Transannular mechanism

In the presence of cationic initiators, these monomers polymerize via a transannular mechanism to give polymers of structures P2eai and P26bi for the bicyclo[6.1.0]non-4-ene, and dihalo-9,9-bicyclo[6.1.0]non-4-ene, respectively (Equation 33). 

Transannular interaction via the electron-delocalization mechanism was found, but lessened by 10-15% for the ligand superhyperfine splitting and 30-35% for the hyperfine splitting (62) in the epr spectrum. The crystal structure of [VOS2CNEt2)2] shows that the molecular core has the expected C2V symmetry [V-0 = 159.1(4), V-S = 138.7(2)-241.0(2) pm] (63). Magnetic and spectral data provided evidence for a tetragonal, pyramidal structure (VII) for these complexes. Like many other coordinatively unsaturated, metal... 

The Hofmann degradation is the most well-known C—N bond cleavage reaction, and its value to structural elucidation of alkaloids has been demonstrated (76). Hofmann degradation of tetrahydroberberine methohy-droxide (1) led to two products base A (2), the C-14—N bond cleavage product, and base B (3), the C-6—N bond cleavage product (Scheme 2) (17,18). The former was the major product when 1 was heated under reduced pressure, but the latter, the thermodynamically controlled product, predominated when the reaction was carried out at atmospheric pressure or in an alkaline medium because base A recyclized back to the starting quaternary base through the transannular reaction. In fact, 2 was heated in aqueous alcohol to afford 1. The mechanism of this recyclization reaction was discussed by Kirby et al. (19). 

Furthermore, the oxy-Cope rearrangement of allenic cycloheptane alcohol 47 (NaOEt, THF, 20 °C, 12 h, 80%) gave rise only to ring-enlarged product 48 without transannular cyclization (equation 15)29. The above transformations can be rationalized either by the fragmentation-recombination mechanism or by a concerted oxy-Cope mechanism29. 

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

The mechanism of the dehydrogenative transannular ring closure of cycloocta-tetraene in the presence of various inorganic reagents to provide complexes of pentalene has been the subject of debate... 

In the carbonylation of unconjugated dienes the nature of the products is influenced by reaction conditions. With Pd halides in ethanol at 100°C and 97 atm CO, 1,5-cyclooctadiene is successively carbonylated to the unsaturated monoester and then to the saturated diester (II). With (Ph3P)2PdCl2 in ethanol-HCl and 300-700 atm CO, the monoester is produced selectively at 60°C and the diester at 100°C (8). Finally, with (Bu3P)2PdI2 in THF at 150°C and 1000 atm CO, 1,5-cyclooctadiene undergoes transannular addition of CO to give a cyclic ketone in 40-45% yield (14, 15). The mechanism proposed involves a a-7r-cyclooctenyl... 

A third possible pathway could yield indan through cyclononane intermediate. We know that cyclononane undergoes transannular dehydrocycli-zation over platinum-on-charcoal catalyst at 300°C, to perhydroindan and then to indan (38) but so far there is no evidence for the direct cyclization of n-nonane to cyclononane. Unfortunately, Il in and Usov used an acidic catalyst and we cannot separate the contributions of acid and metal catalysis to the two mechanisms. Experiments over nonacidic platinum catalysts could show the relative importances of the platinum metal in the two cyclization pathways. 

Electrochemically generated NO3 has been reported to add to medium-ring alkynes and alkynones to furnish bicyclic ketones and epoxy ketones, respectively.50 The postulated mechanism involves the addition of N03 to the alkyne followed by transannular cyclization/elimination of NO3. 

The observed deceleration of the rate of transannular cyclization of a series of olefinic substrates 34 > 36 > 37 > 35 has been rationalized by decreasing electron density on the C-atom undergoing protonation. Negative temperature coefficients for the HCl-catalysed transannular hydrochlorination are consistent with the formation of charge-transfer intermediates. An ion-pair mechanism has been proposed80. 

Molecular mechanics calculations, carried out in order to explain the regio- and stereo-chemistry of the transannular cyclization reactions of 19-nor-5,10-secosteroidal cyclodecenone systems, correlate well with experiments400. 

The reaction of (25 R = Pr1) with HC1 has been found to give (26 X = C1) in quantitative yield as a result of transannular cyclization (Scheme 9) similar reactions occurred with H+-MeOH or H30+. Activation parameters, obtained by investigating the cyclization of (25) at different temperatures, are AH = 21 1 kcal mol-1 and AS = —9 2 cal mol-1 K 1. These results are compatible with an AdE2 mechanism.41... 

The reaction of 13-hydroxysparteine with acetic anhydride in DMSO was shown earlier to give the inverted acetate (axial equatorial) instead of the expected ketone, and this has been confirmed. Application of the reaction to 3-hydroxylupanine (18) (axial OH) and to 13-ep/-hydroxylupanine (19) (equatorial OH) gave the corresponding ketone (10% yield) and an approximately equimolecular mixture of epimeric acetate (20) in each case. The mechanism proposed (Scheme 2) indicates facile formation of a carbonium ion through transannular interactions.23... 

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