Alder-ene

Fig. 9. Reaction of P-methylstyrene with a bismaleimide via a complex Diels-Alder-ENE route.

Examine transition-state structures and bond density surfaces for the Diels-Alder, ene and Cope reactions. 

Scheme 12. Trost s palladium(n)-catalyzed Alder ene cyclization process.

Various oligomers formed by Diels-Alder/ene reactions are observed.333 334 For S-MA11 polymerization Sato et ci//31 used spin trapping to identify the initialing species. On the other hand, in the case of S-AN copolymerization, Ihe... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

Complex 38 also turned out to be an efficient catalyst for cycloisomerization reactions of enynes 41 (Scheme 8) [16, 17]. This seems reasonable if one considers the fact that Fe(0) is isoelectronic to Rh(+1), which is also a catalyst for Alder-ene cycloisomerizations [18, 19]. 

Equally to ferrate 38 ferrates 39 and 40 also catalyze Alder-ene cycloisomeriza-tions [17]. Compared to 38, they require somewhat longer reaction times, possibly due to the chelating cod-hgand, which is more difficult to substitute by the en)me. The presence of cod in the reaction turns out to be of advantage with more demanding substrates like acyclic enynes with a terminal aUcene moiety where ferrate 38 is not reactive. Cod is assumed to stabilize the catalyst in its resting state as ancillary hgand. 

Treating diene-yne derivatives 50 with ferrate 40 does not lead to the expected ene-allenes, instead the [4 + 2]-cycloaddition products 51 are obtained in moderate yields (eq. 1 in Scheme 11). As metal-catalyzed Diels-Alder-reactions of unactivated aUcynes and dienophiles are assumed to proceed via metaUacyclic intermediates, this supports the mechanism for the Alder-ene-reaction discussed before. 

However, in contrast to Fe(0)-ate complexes 38-40 (cf. Sect. 4.2), complex 67 failed to catalyze Alder-ene cycloisomerizations, which may be attributed to the thermal lability of this complex [17]. 

In aqueous media, the addition of unactivated alkynes to unactivated alkenes to form Alder-ene products has been realized by using a ruthenium catalyst (Eq. 3.44).180 A polar medium (DMF H20 = 1 1) favors the reaction and benefits the selectivity. The reaction was proposed to proceed via a ruthenacycle intermediate. 

Reactions of the allylic position of alkenes with carbonyl or imine electrophiles are known as Prins reactions and have been discussed in previous sections (3.2.9). More examples of similar Alder-ene-type reactions (the Prins reaction) will be discussed in Chapter 8. 

Alder-Ene Reactions. An ene-iminium one-pot cyclization proceeds smoothly in a mixture of water-THF (Eq. 12.66). 138 The reactivity of the ene-iminium substrates is highly dependent on the substitution pattern of the ethylenic double bond. This methodology can be used to form homochiral pipecolic acid derivatives. 

The third transformation, by far the most encountered process, is the /3-hydride elimination which is the major and the fastest process in many cases (Scheme 49). The /3-elimination is usually followed by the reductive elimination to give the cycloadduct and regenerate the active metal species. Depending on the regioselectivity of the elimination (Ha or Hb), two dienes, 1,3- and/or 1,4-diene, can be obtained. The products of the latter case formally correspond to Alder-ene adducts (see Chapter 10.12). 

Zhang s group has reported highly enantioselective cycloisomerization processes catalyzed by rhodium(i) chiral complexes (Scheme 53). For instance, (A)-BINAP gives excellent asymmetric induction in the reaction of enediyne 212 to furnish the quasi-enantiopure Alder-ene product 213.219... 

C-C Bond Formation (Part 1) by Addition Reactions Alder-ene Reaction... 

Transition Metal-Catalyzed Intermolecular Alder-Ene Reactions 565... 

Transition Metal-Catalyzed Intramolecular Alder-Ene Reactions 568... 

Applications of the Alder-Ene Reaction to the Synthesis of Biologically Relevant... 

The ene reaction, an electronic relative of the Diels-Alder cycloaddition, is a six-electron process involving the reaction of the n and allylic reactions these reactions are reviewed in detail (for reviews of the Alder-ene reaction and related chemistry, see Refs 1,1a, and lb). Recent developments in the area of hetero-ene chemistry (X1 = H, X2 = N,0) are also surveyed. 

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