A self-condensation

Tire condensation method (Section II,A,1) applied for 1,2,4-thiadia-zolium salts and phenylsubstituted imidazoles leads to the thia-tetraazaful-valenes 91 and 92 (70TL481). Moreover, 2,4,5-tris(ethylsulfanyl)imida-zolium chloride was converted to the unsymmetrical heterofulvalene 93 by a self-condensation reaction in the presence of potassium carbonate and elemental iodine (85PS223) (Scheme 37). 

With excess ketone, the preparation of the aminoketone and subsequent condensation to a pyrrole can be conducted in one pot. In a side-reaction a-aminoketones can undergo a self-condensation to give pyrazines 8 ... 

The number of formed phthalocyanines can be reduced either by using an excess of one phthalo-nitrile317 or by using a phthalonitrile like tetraphenylphthalonitrile which is unable to undergo a self-condensation.111,312,313... 

Unsymmetricaily substituted phthalocyanines are prepared from two differently substituted isoindolinediimines A and B, respectively. If one of the isoindolinediimines cannot undergo a self-condensation due to sterical hindrance, only a reduced number of different phthalocyanines is formed. 

Example 13.3 The conversion of a self-condensing reaction can be limited to give polymers with finite lengths. How does the polydispersity of these polymers compare with those in Example 13.2 where the reaction went to completion with imperfect stoichiometry Make the comparison at the same average chain length. 

Solution The number and weight average chain lengths for a self-condensing polymerization are obtained from Equation (13.18) and (13.19) by setting Sab= E ... 

Example 13.4 Calculate the molecular weight distribution for a self-condensing polymerization with — + Stop the calculations... 

Whereas in alcohols as solvents good yields of esters are obtained a self-condensation of the ester was observed in THF.[123]... 

Likewise, the lactol 62, obtained by addition of methyllithium to 2,3,5-tri-O-benzyl-D-arabinono-1,4-lactone, underwent a self-condensation reaction in the presence of BF3 OEt2-MeCN, to afford an anomeric mixture of two C-disaccharides 63 and 64 in 93% yield (66). 

Self-condensations are another set of important reactions of organolithium compounds. Tamao and Kawachi had reported that [(tert-butoxy diphenyl)silyl]lithium (20) exhibited ambiphUic character, and underwent a self-condensation reaction to give a [2- tert-butoxy)disilynyl]lithium derivative in THF as shown in Scheme 4, and also a nucleophilic substitution reaction with n-butyllithium . 

In the absence of an added nitrile, the 2-aminobenzonitrile undergoes a self-condensation to give 4-amino-2-(2-aminophenyOquinazoline 848 <2000TL2215>. 

Hydroxymethylation of guaiacyl acetone (VII) yielded an amorphous solid, probably a self-condensation product after being methylolated in several positions. Veratyl acetone (VIII) gave a crystalline product in good yield which had no CO group. It can be assumed that the CO group was reduced by a crossed Cannizzaro reaction, and a dimeric product was formed, whose structure is not known yet. 

In a similar way a ketone may undergo a self-condensation reaction to give a ketol, which may be dehydrated in a separate reaction to an a,/ -unsaturated... 

Example 13.4 Calculate the molecular weight distribution for a self-condensing polymerization with k m - ko/(l + m). Stop the calculations when In = 5. Compare the results with those of the Flory distribution. 

In the various reactions involving 4,6-dimethoxyindolin-2- one 73 and phosphoryl chloride, there is the possibility that the likely intermediate, 2-chloro-4,6-dimethoxyindole 102, could react with itself, as it is both electrophilic at C-2 and nucleophilic at C-3 and C-7. Even though no products of such a self-condensation process were observed in any of the reactions with indoles, an investigation of this combination of reactants was carried out. The reaction was indeed quite slow, but eventually very small yields (2 and 1%) of the rather interesting heterocyclic system 103 and its further substituted product 104 were isolated44 (Scheme 29). A possible mechanism is indicated in Scheme 29. A similar 1-methylindole trimer has been synthesized from 2-iodo-1-methylindole by both direct and stepwise routes.39,40 So far attempts to generalize the phosphoryl chloride reaction and improve the product yields have not met with success. 

When two molecules of a Mannich base react together with the formation of a methylenc-bis-derivative (255, Fig. 96), a particular type of C-alkylation lakes place, simultaneously involving deaminomethylation and amine replacement, which could be defined as a self-condensation of the Mannich base. 

Olah (33) has shown that alkyl fluorides undergo a self-condensation reaction (eq. 10) similar to that previously described for the ionization and condensation of methane so that this additional pathway is not an unexpected one. 

Direct disconnection of compound C suggests a crossed Claisen ester condensation but, if we —.ove one methyl group first, a great simplification results as we can use a self-condensation. 

Disconnection b solves no problems - we still have to make the a,P-unsaturated carbonyl r and now we have to make one particular geometrical isomer (cis). Disconnection a reveals 1,5-diCO relationship. Disconnecting this at the branchpoint we get another simple available and a keto-ester (ethyl acetoacetate) that is the product of a self-condensation of ethyl ace -... 

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

By the malonic ester and acetoacetic ester we make a-substituted acids and a-substituted ketones. But why not do the job directly 1 Why not convert simple acids (or esters) and ketones into their carbanions, and allow these to react with alkyl halides There are a number of obstacles (a) self-condensation—aldol condensation, for example, of ketones (b) polyalkylation and (c) for unsym-metrical ketones, alkylation at both a-carbons, or at the wrong one. Consider self-condensation. A carbanion can be generated from, say, a simple ketone but competing with attack on an alkyl halide is attack at the carbonyl carbon of another ketone molecule. What is needed is a base-solvent combination that can convert the ketone rapidly and essentially completely into the carbanion before appreciable self-condensation can occur. Steps toward solving this problem have been taken, and there are available methods—so far, of limited applicability— for the direct alkylation of acids and ketones. 

Figure 1.18. Branching during step polymerization, (a) Addition of a trifunctional agent (e.g. a triol, BBB) to a self-condensing difunctional system (e.g. a hydroxy acid, BA), (b) Hyperbranching from a system AB2. In both cases A may react only with B.

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