Carbocations pentadienyl

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

An alternative approach for generating the pentadienyl carbocation that is needed for the Nazarov cyclization has been demonstrated by de Lera and co-workers [20, 21] (Eq. 13.18). Vinylallene acetal 56 is converted to a ca 1 1 mixture of cyclopentenes 57 and 58 upon exposure to toluenesulfonic acid in acetone at room temperature. The reaction presumably involves initial generation of carbocation 59 that undergoes conrotation to give 60. Intramolecular trapping of the carbocation by the pendant hydroxyl group leads to the observed product. Depending on whether the conrotation in 59 takes place clockwise or counterclockwise, E- (57) or Z-(58) products are formed. 

Coordination to the ketone makes it more of a carbocation, and hence the conjugated system more of a pentadienyl cation 4.90. The cyclization takes place to give the cyclopentenyl cation 4.91, which loses the silyl group and picks up a proton to give the ketone 4.92. The relative stereochemistry at C-l and C-5 has the two hydrogen atoms trans, proving that the cyclization has... 

Draw all of the contributing resonance structures and the resonance hybrid for the carbocation that would result from ionization of bromine from 5-bromo-1,3-pentadiene. Open the computer molecular model at the book s website depicting a map of electrostatic potential for the pentadienyl carbocation. Based on the model, which is the most important contributing resonance structure for this cation Is this consistent with what you would have predicted based on your knowledge of relative carbocation stabilities ... 

Early attempts to verify the stereochemical predictions of orbital symmetry control were hampered by carbocation rearrangement reactions/ such as Wagner-Meerwin shifts, although the very presence of these anomalous pathways is consistent with a cationic pathway. It is now well established that the Nazarov cyclization occurs via a pentadienyl cation 10,... 

Gycloheptatrienes in protic solvents are reported to react with Fe(GO)s and a catalytic amount of NaBH4 to produce ( 7" -l,3-diene)iron tricarbonyl complexes. Pearson and Ghidu have demonstrated that stereospecific cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes proceeds via protonation of a double bond vicinal to the iron tricarbonyl diene moiety. This methodology has been used to diastereoselectively produce polycycles from iron tricarbonyl-stabilized pentadienyl carbocations. " ... 

The increased reactivity of this extended tt system is due to the low activation barriers for electrophilic additions, which proceed through highly delocalized carbocations. For example, the bromination of 1,3,5-hexatriene produces a substituted pentadienyl cation intermediate that can be described by three resonance structures. 

The map of electrostatic potential for the pentadienyl carbocation shows greater electron density or less positive charge (less blue color) in the vicinity of the Cl—C2 and C4—C5 bonds, suggesting that the most important contributing resonance structure is the one with the most positive charge near C3, a secondary carbon. The other contributing resonance structures have a positive charge at primary carbon atoms. 

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