Dienes cationic

The publications of Bauld et al., which were reviewed earlier in Section 7.4.1, deal only with reactions in which a cation-radical (readymade) acted as a reactant. However, there can be cases where a cation-radical is formed in the course of donor-acceptor interaction between initially neutral molecules. Then the rigid or sharply enhanced selectivity of the reaction acquires diagnostic significance. For such cases, there is a general rule—condensation is permissible only for the (dienophile cation-radical -l- diene) pair and forbidden for the (dienophile -l- diene cation-radical) pair. 

The simplest example is represented by pairs in which one-electron oxidation of a dienophile proceeds easier than that of a diene (Jia et al. 2003, Zhon et al. 2005). If cation-radicals of both the diene and dienophile can be formed on the action of a cation-radical initiator, some kind of separation operates. Each of these cation-radicals can exchange an electron with any participant in the reaction. However, since only the diene cation-radical is consnmed, the equilibrinm of the electron transfer is gradually shifted toward this particnlar cation-radical. The diene depicted in Scheme 7.22 enters the reaction in its i-cis form. If the diene cation-radical is in i -trans form, a cylobutane product forms (Remolds et al. 1989, Botzem et al. 1998). 

Without ion-radical initiation, the yield of the resulted product reaches 50% for 24 h. Practically the same yield can be achieved for the same time in the presence of tris(4-bromophenyl)ammoniumyl hexachloroantimonate and for only 6 h on sonication (Nebois et al. 1996). Sonication accelerates the rate-determining formation of the diene cation-radical. Of course, hydroxynaphthoquinone is strong enough as an electron-acceptor with respect to 2-butenal Af,Af-dimethylhydrazone. Therefore, the question remains whether sonication is more or less the general method for the initiation of ion-radical cycloaddition. A possible role of sonication in optimization of ion-radical reactions was considered in Section 5.2.5. 

Tungsten j4-diene cations in both s-trans and s-cis forms have been synthesized and the influence of the diene conformation on the regiochemistry of nucleophilic attack has been demonstrated488. Application of nucleophilic additions to Mo-complexed olefins in the construction of quaternary carbon centres has been summarized489. 

If cation radicals of both the diene and the dienophile can be formed upon the action of the cation radical initiator, some kind of separation operates. Each of these cation radicals can exchange an electron with any participant in the reaction. However, since only the diene cation radical is consumed, the equilibrium of the electron transfer is gradually shifted toward this particular cation radical. 

The diene depicted in Scheme 6-18 enters the reaction in its s-cis form. If the diene cation-radical is in the s-trans form, a cylobutane product forms (Reinolds et al. 1989 Botzem et al. 1998). 

The attacking electrophile E + first interacts with Fe atom to give a p diene cationic intermediate which then loses a proton to give the final product, as shown... 

Of further interest is the observation that the same reaction is not observed at all under PET conditions, which should reliably furnish the same diene cation radical [50]. Further, the normally efficient cation radical cyclodimerization of 1,3-cyclohexadiene is completely inhibited in the presence of the hindered diene, so that... 

C), the s,-trans pentadiene cation 58a irreversibly rearranges to the %-cis diene cation The s-trans geometry of cations 58 has been ascertained by NMR spec-... 

The chloride will react with the diene cation to form two products. This is because the positive charge is distributed between two carbons ( 2 and 4), hence there are two sites to which the chloride could attach. 

The most important feature that makes CpMo(CO)2(7i-allyl) useful in organic synthesis is its facile transformation to the CpMo(CO)2(Jt-di-ene) cation. This reaction is particularly suitable for ring systems with six or seven members because their a-methylene protons adjacent to the Mo-Jt-allyl moiety are subject to Ph3CBF4-promoted hydride abstrac-tion. For an acyclic system, this method is successful in only a few cases, mainly on simple molybdenum (ji-onfi-crotyl) compounds (Eq. 2, Scheme 15). The molybdenum-Jl-diene cation is useful because of its reasonable reactivity to nucleophilic attack on carbanions and common... 

Intramolecular nucleophilic attack at the 7c-diene cation proceeds relatively smoothly providing that the products have a five- or six-mem-bered ring. Green reported representative cases shown in Scheme 16. (ri -Indenyl)Mo(CO)2(CH3CN)2 55 proved a useful reagent for intramolecular cyclization of the molybdenum-t -dienes 56 and 57 to the Tj -tetrahydrofuran 58 and -pyran 59 compounds. A molybdenum-Jt-complex of bicyclic spiroether 61 is likewise generated from diene 60 by desilylation. In the presence of DBU, the branched CRjfR = COOMe,... 

With various versatile demetallation methods for compounds of CpMo(CO)2(it-alIyl) and their facile transformation to 7t-diene cations, implementing stereocontrolled multiple functionalization of these compounds appears ultimately important for application to organic synthesis. 

Addition of chiral oxazolidinone enolates to two molybdenum-diene cations 66 and 67 was investigated, and the results were more satisfactory for the cyclohexadiene system than for its seven-membered counterpart. 

The a-hydroxyl allyl compounds presented in Scheme 30 are easily ionized selectively by (CF3S02)20 at -78 °C in anhydrous ether to generate s-trans-cis-diene cationic precipitates via an intramolecular S 2 mechanism. The resulting s-rran -cw-1,3-diene cationic precipitates are thermally unstable as temperatures increase to 25 °C and decompose liberating free dienes. We utilized this method for chemoselectively synthesizing cis-1,3-dienes and cis-l,3,5-hexatrienes ° with 50-70%... 

Addition of organocopper reagents to j-trans-diene cation A in Scheme 31 gives u-l-anti- i-syn-dWyl compounds 143 with ca. 50%... 

Addition of CF3SO3H to the q -2-carbomethoxylpentadienyl compounds of molybdemrm and tungsten compounds 153 in diethyl ether at -40 °C generates j-tran -diene cation 154a that is stable at this tern-... 

To account for the stereochemical outcome, we propose a mechanism in Scheme 38 that involves a metal-5-fra j -diene cationic intermediate I. A near chairlike transition state structure is proposed to account for the stereochemistry. The states A and B represent the most likely transition states that have the most bulky complexed RCO(BF3) group situated at the equatorial site to avoid 1,3-diaxial steric hindrance. A further comparison between the two stractures indicates that A is the preferred structure because it avoids steric hindrance between two neighboring equatorial substituents, as in B. The presence of an electron-withdrawing group, such as carbomethoxyl, is required for this cyclization. No cycli-zation occurs for other T) -pentadienyl compounds, including that having the substituent CH=CH2 at the C(2)-allylic carbon position. 

Electrophilic addition of aldehydes to [CpMo(CO)2(Tl -6-R-cyclohexadien-l-yl)] (R = H, Me, Ph) catalysed by boron trifluoride was reported to afford isolable (Tj -diene) cationic salts of molybdenum. Demetallation by trimethylamine-N-oxide afforded functionalised cyclohexadiene compounds in good yields. The stereochemistry of the carbon-carbon bond forming reaction was clarified. 

Reaction of 1,8-naphthyridine with [Rh (diene)(He2CO) ] yields [Rh-(u-naphthyridine) - (diene) cations (27) [diene = nbd, (25)]... 

Kinetic results for the replacement of X (F, Cl, or Br) in [RuX2(bipy)2] by pyridine in a variety of aprotic solvents indicate a dissociative mechanism with a transient [RuX(bipy)a]+ intermediate. The range of rate constants for nucleophilic substitution in [PdX(dien)]+ cations by a given series of incoming groups shows that there is a marked decrease in nucleophilic discrimination in going from, e.g., water or methanol to the dipolar aprotic solvents DMF or DMSO. ... 

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