Ethene Diels-Alder cycloaddition

As it happens, the frontier orbital interactions in the Diels-Alder cycloaddition shown above are like those found in the middle drawing, i.e., the upper and lower interactions reinforce and the reaction proceeds. The cycloaddition of two ethene molecules (shown below), however, involves a frontier orbital interaction like the one on the right, so this reaction does not occur. 

In the intermolecular series, Diels-Alder cycloaddition of ethene to the pyrazi-none heterodiene led to the expected bicyclic cycloadduct (Scheme 6.95 b) [195], The details of this transformation, performed in pre-pressurized reaction vessels, are described in Section 4.3.2 [196], Similar cycloaddition reactions have also been studied on a solid phase (Scheme 7.58) [197]. 

For example, the Diels-Alder cycloaddition results in an adduct of buta-1,3-diene and ethene, but the reaction cannot be described as an attachment, since bonds are formed between more than two centers.)... 

Heating the diethanolamine or bis(2-chloroethyl)amine hydrochlorides with aniline derivatives gave 1-arylpiperazine derivatives. The 1-substituted piperazines were deuteromethylated. 1,4-Dithiocarbonyl piperazine was obtained from reaction of benzaldehyde with piperazine and sulfur. l,4-Diacetylpiperazine-2,5-dione were reacted with aldehydes to give the monoarylidene products and then 73. 4-Benzylpiperazine-2,5-dione was reacted with bromobenzene to give 74. Diels-Alder cycloaddition of pyrazinone with ethene gave 75 . ... 

Recently, a tandem Diels-Alder Ramberg-Backlund reaction was developed by Block and co-workers.29 Using chloromethanesulfonyl-ethene (35) as the dienophile and cyclohexa-1,3-diene as the enophile, the Diels-Alder cycloaddition produced adduct 36. A subsequent Ramberg-B3cklund reaction then converted 36 to diene 37.29... 

S.l The Diels-Alder cycloaddition reaction that is obtained is formally the product of using ethene as the dienophile. 

Explain why intermolecular Diels-Alder cycloaddition reactions usually fail with unactivated dienophiles such as ethene. 

Diels-Alder cycloaddition of pyrazinone 687 with ethene in the presence of 1,2-dichlorobenzene (DCB) under MWI gave the bicyclic cycloadducts 688 in 86-89% yields within 40-140 min (Scheme 132). Under conventional heating, these reactions were carried out in an autoclave for 12 h (02JOC7904). 

In certain cases, multiple frontier orbital interactions must be considered. This is particularly true of cycloaddition reactions, such as the Diels-Alder reaction between 1,3-butadiene and ethene. 

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. 

The well-known Diels-Alder reaction [95,104-106] is a standard method for forming substituted cyclohexenes through the thermally allowed 4s + 2s cycloaddition of alkenes and dienes. In particular, the reaction between ethene and 1,3-butadiene to yield cyclohexene is the prototype of a Diels-Alder reaction (Scheme 28.4). It is now well recognized that this reaction takes place via a synchronous and concerted mechanism through an aromatic boatlike TS [105]. 

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

The SC descriptions of the electronic mechanisms of the three six-electron pericyclic gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition offulminic acid to ethyne [12], and the disrotatory electrocyclic ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments of pericyclic reactions, including the Woodward-Hoffmarm mles [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman model [4—6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is very much easier to interpret and to visualize directly. 

A Furan enters into a cycloaddition reaction with the dienophile diethyl acetylcnedicarboxylate to form an adduct. Catalytic reduction occurs preferentially at the more electron-rich double bond of the adduct, and the dihydro product, upon heating, undergoes a reverse Diels-Alder reaction to release ethene and the diesier (Scheme 6.29),... 

As can be seen in the intramolecular cycloaddition (Section 8.03.5.1), the intermolecular Diels-Alder reactions between functionalized 2(l/f)-pyrazinones 83 and dimethyl acetylenedicarboxylate (DMAD) forming bicyclo adducts 84 has been shown to be significantly rate enhanced and increased in yields by using controlled microwave irradiation compared to the conventional thermal protocols (Scheme 21) <2002JOC7904>. The microwave-assisted Diels-Alder reactions of substituted 2(l//)-pyrazinones with ethene are significantly more effective utilizing prepressurized (up to 10 bar) reaction vessels <20040BC154>. 

The Diels-Alder reaction is both a 1,4 addition of ethene to 1,3-buta-diene and a 1,2 addition of butadiene to ethene. It can be called a [4 + 2] cycloaddition and as such results in the formation of a six-membered ring. Many other cycloadditions are known, such as [2 + 1], [2 + 2], [3 + 2], [4+1], [2 + 2 + 2], and so on, which give different sizes of rings. Some specific examples follow ... 

The left division corresponds to a simple [4 + 2] cycloaddition, whereas the right division corresponds to a complex reaction involving formation of three ring bonds at once. Actual Diels-Alder reactions require diene and dienophile starting materials, and two possibilities, using 1,3-cyclopentadiene as the diene and ethene or ethyne as dienophile, follow ... 

The cycloaddition reactions that we have discussed so far in this chapter ([2 + 2], [4 + 2], etc.) have involved ring formation by bringing two unsaturated molecules together. Thus [4 -r 2] addition is represented by the Diels-Alder reaction of ethene and 1,3-butadiene ... 

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