,5-carbon-hydrogen insertion providing a useful route for the construction of substituted cyclopentenes a competing intramolecular pathway is rearrangement to alkynes. 

The possibility for the existence of two different metal-inserted BT products stems directly from the orbital structure of the BT LUMO and second LUMO (SLUMO) (Table 5). On the basis of Sargent s predicted insertion mechanism, the character of the LUMO is consistent with S-Cy insertion while the character of the SLUMO coincides with S-C insertion. While occupation of the LUMO is energetically preferred, the calculated LUMO/ SLUMO gap is small the small gap is consistent with the intramolecular pathway between the S-Cy and S-C products of the rhodium-inserted complex (CsMes)Rh(PMe3)(r -(7,y-2-MeC8HsS). Occupation of the SLUMO, therefore, is readily achievable. 

Historically, the possibihty of obtaining intramolecular rearrangement came from the tautomeric irreversible rearrangements observed by Chapman (Scheme 2) [4,5]. In his early work. Chapman was able to demonstrate that the reaction undertakes an intramolecular pathway, i.e. via a spirocyclic intermediate. Not much later Smiles foimd another reaction causing the 1,3 shift of the substituent in the aromatic ring, involving a similar intermediate [6,7]. 

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