Base-catalysed Dimerization

Base-catalysed Dimerization. iV-Alkyl-3-isothiazolones (116) having a free 5-position are readily dimerized by base to 2,4-bismethylene-l,3-dithietans 

5 -Oxides and -Dioxides.—Isothiazol-4-in-3-ones (118) are convertible into the 1-oxides and 1,1-dioxides by the action of the correct proportions of m-chloroperbenzoic acid. Other oxidizing agents, including nitric acid, dinitrogen tetroxide, and chromic acid, are effective in forming 1-oxides from 3-hydroxyisothiazoles (118 R = H).  

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The base-catalysed intermolecular condensation of nitriles of the type RCH2CN is one of the oldest known methods for the preparation of / -enaminonitriles, and in the case of simple self-condensation it leads to aliphatic analogues of the cyclic / -ena-minonitriles formed in the classical Thorpe-Ziegler cyclization. For example, the base-catalysed dimerization of acetonitrile with sodium gives 3-amino-crotononitrile661. 

Dimethylaminomaleonitrile is readily formed in dilute aqueous solution of HCN at room temperature666. The base-catalysed dimerization of malononitrile gives the dimer l,l,3-tricyano-2-aminopropene, which exists mainly in the enamine form667-670. 

Called the aldol reaction, base-catalysed dimerization i a gene reaction for all aldehydes and kcUmes with an r hydrogen atom. If the sMe hyde or ketwie does not have an a hydri en atom, however, aldol coiukt-... 

Iminoazetidines have been prepared by either the thermal dimerization of iV-methyIdiphenylketenimine (81) or the base-catalysed dimerization of bis (trifluoromethyl)ketenimine (82). 

Furthermore, although the intercepts k kiK/k- ) and the slope (kikjK/k-i) are equally influenced by the dimerization constant K in equation 28, this does not imply that they should show the same effect on changing the solvent. According to the dimer mechanism , it could be expected that the base catalysed decomposition of the transition state SB2, measured by Ag, should be more depressed by small additions of protic solvents than the spontaneous decomposition measured by Ag. Indeed, the overwhelming evidence on the classical base catalysis by amines shows that usually Ag is more important in aprotic than in protic solvents1. 

Base-catalysed equilibration of 6a-nitrocholest-4-ene and the 6j8-isomer led to a 1 1 mixture in which the unusual stability of the 6j3-isomer was tentatively attributed to homoconjugation.The 20-azidopregnane (96) was converted into the imino-dimer (97) by reaction with Bp3-Et20 according to the Scheme 2. In a study of the reactions of halogens with 5a-conanine (98) it was observed... 

The base-catalysed converdori of substituted ketene dimers to ketene trimers has been described. 4< t4 s ... 

Unlike COClj (see Section 9.1.3.1), carbonyl difluoride can be dimerized to trifluoromethyl fluoroformate, FC(0)0CF3, and trimerized to bis(trifluoromethyl)carbonate, 0=C(OCF3)j, by means of a base-catalysed reaction conducted at -50 to -110 C in the presence of activated charcoal [45b]. The dimer is formed in 45-60% conversion, whereas the trimer is only formed in about 1% yield [45b]. 

A. Yoshida, S. Hikichi, N. Mizuno, Acid-base catalyses by dimeric disilicoicosatungstates and divacant y-Keggin-type silicodecatungstate parent Reactivity of the polyoxometalate compounds controlled by step-by-step protonation of lacunary W=O sites, J. Organomet. Chem. 692 (2007) 455. 

Synthetic work in this area includes a new, stereospecific synthesis of sparteine and a synthesis of isosophoramine. The reduction of quinolizidone (16) by means of lithium aluminium hydride affords, as noted previously, a dimeric product (17), presumably via condensation of the initially formed enamine (18) with the corresponding immonium cation (19). With di-isobutyl aluminium hydride the yield of (17) can be increased to 85%. Analogous reduction of the dilactam (20), prepared by base-catalysed condensation of the bromo-lactam (21) with a-piperidone, gave an intermediate (22), in which the enamine and immonium ion units are present within the same molecule. Intramolecular cyclization of (22), via a transition state derived from the conformation (23), spontaneously gave the... 

A variation of this type of reaction involves interaction of two alkyne groups via radical formation. The mechanism proposed by Garratt involves initial base-catalysed isomerization to the bis allene, which then undergoes an intramolecular allene dimerization to give the bis methylene diradical (67). 

It is well known that olefins dimerize on contact with a nickel oxide catalyst at elevated temperature. Since the discovery of their specific activity in catalysing dimerization reactions, nickel-based catalysts have been the subject of intensive mechanistic and kinetic studies directed toward identifying and understanding the active sites responsible for this property. Earlier nickel catalysts were homogeneous however, nickel deposited on different supports has shown high selectivity towards alkene dimerization (see Table 5, page 276). 

The hydroxy-nitrone (8a) gave the isomeric hydroxy-nitrone (9) on treatment with acid. The latter hydroxy-nitrone underwent base-catalysed deuteriation at C-21, presumably via the dihydroxy-enamine tautomer. Treatment of either hydroxy-nitrone with methanolic sulphuric acid gave the unstable methoxy-nitrone (10), which yielded the dimeric compound (11) on attempted crystallization. The reaction of nitrone (8b) with dialkyl phosphite derivatives was reported earlier details of this study have now been published. ... 

Levoglucosenone s conjugated system undergoes base catalysed oligomerization (64-69) with the formation of variety of complex dimers, Uimers and oligomers. 

Formation of A -pyrazolines by base-catalysed nitrile dimerization evidently proceeds via a methoxycyclopropane intermediate (Scheme 64). ... 

Application of the Grenacher reaction (condensation of aldehydes with rho-danine) to di-(9-isopropylidene derivatives of a/de/zydo-L-arabinose, -D-arabinose, and -L-xylose followed by base-catalysed hydrolysis gave sugar derivatives of the type RCH=CH(SH)C02H where R = glycosyl. Photoirradiation of phenyl 2,3,5-tri-(9-acetyl-l-seleno-a, 3-ribofuranose results in migration of the sugar residue to the ortho-position with concomitant formation of the diselenide dimer. ... 

CHMel/ thus supporting the interpretation of rearrangements of alkenes in liquid SO 2 that is based on the instability of allyl sulphinic acids. Sodium salts of sulphinic acids take the role of co-catalyst in PdClj-catalysed dimerization of butadiene in an alcohol solvent, leading to octa-2,7-dienyl alkyl ethers, and act as arylating agents towards cyclo-octa-1,5-diene in the presence of PdClj. ... 

The mechanism for copper(n)-catalysed dimerization of pyruvate (P ) proceeds by a CuP+ complex which undergoes a general-base catalysed enolization process,... 

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