4+2 Diels-Alder-like

On a first, very broad, approximation singlet oxygen behaves somehow like ethylene. Three types of reactions of 102 are usually observed and have been utilized in organic synthesis 606 608) a) the Diels-Alder like cycloaddition to dienes (6.1) b) the ene reaction with olefins (6.2) and c) cycloaddition to activated double bonds (6.3). 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

A new diastereoselective and enantioselective synthesis of a-amino-y-oxo acid esters has been reported involving the alkylation of enamines with acyliminoacetates (78). The stereocontrol is attributed to formation of a Diels-Alder like transition state (79). Ring opening of the adduct leads to a zwitterion or alkylated enamine, hydrolysis of which gives the single diastereoisomer (80 de > 96%)174 (Scheme 71). The use of a chiral ester [R = ( + )- or ( —)-menthyl or (—)-8-phenylmenthyl] converted this process into an enantioselective reaction (de and ee 24-67%). Since the reaction proceeds with complete anti-diastereoselectivity the two stereoisomers, enantiomeric at the two new stereogenic centres, could readily be separated by fractional crystallization. The main isomer of 80 (X = CH2), obtained in 80% yield, was shown to have the (l S, 2R)-configuration174. 

Cleavage of imidazoles dehydroamino acids,3 2,4-Disubstituted imidazoles4 undergo a Diels-Alder-like reaction with singlet oxygen in the presence of DBU (1-2 equiv.) to provide imine diamides, which isomerize to dehydroamino acid derivatives in the presence of base. Hydrogenation in the presence of catalysts with chiral phosphine ligands results in optically active amino acid diamides. The overall process is illustrated for the synthesis of the N-benzylamide of N-acetylleucine (equation I). 

X = CH or CPh) electrocyclic cleavage is more commonly observed. Of such examples the domino-Diels-Alder-like synthesis of semibullvalenes deserves... 

In a Diels-Alder-like (4 + 2) cycloaddition followed by a 1-3 H-shift the benzoxaphosphinine is formed (Scheme 55) <91BCJ713>. Originally (278) was formulated as a 1,2-oxaphosphetane <73AG(E)1010, 80CB3303). 

A few examples have been described of Diels-Alder-like reactions with imides formally derived from sulfur trioxide.58 N-Sulfonylimides are quite reactive in a [2 + 2] fashion with alkenes, but their ability to generally act as dienophiles is yet to be established. Sulfonylimines do react with highly oxygenated dienes to give adducts after hydrolysis [Eq. (35)]. 

Scheme 8. Synthesis of an unsymmetrical pyrazine via a Diels-Alder like reaction.

A quite different product (152) is formed in 70% yield by photolysis of diphenyl derivative (150). Formation of this compound is explained as a result of an electrocyclic conrotatory ring opening leading to (151) and subsequent Diels-Alder-like intramolecular cycloaddition of a carbonyl group to the aza diene moiety. The possibility of this unusual Diels-Alder-like reaction is explained by a favorable position of the CO group in (151) (Scheme 23) <86J0C3123). 

A Diels-Alder-like cycloaddition reaction, involving one As-N bond as the olefin, of l,3-dimethyl-l,3,2-diazarsolidinium tetrachlorogallate 14 with 2,3-dimethylbutadiene gave l,4,6,7-tetramethyl-l,2,3,4,5,8-hexahydro-[l,3,2]diazarsolo[l,2- ]-[l,2]azarsinin -ium tetrachlorogallate 15 (Scheme 7) <1996CJC2209, 1994AGE1267>. 

MO calculations have been carried out for the [4-f2] Diels-Alder-like cyclization of aminoboranes R2B = NR 2 with substituted m-1,3-butadienes <2003OM3748>, and for the [4-1-2] ene-type reactions between iminoboranes... 

The computational studies disclosed that Diels-Alder-like transition states could account for the high diastereoselectivity obtained in these allylic substitution reactions (eqn. (2) in Scheme 6.6). 

Cyclobutadiene undergoes rapid dimerization in a formal 2-1-2 fashion but more likely the reaction is a 4 -h 2, Diels-Alder-like process. Most interesting is the fact that dimerization cannot occur when cyclobutadiene is generated within the cavity of a hemicarcerand. ... 

The title reaction is not an isomerization but an interesting dehydrogenation reaction. 1,4-Cyclohexadiene undergoes dihydrogen loss with log k = 12.02 - 42100/23RTThe reaction would appear to be a syn elimination by a retro Diels-Alder-like pathway since ra/i5 -3,6-dideuterio-l,4-cyclohexadiene gives benzene containing 94% of only one deuterium (Scheme 7.44). 

A copper enolate is also regarded as an intermediate in a hetero-Diels-Alder-like sequence between a ketone and an analog of Danishefsky s diene, modified at silicon ii.e., (0Et)3 in place of Mes Scheme 1-12)J A fluoride-initiated (tetrabutylammonium difluorotriphenylsilicate [TBAT]) cleavage of the starting silyl enol ether leads to a nonracemically ligated (with Walphos) copper enolate, which condenses and then closes to the observed dihydropyranones in modest ee s. Noteworthy in this scheme is generation of the quaternary center. 

Evans pointed out that one would therefore expect the Diels-Alder reaction to be a concerted one-step process because the activation energy for this should be less than that involving an intermediate biradical. This prediction has proved to be correct. He also pointed out that a one-step Diels-Alder-like dimerization of ethylene to cyclobutane. 

So far we have considered only reactions in which the pericyclic ring contains an even number of atoms. Reactions of this kind are, however, known in which an odd-numbered ring is involved. A simple example is the Diels-Alder-like addition of 2-methylallyl cation (148) to cyclopentadiene (149) to form the methylbicyclooctyl cation (150). The transition state for this reaction is easily seen to be of Hiickel type (151) and so isoconjugate with tropylium. Since the allyl cation contains only two n electrons, we are dealing here with a six-electron system isoconjugate with the tropylium cation (147) and hence aromatic. In reactions of this kind, both the reactants and the transition state are odd. The reactions are therefore of 001 type. Since, moreover, the aromaticity or antiaromaticity of the transition state is again unrelated to the structures of the reactants or products, the reactions are of anti-BEP type and are consequently classed as 00 J. 

T1 an endo Diels-Alder like transition state model... 

CH = CH — CH = CH — are said to have conjugated double bonds and react somewhat differently from the other diolefins. For instance, bromine or hydrogen is often added so that a product of the type -CHBr-CH=CH-CHBr- is formed. Also, these hydrocarbons participate in the Diels-Alder reaction see diene reactions). They show a tendency to form rubber-like polymers. Hydrocarbons not falling into these two classes are said to have isolated double... 

In summary, it seems that for most Diels-Alder reactions secondary orbital interactions afford a satisfactory rationalisation of the endo-exo selectivity. However, since the endo-exo ratio is determined by small differences in transition state energies, the influence of other interactions, most often steric in origin and different for each particular reaction, is likely to be felt. The compact character of the Diels-Alder activated complex (the activation volume of the retro Diels-Alder reaction is negative) will attenuate these eflfects. The ideas of Sustmann" and Mattay ° provide an attractive alternative explanation, but, at the moment, lack the proper experimental foundation. 

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

The most effective Lewis-acid catalysts for the Diels-Alder reaction are hard cations. Not surprisingly, they coordinate to hard nuclei on the reacting system, typically oxygen atoms. Consequently, hard solvents are likely to affect these interactions significantly. Table 1.4 shows a selection of some solvents ranked according to their softness. Note that water is one of the hardest... 

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