Reactions of Diene-derived Anions

Bimolecular ion/molecule reactions of dienes and polyenes have been extensively studied for several reasons. Some of them have been mentioned implicitly in the previous sections, that is, in order to structurally characterize the gaseous cations derived from these compounds. In this section, bimolecular reactivity of cationic dienes, in particular, with various neutral partners will be discussed, and some anion/molecule reactions will be mentioned also (cf Section IV). In addition, the reactions of neutral dienes with several ionic partners will also be discussed. Of this latter category, however, the vast chemistry of reactions of neutral dienes with metal cations and metal-centred cations will not be treated here. Several reviews on this topic have been published in the last decade178. 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

Nucleophilic attack occurs at C(2) of the diene. The 1,3-cyclohexadiene complex 66 is converted to the homoallyl anionic complex 67 by nucleophilic attack, and the 3-alkyl-1-cyclohexene 68 is obtained by protonation. Insertion of CO to 67 generates the acyl complex 69, and its protonation and reductive elimination afford the aldehyde 70 [20]. Reaction of the butadiene complex 56 with an anion derived from ester 71 under CO atmosphere generates the homoallyl complex 72 and then the acyl complex 73 by CO insertion. The cyclopentanone complex 74 is formed by intramolecular insertion of alkene, and the 3-substituted cyclopentanone 75 is obtained by reductive elimination. The intramolecular version, when applied to the 1,3-cyclohexadiene complex 76 bearing an ester chain at C(5), offers a good synthetic route to the bicyclo[3.3.1]nonane system 78 via intermediate 77 [21]. 

However, reaction of acyclic dienamines with hydrazoic acid gives a mixture of products derived by 1,2-, 1,4- and 3,4 + 1,2-addition of HN3 to the diene system. In this case C-protonation is followed immediately by addition of the strongly nucleophilic azide anion, so that equilibrium of the C-protonated enamines cannot occur3c. Treatment of the morpholine dienamine of isophorone with trichloroacetic acid in boiling benzene resulted in decarboxylation and the 1,4-addition of a proton and the trichloromethyl anion. Basic hydrolysis of the adduct gave dienoic acid 54 (Scheme 4). 

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

There are a number of examples of chiral ferrocene derivatives prepared by reaction of substituted cyclopenta-diene anions with FeCl2 or a monocyclopentadienyl iron species rather than using the strategy of introduction and modification of a side chain on ferrocene itself. Addition... 

It was determined that carbon nucleophiles derived from carbon acids with p/fa > 22 or so are sufficiently reactive to combine with the diene ligand rapidly at —78°C to produce an anionic intermediate (Scheme 25). With a few exceptions, the regioselectivity favors formation of the homoallyl anionic complex from addition at C-2, by kinetic control. This intermediate can be quenched with protons to give the terminal alkene, or can react with excess CO to produce an acyl iron intermediate. Following the recipes of Collman s reaction, the acyl iron intermediate can lead to methyl ketones, aldehydes, or carboxylic acids. The processes are illustrated with the 1,3-cyclohexadiene complex (Scheme 25). ... 

Later, the dimethylphenylsilyl adduct 75 d was not only converted to the vinyl-ketene acetal 81 by a Wittig-Horner reaction via the ester 80, but also by reduction of 80 to the aldehyde 84 followed by silylation of the corresponding anion to the silylenol ether 85. Vinylcyclohexenes without terminal substitution (e. g., 83) was prepared via an alcohol derived from 80 or 84 [73,74]. The least substituted diene 83 was alternatively prepared from the triflate 82 in a Stille coupling with trialkylvinyl stannane [75] (Scheme 24), a reaction also used by Toshima etal.[76]. 

The radical anions of buta-1,3-diene and its methyl substituted derivatives are studied electrolytically in liquid ammonia [393] or in tetrahydrofuran [394]. In general, reaction of these compounds with an alkali metal in ethereal solvents fails to... 

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