P dianion

A higher-order cyanocuprate, R2Cu(CN)Li2, reacts with a,P-unsaturated acylzirconocene chlorides at — 78 °C in THF to afford saturated ketones without giving a Michael-type product (Scheme 5.39). Treatment of the reaction mixture with D20 gives the [Pg.174]

Compounds of this type can serve as a synthon for an a,p-dianion of cyclohexanone. 

Table 2 lists examples of monoalkylation reactions of ketone a,p-dianions. The dianions always react with alkylating agents at the /3-carbon atom to generate the corresponding lithium enolates, which can be quenched by water or electrophiles such as TMSC1 and PhSSPh. The last two examples demonstrate the results of the single alkylation reaction of a ketone a,5-dianion with hexyl and allyl bromides, which occurs at the 5-position to give lithium Z-enolates. 

The double deprotonation of 2-arylnitroethanes followed by treatment with electrophiles led to 2-sub-stituted-2-arylnitroethanes.73 This fact suggested that the strong base sequentially abstracted a- and then P-protons of these substrates giving a,P-dianions (57). The same behavior has been observed with primary nitroalkanes having a vinyl or carbonyl group on the P-carbon. If there is only one a-nitro CH as in open-chain and cyclic secondary nitroalkanes, bis(lithioxy)enamines (58) and (59) (dianion derivatives of a-nitroalkenes, also called super enamines) are generated and exclusively those with a terminal double bond in the case of 2-nitroalkanes (equations 22 and 23). These dianion derivatives react with aromatic and aliphatic aldehydes as well as ketones to give 1,3-difunctional derivatives. In contrast to the lack of... 

The formation of the complex between the free base (PH2) and the metal ion has been extensively studied. It was found that the complicated metal incorporation mechanism can be best interpreted for the reaction of the dipositive metal ion and the neutral chelating ligand. However, this does not mean that the anionic porphyrin species (PH and P ) are not reactive. In fact, they are present in very small concentrations, so small that they can hardly be detected. On the other hand, the cationic porphyrin species are definitively unreactive. Earlier works, of some twenty years ago, preferred the mechanism of metal incorporation which assumes a pre-equilibrium between the free base (PH2) and the dissociated protons of pyrrole nitrogens, so that the metal cation would actually react with the P " dianion. Later works, however, indicate the existence of the so-called SAT (sitting-atop) complex intermediate, which was already foreseen in 1960 by Fleischer and Wang. It was later shown that the SAT metal-ion intermediate can deform porphyrin, thus... 

Ryu I, Ikebe M, SonodaN, Yamato S, Yamamura G, Komatsu M (2000) Conjugate addition of zincates derived fiorn ketone o, p-dianions to enones. An access to unsymmetrical 1,6-diketones. Tetrahedron Lett 41 5689 5692. doi 10.1016/S0040-4039(00)00925-4... 

Epoxides provide another useful a -synthon. Nucleophilic ring opening with dianions of carboxylic acids (P.L. Creger, 1972) leads to y-hydroxy carboxylic acids or y-lactones. Addition of imidoester anions to epoxides yields y-hydroxyaldehyde derivatives after reduction (H.W. Adickes, 1969). 

The addition of acetylides to oxiranes yields 3-alkyn-l-ols (F. Sondheimer, 1950 M.A. Adams, 1979 R.M. Carlson, 1974, 1975 K. Mori, 1976). The acetylene dianion and two a -synthons can also be used. 1,4-Diols with a carbon triple bond in between are formed from two carbonyl compounds (V. Jager, 1977, see p. 52). The triple bond can be either converted to a CIS- or frans-configurated double bond (M.A. Adams, 1979) or be hydrated to give a ketone (see pp. 52, 57, 131). 

Cydopentane reagents used in synthesis are usually derived from cyclopentanone (R.A. Ellison, 1973). Classically they are made by base-catalyzed intramolecular aldol or ester condensations (see also p. 55). An important example is 2-methylcydopentane-l,3-dione. It is synthesized by intramolecular acylation of diethyl propionylsucdnate dianion followed by saponification and decarboxylation. This cyclization only worked with potassium t-butoxide in boiling xylene (R. Bucourt, 1965). Faster routes to this diketone start with succinic acid or its anhydride. A Friedel-Crafts acylation with 2-acetoxy-2-butene in nitrobenzene or with pro-pionyl chloride in nitromethane leads to acylated adducts, which are deacylated in aqueous acids (V.J. Grenda, 1967 L.E. Schick, 1969). A new promising route to substituted cyclopent-2-enones makes use of intermediate 5-nitro-l,3-diones (D. Seebach, 1977). 

Nonanedione, another 1,3-difunctional target molecule, may be obtained from the reaction of hexanoyl chloride with acetonide anion (disconnection 1). The 2,4-dioxo substitution pattern, however, is already present in inexpensive, symmetrical acetylacetone (2,4-pentanedione). Disconnection 2 would therefore offer a tempting alternative. A problem arises because of the acidity of protons at C-3 of acetylacetone. This, however, would probably not be a serious obstacle if one produces the dianion with strong base, since the strongly basic terminal carbanion would be a much more reactive nucleophile than the central one (K.G. Hampton, 1973 see p. 9f.). 

Diethyl 3-oxoheptanedioate, for example, is clearly derived from giutaryl and acetic acid synthons (e.g. acetoacetic ester M. Guha, 1973 disconnection 1). Disconnection 2 leads to acrylic and acetoacetic esters as reagents. The dianion of acetoacetic ester could, in prin-ciple,be used as described for acetylacetone (p. 9f.), but the reaction with acrylic ester would inevitably yield by-products from aldol-type side-reactions. 

In a mixture of ether and tetrahydrofuran.In HMPT the same barrier is obtained for the 3,5-dimethylpyrazole. Tn DMSO. Pr TPP = tetrakis(p-isopropylphenyl)porphinate dianion. 

This works well for all nido- and arachno-boranes but not for the c/o o-dianions, which are much less reactive. Reactions of B2H6 with NH3 are complex and, depending on the conditions, yield aminodiborane, H2B(/r-H)(/r-NH2)BH2, or the diammoniate of diborane, [BH2(NH3)2]-[BH4] (p. 165) at higher temperatures the benzene analogue borazine, (HNBH)3, results (see p. 210). 

During the past decade there has been an astonishing proliferation of further binary earbon-sulfur speeies, both anionie and neutral. Of the anions, the beige coloured dianion (made from tctrachloroeyelopropenc) has the /J3/, strueturc (I) and the yellow QS4 (made from squarie aeid, p. 312) has the D, slrueture (2). The off-while CfiSf, ", (3), (made from... 

Se3Bri3- < > SeCls , TeClj-, TeCle ", etc.< > The anion structures are much as expected with the Se species featuring square planar (pseudo-octahedral) units, and the trinuclear Se " anions as in the tellurium analogue above. See also p. 776. There are, in addition, a fascinating series of bromoselenate(II) dianions based on fused planar SeBr4 units, e.g. Se3Brg ", Se4Bri4 ,... 

Williams and McClymont have observed that acylation reactions of the dianion of 2-(5-oxazolyl)-l,3-dithiane (15) lead to formation of 4,5-disubstituted oxazole products through a Comforth rearrangement pathway under base-induced, low-temperature conditions. For example, deprotonation of 15 with LiHMDS (3.0 equivalents) at -78°C, followed by addition of benzoyl chloride or p-chlorobenzoyl chloride and warming to 0°C, provided 16 in 74% and 47% yield, respectively. 

A general method for malting Camps precursors has been developed/ Treatment of an anthranilic acid 15 with an acid anhydride or chloride in the usual way results in the corresponding benzoxazinone (16). Subsequent treatment with the dianion of an N-substituted acetamide furnishes P-keto amide 17. The reactions were run with crude 16, yields typically 50-80% overall. The effect of substituents on the reaction has not been extensively investigated. 

Ethynyl derivatives of 2-aryl-4,5,6,7-tetrahydroindazole were prepared from the p-chlorophenyl hydrazone of cyclohexanone. The hydrazone was treated with two equivalents of -butyllithium at —78°C to generate the dianion, which was then quenched with the appropriate substituted ethyl ester (94MT29). 

Dicarboxylic acids have two dissociation constants, one for the initial dissociation i into a monoanion and one for the second dissociation into a dianion, i-or oxalic acid, H02C—COoH, the first ionization constant has p/Cal = 1.2 and the second ionization constant has pKa2 = 4.2. Why is the second carboxyl group so much less acidic than the first ... 

Reaction of the starting cycloadducts 5 with potassium /cri-butoxide leads to pyridines 6, probably via the diazocinedione dianion, which eliminates cyanate anion via an alternative bicyclic tautomer (cf. Houben-Weyl, Vol. E7b, p 514). 

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