Cyclopentadiene, Diels- Alder cycloaddition reaction

In contrast to those unreactive dienes that can t achieve an s-cis conformation, other dienes are fixed only in the correct s-cis geometry and are therefore highly reactive in the Diels-Alder cycloaddition reaction. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cycIopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self Diels-Alder reaction. 

Cyclopentadiene is very reactive in Diels-Alder cycloaddition reactions, but 1,3-cyclohexadiene is less reactive and 1,3-cycloheptadiene is nearly inert. Explain. (Molecular models are helpful.)... 

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

A rapid access to carbocyclic nucleosides, containing a fused isoxazoline ring has been proposed, starting from cyclopentadiene. The route involves a het-ero Diels-Alder cycloaddition reaction of nitrosocarbonylbenzene followed by a 1,3-dipolar cycloaddition of nitrile oxides, cleavage of the N-0 tether and transformation of the heterocyclic aminols into nucleosides via construction of purine and pyrimidine heterocycles (457). 

The phosphino-oxazoline copper(II) complex (55) has also been found to be an effective catalyst[136] as have some titanium complexes, such as the extensively researched titanium-TADDOL system (56)[137]. A modified Ti(IV)-TADDOL compound is the catalyst of choice to promote Diels-Alder cycloaddition reactions between cyclopentadiene and alk-2-enyl phenylsulfonylmethyl ketones[138]. 

Oppolzer sultam-like chiral auxiliary (e.g., Xc in 304) has been studied in Diels-Alder cycloaddition reactions (Scheme 43) <2003JP0700>. The TiCU-promoted reaction of dienophile 304 and 1,3-cyclopentadiene 305 in DCM is complete within 18h and excellent diastereoselectivity of product 306 is observed. The same reaction in the absence of Lewis acid provides product 306 in very low yield. However, switching to trifluoroethanol as the solvent, the cycloaddition reaction proceeds to completion, albeit with slightly diminished levels of diastereoselectivity for Diels-Alder adduct 306. Surprisingly, the use of hexane as the solvent affords the opposite (23, J 31-diastereomer of 306 as the major product. 

An AMI semiempirical method was used to investigate the Diels-Alder cycloaddition reactions of vinyl sulfenes with buta-1,3-dienes.156 The reactivity and stereoselectivity of vinyl boranes have been reviewed.157 Aromatic methyleneamines undergo reverse-electron-demand Diels-Alder reactions with cyclopentadiene, norbom-ene, and vinyl sulfides.158... 

The cyclopent-2-enone required for the photodimerisation is prepared by the hydrolysis and oxidation of 3-chlorocyclopentene, which is obtained by the low temperature addition of hydrogen chloride to cyclopentadiene. The latter is obtained by heating dicyclopentadiene. This depolymerisation is an example of a reverse (or retro) Diels-Alder cycloaddition reaction the diene readily reforms the dicyclopentadiene on standing at room temperature. 

Table 5-1. Comparison of the activation energies and specific rate constants for the bimolecular Diels-Alder cycloaddition reaction of cyclopentadiene giving endo-dicyclopentadiene in the gas phase and in solution at 20 °C [3, 33, 34],...

Solvent-controlled diastereoselectivities have also been observed in Diels-Alder cycloaddition reactions of cyclopentadiene with bis-(—)-menthyl fumarate [707] and with the acrylate of (S)-ethyl lactate, CH2=CH-C0-0CH(CH3)-C02Et [708]. In the latter reaction, giving four diastereomeric cycloadducts, diastereoselectivities of up to 85 15 have been obtained in n-hexane [708], The diastereoselectivities decrease with increasing solvent polarity, while the endojexo selectivity increases. This is in agreement with the pattern found for simple achiral acrylates [124] cf. Eq. (5-43) in Section 5.3.3. We shall conclude this section with reference to the E)I Z) isomerization of R ... 

Diels-Alder Cycloaddition Reactions. Catalysts 1 and 2 facilitate asymmetric Diels-Alder cycloaddition reactions between propenyl oxazolidinones (and thio-oxazolidinones) and dienes such as cyclopentadiene (eq 2). Both 1 and 2 display sim-... 

A chiral zw a-metallocene triflate complex catalyzes the Diels-Alder cycloaddition reaction between an oxazolidinone-based dienophile and cyclopentadiene [206]. Triflate in titanocene and zirconocene complexes is labile [207,208] and thus the polarity of solvent influences the reactivity and enantioselectivity. Asymmetric hydrogenation of imines and enamines catalyzed by chiral aw a-titanocene catalyst provides amines with high enantioselectivity [209,210]. 

The butadienylphosphonium salts (120) are formed in the reactions of phosphines with alkenoyl bromides. Diels-Alder cycloaddition reactions of vinyltriphenylphosphonium bromide with cyclopentadiene and anthracene derivatives have been used (together with conventional quaternization procedures)... 

Arynes, such as benzyne (1,2-dehydrobenzene), also undergo Diels-Alder cycloaddition reactions. Benzyne, CelTj, is a highly reactive species and can be prepared by elimination of a suitably substituted benzene derivative. It reacts in situ with various dienes such as furan, cyclopentadiene, cyclohexadiene and even benzene and naphthalene to give bicyclic or polycyclic cycloadducts (3.15). Analogous addition reactions are shown by dehydroaromatics in the pyridine and thiophene series. 

In all of these examples, the reaction products are under kinetic control. That is, the product mixture is determined by which product is made fastest rather than which is thermodynamically most stable. The Diels-Alder cycloaddition reaction gives another example of this phenomenon. The reaction of methyl acrylate with cyclopentadiene gives a mixture of two endo and exo products (see Scheme 2.5). 

Lee and co-workers reported camphor sulfonyl hydrazines 54 as organocatalysts in enantioselective Diels-Alder reactions of cyclopentadiene and a,[L-unsaturated aldehydes, Scheme 3.21 [34], On the other hand, a similar Diels-Alder cycloaddition reactions with camphor-derived sulfonylhydrazines and HC10 in MeNO were also developed by Langlois, et al. [35]. 

Various approaches to the assembly of the 1,3-dienes that engage in the Diels-Alder cycloaddition reactions leading to substrates for DPM and ODPM processes have been reported. Thus, for example. Yen and Liao demonstrated, during the course of a total synthesis of the Lycopodium alkaloid magellanine, that oxidation of acetovanillone (109) rScheme 9.151 with diacetoxyiodobenzene (DAIB) in the presence of methanol afforded the o-benzoquinone monoketal 110. The latter conpound engaged in an in situ Diels-Alder reaction with added cyclopentadiene (111) and the resulting adduct 112 proved to be an excellent substrate for the ODPM rearrangement reaction. Thus, photolysis of 112 as a solution in acetone afforded the pivotal tetracyclic diketone 113 in 92% yield. 

Figure 1.15 Modern variants of the Diels-Alder cycloaddition reaction (a) Inverse-electron-demand Diels-Alder (lEDA) reaction between tetrazine and norbornene/ and (b) hetero Diels-Alder (HDA) reaction between thiocarbonyl and cyclopentadiene. ...

The Diels-Alder cycloaddition reaction is a concerted reaction, with no change in the charge density upon going from reactants to the activated complex and finally to the products. Therefore, from the Hughes-Ingold approach, litde solvent effect is expected upon these reactions. In spite of this, dramatic solvent effects are often observed when a non-symmetric dienophile is used, and a mixture of products is formed. For instance, the reaction of methyl acrylate with cyclopentadiene gives a mixture of two products (see Scheme 10.61. 

An extensive review describing the use of chiral hetero-substituted buta-1,3-dienes in stereoselective Diels-Alder cycloaddition reactions has been published. The axially chiral ligand l,T-(2,2 -bisacylamino)binaphthalene is effective in the ytterbium-catalysed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-1,3-... 

The Diels-Alder cycloaddition reaction with cyclopentadiene carried out by Barhoumi-Slimi et with Z-ethyl-2-chloro-3-formylactylate... 

The ionic aza-Diels-Alder cycloaddition reactions of cyclopentadiene and pro-tonated glyoxylate imines possessing two chiral auxiliaries, N-(R) or N-(S)-l-phenylethyl and 8-phenylmenthyl or 8-phenyln omenthyl, yielded optically pure... 

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