Diels-Alder reactions hexatriene

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

The combination of the spin-coupled formulation of modem valence bond theory with intrinsic reaction coordinate calculations provides easy-to-interpret models for the electronic rearrangements that occur along reaction pathways. We survey here the information revealed by such studies of the mechanisms of various gas-phase six-electron pericyclic reactions the Diels-Alder reaction between butadiene and ethene, the electrocyclization of cis-l,3,5-hexatriene, the 1,3-dipolar cycloaddition between fulminic acid and ethyne, and the 1,3-dipolar cycloaddition of diazomethane. The fully-variational CASVB strategy proves particularly efficient for such studies. 

During the Diels-Alder reaction [1] and in the electrocyclization of cis-1,3,5-hexatriene [2], bonds break and form in a homolytic fashion, with orbitals remaining associated with the same centres throughout the reaction. For such systems, there is a major recoupling of the electron spins. This last takes place most rapidly at or near the transition state. The resonance pattern, taken together with other characteristics, is reminiscent of the spin-coupled description... 

The Site Selectivity of Diels-Alder Reactions. Site selectivity is another kind of regioselectivity, in which a reagent reacts at one site (or more) of a polyfunctional molecule when several sites are, in principle, available. Thus butadiene reacts faster with the quinone 6.209 at C-2 and C-3 than at C-5 and C-6. The cyano groups will lower the coefficients at C-2 and C-3 more than those at C-5 and C-6. The dimer of hexatriene is 6.210 and not 6.211, which we can similarly explain by looking at the coefficients of the frontier orbitals, essentially narrowing the problem down to assessing the Zc2 term in Equation 3.4. 

The modification of polymers by means of Diels-Alder reactions has been previously described in the case of 13 -hexatriene, 1,3,5-heptatriene and 2,4,6-octatriene homopolymers >e ite the tra s-rra s-i, 3-diene structure of the repeat units of 1,6-polytrienes, which is known to react with dienophiles at a rate five times that of cis-trans dienes, the reactions were largely incomplete even after long periods of time when they were carried out at moderate temperature with strong dienophiles. 

Whereas inter- and intramolecular Diels-Alder reactions normally require electron-deficient dienophiles, the 67r-electrocyclization proceeds with a large vaiiety of substituents on a hexatriene. In one such approach, the intramolecular Heck-type reaction of a 2-bromo-1 -en-(ft> — l)-yne 66 is used as a trigger to initiate an intermolecular Heck coupling with an alkene to form the conjugated 1,3,5-hexatriene 67 which eventually cyclizes in a 67T-electrocyclic process (Scheme 3-21) [173]. In many cases, aromatization of the cyclohexadiene 68 formed primarily occurs to yield carbo- and heterobicyclic compounds of type 70 [173a,b]. But with alkyl ethenyl ethers the cyclohexadienes 69 can be obtained in moderate yields [173b]. 

Diels-Alder reaction. Sulfur dioxide adds 1,6 to cu-hexatriene in ether at room temperature to form the adduct 2,7-dihydrothiepin-l,1-dioxide (2).2 The reaction is analogous to the 1,4-addition of sulfur dioxide to butadiene (see Sulfolene, this... 

This scheme involves Diels-Alder reaction of olefin and butadiene as a main reaction, followed by successive dealkylation and/or dehydrogenation, and is in line with that of Wheeler and Wood (24). In this connection, the alternate proposal by Kinny and Crowley (10)—that a hexatriene intermediate is formed by polymerization and cyclics result from cycliza-tion of this triene—is considered less likely. 

Knoke, M. and de Meijere, A. (2003) A versatile access to l-cyclopropyl-2-aryl-1,3,5-hexatrienes - domino Heck-Diels-Alder reactions of l,3-dicyclopropyl-l,2-propadiene. Synlett, 195-8. 

To derive the frontier molecular orbital analysis for any [3,3]-shift, we ll use 1,5-hexatriene as the model, just as we used butadiene and ethylene as models for the frontier molecular orbital analysis of all Diels-Alder reactions. As always, in a frontier molecular orbital analysis, we first identify a proposed geometry for the reaction. Let s propose a chairlike transition state in which the carbons on the ends of the chain react from the top of one ir bond and the bottom of the other. 

The reactions of silenes with aldehydes and ketones is another area whose synthetic aspects have been particularly well-studied4,6 7 10 12. The favoured reaction pathways for reaction are generally ene-addition (in the case of enolizable ketones and aldehydes) to yield silyl enol ethers and [2 + 2]-cycloaddition to yield 1,2-siloxetanes (equation 44), but other products can also arise in special cases. For example, the reaction of aryldisilane-derived (l-sila)hexatrienes (e.g. 21a-c) with acetone yields mixtures of 1,2-siloxetanes (51a-c) and ene-adducts (52a-c) in which the carbonyl compound rather than the silene has played the role of the enophile (equation 45)47,50 52 98 99. Also, [4 + 2]-cycloadducts are frequently obtained from reaction of silenes with a,/i-unsaturated- or aryl ketones, where the silene acts as a dienophile in a formal Diels-Alder reaction6 29,100-102. 

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