Reactions of Dienes

The di-77-methane reaction of diene (79) afforded the tricyclic product (80).57 

A theoretical treatment of the photochemical Diels-Alder reaction has been published.58 Pusset and Beugelmans have measured the fluorescence spectra and singlet energies of several conjugated dienes (81).59 

The text of a lecture on the benzophenone-sensitized c/j-Zraws-photochemistry of penta-1,3-diene and hexa-2,4-diene has been published.80 A quantum-chain process is described. Wrighton and Schroeder61 have reported the photochemical hydrosilylation of dienes by irradiation in the presence of Cr(CO) and a silane. The following example is typical  

Bigwood and Boue 2 have examined the reaction of penta-1,3-dienes with trimethyltin hydride and have found that the irradiation not only yields cyclobutene but also gives the two addition products (82) and (83). The absence of 

2-addition products suggests that the addition is concerted. The authors 2 therefore suggest that a doubly twisted excited state (84) is preferred to the allylmethylene state which had been suggested at an earlier date for the conformation of the relaxed Sx state of dienes. 3 The divinylsilanes (85) have been 

Let this example serve as a warning. The rule that states that the electrophile adds to the sp carbon bonded to the most hydrogens cannot be applied to reactions that form carboca-tions that can be stabilized by electron delocalization. In such cases, you must look at the relative stabilities of the individual carbocations to predict the major product of the reaction. 

What is the major product obtained from the addition of HBr to the following compound  

Predict the sites on each of the following compounds where protonation can occur, a. CH3CH=CH0CH3 + H+ b. + H+ 

Solution to 36a The resonance contributors reveal that there are two sites that can be protonated the lone pair on oxygen and the lone pair on carbon. 

For another example of how delocalized electrons can affect the product of a reaction, we will compare the products formed when isolated dienes (dienes that have only localized electrons) undergo electrophilic addition reactions to the products formed when conjugated dienes (dienes that have delocalized electrons) undergo the same reactions. 

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Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

Oxidative reactions of dienes are accomphshed under similar conditions as those of alkenes. Abicydic diene synthesized from hexafluorobenzene and 1,2-di-chloroethylene is monoepoxidized by triflnoroperoxyacetic acid [43] (equation 35). 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

Photolytic reactions of dienes frequently give complex mixtures of rearranged products. Described here, however, is a photolytic isomerization of 1,5-cyclooctadiene (present in solution, in part, as a complex with cuprous chloride) that affords a good yield of one product. 

Figure 3.82 (a) The effect of or/Ao-substituents on substitution reactions of cis-Pt(PR3)2ArCl complexes (b) the effect of alkyl substituents on substitution reactions of dien complexes. 

The Diels-Alder reactions of dienes 11 and 12 with many dienophiles allowed other fluorinated aromatics to be synthesized [12,13]. For example, diene 11 reacted with dimethylacetylenedicarboxylate and ethylpropiolate (Scheme 2.8) to give trifluoromethyl diethylphthalate and trifluoromethylethylbenzoate, and diene 12 with/ -benzoquinone affords 5-fluoronaphthoquinone (Equation 2.5). 

Tamariz and coworkers [42] have described a versatile, efficient methodology for preparing N-substituted-4,5-dimethylene-2-oxazolidinones 42 (Figure 2.5) from a-diketones and isocyanates and have also studied their reactivity in Diels-Alder reactions. This is a method for synthesizing polycyclic heterocyclic compounds. Some of the reactions of diene 42 are summarized in Scheme 2.18. The nitrogen atom seems to control the regiochemistry of the reaction. 

Dihydro-1-vinylnaphthalene (67) as well as 3,4-dihydro-2-vinylnaphtha-lene (68) are more reactive than the corresponding aromatic dienes. Therefore they may also undergo cycloaddition reactions with low reactive dienophiles, thus showing a wider range of applications in organic synthesis. The cycloadditions of dienes 67 and 68 and of the 6-methoxy-2,4-dihydro-1-vinylnaphthalene 69 have been used extensively in the synthesis of steroids, heterocyclic compounds and polycyclic aromatic compounds. Some of the reactions of dienes 67-69 are summarized in Schemes 2.24, 2.25 and 2.26. In order to synthesize indeno[c]phenanthrenones, the cycloaddition of diene 67 with 3-bromoindan-l-one, which is a precursor of inden-l-one, was studied. Bromoindanone was prepared by treating commercially available indanone with NBS [64]. 

Table 5.2 High pressure Diels-Alder reactions of diene 77 with dienophiles 78...

Indium trichloride [30] and methylrhenium trioxide [31] catalyze the aqueous Diels-Alder reaction of acrolein and acrylates with cyclic and open-chain dienes. Some examples of the cycloaddition of methyl vinyl ketone with 1,3-cyclohexadiene are reported in Scheme 6.18. MeReOs does not give satisfactory yields for acroleins and methyl vinyl ketones with substituents at the jS-position and favors the self-Diels-Alder reaction of diene. 

According to the calculations at high levels of theory, the [4+2] cycloaddition reactions of dienes with the singlet ( A oxygen follow stepwise pathways [37, 38], These results, which were unexpected from the Woodward-Hoffmann rule and the frontier orbital theory, suggest that the [4+2] cycloadditions of the singlet ( A oxygen could be the reactions in the pseudoexcitation band. 

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. 

Diels-Alder reactions in supercritical water have also been investigated.57 Kolis has shown that Diels-Alder reactions of dienes with various electron-poor dienophiles can be performed in supercritical water with high yields of the desired product without the addition of... 

The reaction of dienes bearing an A-dienyl lactam moiety with activated olefins was examined by Smith.94 The lactams were excellent enophiles and provided exclusively the ortho regioisomer with good selectivity for the endo (Z) product (Eq. 12.33). 

The aqueous Diels-Alder reaction has also been used for bioconjugate studies. A Diels-Alder reaction of diene oligonucleotides with maleimide dieneophiles was used to prepare oligonucleotide conjugates in aqueous media under mild conditions (Eq. 12.39).102 A Diels-Alder-type cycloaddition of an electronically matched pair of saccharide-linked conjugated dienes and a dienophile-equipped protein was the... 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

Reaction of dien, dien = diethylene-triamine, with IrCl3-1.51120 yielded [Ir(dien)2]Cl3.157 Separation of the mer-isomer was effected by chromatography on SP-Sephadex, with an X-ray structure... 

Tab. 7.1 Diels-Alder reactions of dienes 1-3 using the GS/MW process [15, 16]. 

This chapter deals with thermal ring-closure reactions of dienes and polyenes resulting in carbocyclic compounds the formation of heterocycles is mentioned only occasionally. The account is highly selective, concentrating on recent work, since two comprehensive general reviews have appeared1,2. Other pertinent reviews are cited at appropriate places in the text. 

Processes with gaseous reactants are excluded here. Due to the large compressibility of gases an increase of pressure (up to 1 kbar) leads essentially only to an increase of gas concentration, and hence to an acceleration of bimolecular processes in which gases are involved as reactants. The effect of pressure on a chemical reaction in compressed solution is largely determined by the volume of reaction (AV) and the volume of activation (AV ). It is not the purpose of this chapter to provide a complete survey of reactions of dienes and polyenes which have been investigated at elevated pressures. There are many excellent monographs (e.g. References 1-4) and reviews (e.g. References 5-16) on this topic which cover the literature up to early 1990. After a short introduction into the basic concepts necessary to understand pressure effects on chemical processes in compressed solutions, our major objective is to review the literature of the past ten years. 

The partially hydrogenated phenanthrene derivative 18 (entry 4) is a very moderate diene due to the steric crowding caused by the substituents and the anulated rings, and it reacts even with highly reactive dienophiles such as maleic anhydride (MA) or N-phenylmaleic imide only at high pressure. The minor product 20 in the reaction with MA obviously stems from diene 21. This can be explained by a double-bond isomerization 18 - 21 prior to the cycloaddition, certainly catalyzed by traces of acid present in the MA. In the absence of acid only the Diels-Alder adduct 22 derived from diene 18 was observed. In the reaction of diene 23 with MA (entry 5) a similar sequence of steps was observed. A [1,5] shift of the C—O bond in 23, again certainly acid-catalyzed, produces the diene 26 followed by the Diels-Alder reaction with MA to give 24 and 25. 

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