1,3-diene cationic reaction

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. 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

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). 

A surprising steric sensitivity is frequently observed for these radical cation reactions. As was shown previously during the discussion of chemoselectivity, the variable positioning of bulky substituents has effects on periselectivity as well. DFT calculations on the influence of diene substitutions for the neutral reaction have demonstrated a behavior similar to that of the radical cation reaction. Although... 

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... 

The radical cation Diels-Alder reaction has been the subject of many mechanistic and theoretical investigations and has been shown to have much synthetic potential. With regard to heteroaromatics, the reaction has been exploited by Steckhan in the cycloaddition of indoles and 1,3-dienes. This reaction occurs smoothly upon photosensitization by triarylpyrrilium tetrafluoroborates. The reaction is satisfactory rationalized as involving addition of the indole radical cation to electron-rich dienes (Scheme 35), and the regioselectivity is in accord with theoretical predictions [104]. The reaction with exocyclic dienes has been developed for the synthesis of carbazole derivatives such as 52 and 53 [105]. 

Novel types of polyamines and cationic polyelectrolytes form from polymers of conjugated dienes in reactions with carbon monoxide, amines, and water at 150 °C and 1000-1500 psi pressure. The reaction can be illustrated as follows ... 

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... 

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. 

The Bradsher reaction is formally a [4 + 2] Diels-Alder reaction. However, as a consequence of the aza cationic nature of the diene, this reaction proceeds by the inverse electron demand manifold. The classical Diels-Alder reaction employs the partnering of an electron-rich diene and an electron-deficient dienophile to provide the proper interaction of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as prescribed by frontier molecular orbital theory (FMO) to generate the observed adducts. Thus FMO theory interprets this reaction proceeding via the HOMO of the diene with the LUMO of the dienophile. In the case of the inverse electron demand reaction, the electronics of the reaction are inverted. Therefore, in the Bradsher reaction, the electron-deficient aza cation diene s LUMO interacts with the HOMO of an electron rich dienophile. This mechanistic pathway provided a rationalization for the lack of reactivity of the electron-deficient tetracyanoethylene (TCNE), while electron-rich styrenes afforded the predicted product from reaction of 1 to generate 2. ... 

Another type of ring formation is the so-called palladium-ene cyclizations - developed by Oppolzed (Scheme 55). The presence of an olefin near the cationic 17 -allylpalladium complex facilitates its insertion into the C—Pd bond. Reductive elimination and proton elimination result in the formation of cyclic dienes. Similar reactions have been reported by others. " p i... 

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

Experimental and computational approaches have been used to investigate the gold-catalysed intermolecular cycloadditions of allenamides and 1,3-dienes. In reactions catalysed by AuCl, the electron-neutral dienes favour a concerted 4 + 3-cycloaddition followed by a ring contraction, while electron-rich dienes prefer a stepwise cationic pathway to yield the same formal 4+2-cycloadduct. ... 

Substitution. The kinetics of reactions of /ra/t -CPdClgCR SR ) ] complexes with a series of amines of various basicities and steric requirements have been studied in 1,2-dimethoxyethane at 298 K. In fact the first step is fast, and it is the second step whose kinetics have been examined here. Amine basicity is an important factor logarithms of second-order rate constants correlate linearly with amine pKa values in the absence of steric hindrance. Steric factors have a large effect when the incoming amine e.g. 2-methylpyridine) or the leaving sulphide (e.g. phenyl isopropyl sulphide) is bulky. The kinetic pattern for the reaction of [PdCl2(cod)] with 2,2 -bipyridyl in aqueous methanol has been interpreted by the reaction sequence shown in Scheme 3. The mechanism is dissociative with respect to the palladium centre. The rate law for the reaction of the [PdBr(dien)]+ cation with inosine has already been discussed in the introductory section to this chapter. 

Apart from the role of bonding with and stabilizing the cyclopentadienyl radicals, the iron atom may also take a direct part in the chemistry of ferrocene substituents, and in the mechanism of substitution. It has been proposed that electrophilic substitutions occur by a general reaction path in which the attacking electrophilic group interacts first with the iron atom to give a ir-diene cationic intermediate ... 

The Diels-Alder reaction of dienophiles 5.1a-e, containing neutral, cationic or anionic substituents, with diene 5.2 in the absence of Lewis acids is retarded by micelles of CTAB, SDS and C12E7. In the situation where the dienophile does not bind to the micelle, the reaction is inhibited because uptake of... 

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