Symmetrical or Intramolecular Coupling

See also page 168, Section 3 page 175, Section 5 and page 185, Section 7. 

Taylor, A. I. Battersby, Oxidative Coupling of Phenols, M. Dekker, New York (1967) Russ Chem Rev 47 649 (1978) 

BF3OEt2, CF3SO3H, (CFjSOj O JOC 53 224 (1988) (intramolecular) Tetr 47 3787 (1991) 

2ROjCCH=CHC02R eleclr°lysis. [CH(C02R)CH2C02R]2 Org Syn Coll Vol 7 479 (1990) 

Ni(CK 4)2, electrolysis Mi(acac)2, electrolysis NiCl2(PPh3)2, electrolysis 

Fe(C104)36H20, CH3CN Chem Pharm Bull 33 3599 (1985) 

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The intramolecular coupling of the two OH vibrations of a water molecule to a symmetric (vi) and an antisymmetric (V3) stretching mode is removed for strongly distorted H2O molecules, where the separation between Vi and V3 has increased compared to that of free H2O, viz. 99 cm, (see Sect. 4.2.2 and Table 3). In many hydrates with symmetric water molecules - site symmetry C2V (or an effective local potential of C2V or nearly C2V symmetry) - this separation is reduced (see also Sect. 4.2) to <40 cm as in the case of CUCI2 2 H2O ) and Bal2 H20 >. 

By anodic decarboxylation carboxylic acids can be converted simply and in large variety into radicals. The combination of these radicals to form symmetrical dimers or unsymmetrical coupling products is termed Kolbe electrolysis (Scheme 1, path a). The radicals can also be added to double bonds to afford additive monomers or dimers, and in an intramolecular version can lead to five-membered heterocycles and carbocycles (Scheme 1, path b). The intermediate radical can be further oxidized to a carbenium ion (Scheme 1, path c). This oxidation is favored by electron-donating substituents at the a-carbon of the carboxylic acid, a basic electrolyte, graphite as anode material and salt additives, e.g. sodium perchlorate. The carbocations lead to products that are formed by solvolysis, elimination, fragmentation or rearrangement. This pathway of anodic decarboxylation is frequently called nonKolbe electrolysis. 

In a subsequent study (21) energy was localized on one end of the molecule, all in either symmetric or antisymmetric stretch. Antisymmetric stretch excitation was found to produce reaction of nearly all molecules by C—scission within a couple of vibrational periods. The same amount of energy placed in symmetric stretch resulted in an induction period of several vibrational periods in which no reaction occurred at all, followed by an enhanced C—H reactivity and, after a bit longer induction, a retarded C—C contribution. This work confirmed earlier suggestions of restricted intramolecular energy transfer by direct observation. 

An interesting strategy for the construction of 2,5-dihydrothiophenes employed nucleophilic displacement to introduce the sulfur atom into a diketone intermediate (9) or (12). This was followed by an intramolecular reductive coupling of the diketosulfide, using a low-valent titanium reagent generated in situ [88] (Scheme 6.4). Either symmetrical or unsymmetrical compounds could be prepared and converted to sulfolenes by treatment with mCPBA. 

In 2008, Shimizu and coworkers developed a novel approach of palladium-catalyzed intramolecular coupling of readily available 2-(arylsilyl)aryl triflates (Scheme 4.32) [47]. The key to this reaction was the installation of bulky substituents on silicon and the use of Et2NH as a base. Symmetrical and asymmetrical 9-silafluorenes, substituted with electron-donating and/or electron-withdrawing groups, such as NMc2, OMe, CFg, and CN, were synthesized from the... 

The reductive cross-coupling reaction—i.e., the reaction of two C-X moieties to give a new C-C-bond—favorable competes with other cross-coupling reactions when symmetrical products or intramolecular reactions are required (Scheme 5-141). Both intermolecular as well as intramolecular variants are known. 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

While the syntheses of the acyclic precursors in the examples above each require a couple of steps, symmetrical dienynes with a central triple bond and heteroatoms in the tethers are more easily accessible. They can yield heterotricyclic compounds by the same reaction mode, for example, the diaza- and dioxatricycles 121 are obtained starting from dienynes 119 (Scheme 18) [73]. Yields were best (90%) with N-tosyl linkers, with N-Boc groups the reaction was slower (41% yield), and with N-benzyl linkers only decomposition occurred. This may be due to coordination and blocking of the catalyst by the more Lewis-basic amines. The cis- and frans-diastereomers of 121 were formed in a ratio of 1.8 1, and this ratio did not change in other solvents, at different temperatures, with other catalyst precursors or under high pressure (10 kbar). In view of the apparent influence of the tether, the unsymmetrical oxazaprecursor 122 gave a 7 3 mixture of tricycles 123 and 124. Obviously, the hydridopalladation of the triple bond occurred with some regioselectivity such that intramolecular carbopalladation of the allyl-amine predominated. It is noteworthy that in these cases the intramolecular Diels-Alder reactions of the intermediate trienes 120 already occur under the employed conditions, i.e. at 80 °C. 

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