Dienes unsymmetric

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

Unsymmetrical dienes in this synthesis are often capable of high regioselectivity (eqs. 4 and 5) (82). Reaction of (81) with 2-methoxycarbonyl-l,4-ben2oquinone [3958-79-0] yields 97% of (82) [80328-15-0]. Reaction of (81) with 2,3-dicyano-l,4-ben2oquinone [4622-04-2] yields 58% of (83) [80328-16-1]. 

HCl addition to unsymmetrical dienes can be even more complicated. For example, HCl addition to isoprene (2-methyl-1,3-butadiene) might give four different 1,2-addition products and three different 1,4-addition products. 

From reaction of an unsymmetrically substituted diene 11a, b and dienophile 12, different regioisomeric products 13a, b and 14a, b can be formed. The so-called ortho and the para isomer 13a, resp. 13b, is formed preferentially. 

However, if the azepine is C-monosubstituted, e.g. 14, or unsymmetrically substituted, then two isomeric 2-azabicycloheptadienes, e. g. 15 and 16, may result corresponding to electrocyclic ring closure involving C2-C5 or C4-C7 of the azepine ring. In practice, the ratio of the two isomers formed (which may be separated by vapor phase chromatography) varies with the position of the substituent.236 In contrast, irradiation of methyl 2,5-di-tm-butyl-l//-azepine-l-carboxylatein methanol yields only methyl 3,5-di-tert-bulyl-2-azabicycIo[3.2.0]hepta-3,6-diene-2-carboxylate (81 %).70... 

When used in conjunction with unsymmetrical dienes with substituents in the 2-position, the term tail addition has been used to refer to addition to the methylene remote from the substituent. Head addition then refers to addition to... 

Birnbaum, G. I. (1972) Structure of the unsymmetrical 12-oxa[4.4.4jpropella-2,4,7,9-tetraene bis(iron tricarbonyl). An example of an oxygen-diene interaction through space, J. Am. Chem. Soc. 94,2455-2459. 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

When an unsymmetrical diene reacts with an unsymmetrical dienophile, two regioisomer adducts can be formed depending on the orientation of the substituents in the adduct [37] (Equations 1.16 and 1.17). 

The orbital interaction depicted in Scheme 1.15 shows that the two cr-bonds form at the same time but do not develop to the same extent. The Diels-Alder cycloaddition of unsymmetrical starting materials is therefore concerted but asynchronous. A highly unsymmetrical diene and/or dienophile give rise to a highly unsymmetrical transition state and a stepwise pathway can be followed. 

Conjugated dienes can be acylated by treatment with acyl- or alkylcobalt tetracarbonyls, followed by base-catalyzed cleavage of the resulting jt-allyl carbonyl derivatives. The reaction is very general. With unsymmetrical dienes, the acyl... 

If the diene is unsymmetrical, there may be two 1,2-addition products. The competition between two types of addition product comes about because the carbocation resulting from attack by Y is a resonance hybrid, with partial positive charges at the 2 and 4 positions ... 

When an unsymmetrical diene adds to an unsymmetrical dienophile, there are two regioisomeric products (not counting stereoisomers) ... 

Keywords Facial selection. Orbital phase, Secondary orbital interaction. Orbital unsymmetrization. Ketones, Olefins, Diels-Alder dienophiles, Diels-Alder dienes, Michael acceptor. Amine nitrogen atom... 

In this review we will focus on the unsymmetrization of the orbital phase environment in the vicinity of reacting n systems, and its effect on facial selectivities. This idea can be applied to many kinds of recently observed facial selectivities, such as those involving ketones [10-21], olefins [22-31], dienes [32-46] and others [47-49]. 

Diels-Alder cycloadditions involving norbomene 57 [34], benzonorbomene (83), 7-isopropylidenenorbomadiene and 7-isopropylidenebenzonorbomadiene (84) as dienophiles are characterized as inverse-electron-demand Diels-Alder reactions [161,162], These compounds react with electron-deficient dienes, such as tropone. In the inverse-electron-demand Diels-Alder reaction, orbital interaction between the HOMO of the dienophile and the LUMO of the diene is important. Thus, orbital unsymmetrization of the olefin it orbital of norbomene (57) is assumed to be involved in these top selectivities in the Diels-Alder cycloaddition. 

The sterically unbiased dienes, 5,5-diarylcyclopentadienes 90, wherein one of the aryl groups is substituted with NO, Cl and NCCHj), were designed and synthesized by Halterman et al. [163] Diels-Alder cycloaddition with dimethyl acetylenedicarbo-xylate at reflux (81 °C) was studied syn addition (with respect to the substituted benzene) was favored in the case of the nitro group (90a, X = NO ) (syrr.anti = 68 32), whereas anti addition (with respect to the substituted benzene) is favored in the case of dimethylamino group (90b, X = N(CH3)2) (syn anti = 38 62). The facial preference is consistent with those observed in the hydride reduction of the relevant 2,2-diaryl-cyclopentanones 8 with sodium borohydride, and in dihydroxylation of 3,3-diarylcy-clopentenes 43 with osmium trioxide. In the present system, the interaction of the diene n orbital with the o bonds at the (3 positions (at the 5 position) is symmetry-forbidden. Thus, the major product results from approach of the dienophile from the face opposite the better n electron donor at the (3 positions, in a similar manner to spiro conjugation. Unsymmetrization of the diene % orbitals is inherent in 90, and this is consistent with the observed facial selectivities (91 for 90a 92 for 90b). 

Abstract Diels-Alder reaction is one of the most fundamental and important reactions for organic synthesis. In this chapter we review the smdies of the rr-facial selectivity in the Diels-Alder reactions of the dienes having unsymmetrical rr-plane. The theories proposed as the origin of the selectivity are discussed. 

It becomes intriguing to inquire what leads to the observed contrasteric reactivity. Intensive studies to disclose the origin of Tt-facial selectivity examined various dienes having unsymmetrical 7t-plane, since their reactions potentially generate five or more consecutive stereocenters with one operation. In this chapter, we review the theories to disclose the origin of 7t-facial selectivity in Diels-Alder reactions of the substrates having unsymmetrical 7t-planes. Recent works are discussed. 

Ohwada extends his theory, unsymmetrization of n orbitals, to Orbital Phase Environment including the secondary orbital interaction (Chapter Orbital Phase Environments and Stereoselectivities by Ohwada in this volume). The reactions between the cyclopentadienes bearing spiro conjugation with benzofluorene systems with maleic anhydride exemplified the importance of the phase environment. The reactions proceed avoiding the out-of-phase interaction between dienophile LUMO and the HOMO at the aromatic rings. The diene 34 with benzo[b]fluorene favored syn addition with respect to the naphtalene ring, whereas the diene 35 with benzo[c]fluorene showed the reverse anti preference (Scheme 22) [28]. 

Overman, Hehre and coworkers reported anti rr-fadal selectivity in Diels-Alder reactions of vinylcyclopenten 73, 74 and 4,5-dihydro-3-etliynylthiophen S-oxide 75 [38] (Scheme 31). These results are not in agreement with the Cieplak effect, at least in Diels-Alder reactions of the dienes having unsymmetrical rr-plane. Yadav and coworkers reported that the reactions between the vinylcyclohexene 76 and dienophiles favor the reactions syn to oxygen, while 77 and 78 favor the reaction anti to oxygen substituents [39], They discuss the Cieplak effect but the reactions are not suitable. 

Anti TT-facial selectivity with respect to the sterically demanded substituent in the Diels-Alder reactions of dienes having unsymmetrical tt-plane has been straightforwardly explained and predicted on the basis of the repulsive interaction between the substituent and a dienophile. However, there have been many counter examples, which have prompted many chemists to develop new theories on the origin of 7t-facial selectivity. We have reviewed some theories in this chapter. Most of them successfully explained the stereochemical feature of particular reactions. We believe that the orbital theory will give us a powerful way of understanding and designing of organic reactions. 

In the case of an unsymmetrical diene such as isoprene, different orientations of the structural units are possible depending on which end of the diene unites with the chain radical. Of the two competing reactions (6) and (7) shown on page 241, the former would appear to be the more probable one on account of the influence of the methyl substituent in stabilizing to some extent one of the resonance hybrid structures which are shown. 

Frontier orbital theory can also explain the regioselectivity observed when both the diene and alkene are unsymmetrically substituted.4 Generally, there is a preference... 

It was also well established that silenes could take part as the dienophile in Diels-Alder reactions. In many cases, particularly with unsymmetric dienes such as isoprene, the reactions were not clean because, in addition to formation of the [2+4] cycloadduct 61, the possibility exists for the formation of it regioisomer 62, products of an ene reaction 63, and conceivably the [2+2] cycloaddition product 64, as shown in Eq. (23). Wiberg... 

With unsymmetrical dienes (74a and 74b) and unsymmetrical adducts, the problem of orientation of addition (cf. p. 184) arises. Initial attack will still be on a terminal carbon atom of the conjugated system so that a delocalised allylic intermediate is obtained, but preferential attack will be on the terminal carbon that will yield the more stable of the two possible cations i.e. (75) rather than (76), and (77) rather than (78) ... 

Cycloaddition of unsymmetrical 1,3-dienes, such as isoprene 41b, proceeded regiospecifically to give only a single regioisomer 44 (see Equation (13) and Table 7). 

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