Cyclohexadiene cationic

The cationic pathway allows the conversion of carboxylic acids into ethers and acetals. The decarbox-ylative elimination of vicinal diacids or f3-silylcarboxylic acids in combination with cycloaddition reactions leads efficiently to cyclobutenes or cyclohexadienes. Cationic rearrangements or fragmentations initiated by anodic decarboxylation allow the synthesis of specifically substituted cyclopentanes and ring extensions by one or four carbon atoms. 

Because it is similar to RjSi as an electron-donating group, the CpMo(CO)2 fragment is cap le of stabilizing a P-carbocation center. By analogy with the allyl compounds of silane, borane, and tin, CpMo(CO)2(Tl -cyclohexadienyl) and its 6-substituted derivatives 106 a-c undergo BFj-promoted electrophilic addition to aldehydes and a,P-unsaturated enones by generating cyclohexadiene cationic precipitates 107, as shown in Scheme 22. 

Further elaboration of these molybdenum-7i-cyclohexadiene cations for stereoselective synthesis of cyclohexene compounds was achieved. Addition of NaBH, RMgBr, and LiCH(COOMe)2 to diene 107 gives the Ti -4,6-cw-disubstituted cyclohexadienyl compounds 110 a-d. After conversion of 110b to its acid form 111, a NOBp4-promoted intramolecular cyclization of this monoacid compound delivers bicyclic lactone 112 with a 50% yield. 

Protonation of neutral diene complexes [Fe( -diene)(CO)3] (diene = butadiene or cyclohexa-1,3-diene) gives hydrido-complexes containing (T, 7r-bonded ligands. The cyclohexadiene cation (33) undergoes the interconversion of Scheme 11 with AG (- 50 O 11 kcal moH. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

The first studies on cation-radical Diels-Alder reactions were undertaken by Bauld in 1981 who showed [33a] the powerful catalytic effect of aminium cation radical salts on certain Diels-Alder cycloadditions. For example, the reaction of 1,3-cyclohexadiene with trans, iraw5-2,4-hexadiene in the presence of Ar3N is complete in 1 h and gives only the endo adduct (Equation 1.14) [33]. 

Another model cationic species [92] that has been studied is cation 99 which is obtained from 2-oxopropylacrylate 98. By treating compound 98 with equimolecular amounts of trimethylsilyltrifluoromethane sulfonate and methox-ytrimethylsilane in the presence of 1,3-cyclohexadiene, a cycloadduct is produced... 

Ring-member substitution, a very characteristic reaction of some 18-e borabenzene complexes (see Section VII,B), can also occur with 1,4-dibora-2,5-cyclohexadiene complexes. The cobalt complex 53 cleanly reacts with MeCOCl/ A1C13 to give the cation 54 (Scheme 7) (75). The Rh complex (C5Me5)Rh[MeB(CHCH)2BMe] reacts analogously (75). 

Irradiation of mixtures of cyclohexene with 1,3-cyclohexadiene leads to high yield of dimers (G = 6.3)86. Schutte and Freeman87 found that radiolysis of 1,3-cyclohexadiene dissolved in various solvents gives dimers mainly via cationic Diels-Alder addition,... 

Cyclopentadiene behaves differently than the cyclohexadienes in that its radiolysis leads to high molecular weight polymer via a cationic mechanism89, whereas such compounds are not formed in high yield from cyclohexadienes irradiated in the liquid phase. 

Dialkynylstannanes were synthesized and characterized by their NMR spectra. These compounds react with boranes yielding l-stanna-4-bora-2,5-cyclohexadienes (97), stan-noles (98), as shown in reaction 24, and stannolines (99), as shown in reaction 25. When R 4 H it is possible to isolate at low temperature the tautomeric intermediates 95 and 96 of reaction 23, one of which is a TT-stabilised stannyl cation complex279. Application of the same borane derivatization scheme to tetraalkynylstannanes leads to formation of 1-stannaspiranes where the rings are combinations of 97, 98, and 99 structures116,280. 

Two types of derivatives of 1,2-cyclohexadiene with two heteroatoms were proposed as reactive intermediates more than 20 years ago. Lloyd and McNab [168] observed the reaction of the 5-bromo-l,2-dihydropyrimidinium ions 411 with thiourea in refluxing ethanol to give the bromine-free cations 413. Suspected as intermediates, the 5d2-dihydropyrimidines 412 were initially considered as zwitterions of the type 414-Zj. However, quantum-chemical calculations on the parent systems suggested an unambiguous preference of the allene structure 414 over the zwitterion 414-Za [169]. 

The trianionic cobalt catalyst has been successfully employed in the hydrogenation of 1,3-butadiene in [bmim][BF4] [10], The product from this reaction is 1-butene which is formed with 100% selectivity. Unfortunately the catalyst undergoes a transformation to an inactive species during the course of the reaction and reuse is not possible. The cationic rhodium catalyst together with related derivatives have been used for numerous reductions, including the hydrogenation of 1,3-cyclohexadiene to cyclohexane in [bmim][SbF6] [11],... 

The enamide 166 react with the cation radical of cyclohexadiene, which is generated by sensitized electron transfer with the photoexcited dicyanobenzene (DCB), to generate a Diels-Alder type adduct (equation 110)167. 

The 1,3-cyclohexadiene complex 64 may be prepared by addition of two equivalents of hydride to the (C6H6)Mn(CO)3+ cation 65 (R = H, Scheme 16)90. The first equivalent of hydride generates the neutral ( j5-cyclohexadienyl)Mn(CO)3 complex... 

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