Dimerization substituents

While both 2- and 3-vinylindole have been synthesized and characterized[l,2], they arc quite reactive and susceptible to polymerization. This is also true for simple l-alkyl derivatives which readily undergo acid-catalysed dimerization and polymerization[3]. For this reason, except for certain cases where in situ generation of the vinylindoles is practical, most synthetic applications of vinylindoles involve derivatives stabilized by EW-nitrogen substituents[4]. 

Auto-association of A-4-thiazoline-2-thione and 4-alkyl derivatives has been deduced from infrared spectra of diluted solutions in carbon tetrachloride (58. 77). Results are interpretated (77) in terms of an equilibrium between monomer and cyclic dimer. The association constants are strongly dependent on the electronic and steric effects of the alkyl substituents in the 4- and 5-positions, respectively. This behavior is well shown if one compares the results for the unsubstituted compound (K - 1200 M" ,). 4-methyl-A-4-thiazoline-2-thione K = 2200 M ). and 5-methyl-4-r-butyl-A-4-thiazoline-2-thione K=120 M ) (58). 

In the commercial Gorham process (2), the extremely reactive PX is conveniendy generated by the thermal cleavage of its stable dimer, Vo-di- -xyljIene (DPX), a [2.2]paracyclophane [1633-22-3] (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becorning substituents of the polymer in the coating. 

DPXC ndDPXD. The economic pressure to control dimer costs has had an important effect on what is in use today (ca 1997). Attaching substituents to the ring positions of a [2.2]paracyclophane does not proceed with isomeric exclusivity. Indeed, isomeric purity in the dimer is not an essential requirement for the obtaining of isomeric purity, eg, monosubstituted monomer, in the pyrolysis. Any mixture of the four possible heteronucleady disubstituted dichloro[2.2]paracyclophanes, will, after all, if pyrolyzed produce the same monomer molecule, chloro- -xyljIene [10366-09-3] (16) (Fig. 4). 

Dimeric aldoketenes and ketoketenes of P-lactone stmcture show a chemical behavior which is not much different to that of diketene. Thus nucleophiles add ia similar fashion to give derivatives of 3-ketoacids which are mono- or dialkylated at C-2 (aldo- and ketoketene dimers, respectively), but the reaction can often be slower than with the parent compound and, ia case of long-chain or bulky substituents, may not proceed at all. Other reactions can proceed differendy than those with diketene. For an overview of important reactions of aldoketene and ketoketene dimers see Reference 122. 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

Substituent R Approximate time for complete dimerization at 18 C Substituent R Approximate time for complete dimerization at 18 °C... 

Additional work showed that the dimerization of arylazirines to 1,3-diazabicy-clo[3.1.0]hex-3-enes is a general reaction which is independent of the nature of the substituent groups attached to the C atom of the azirine ring. Care is required in the choice of solvent, photolysis time and substituents since the 1,3-diazabicyclohexenes are themselves photochemically labile (72JA7788). 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

Type 125 0x0 forms are characteristic for 5-hydroxy-l,2,4-triazoles [76AHC(S1), pp. 379, 388], These forms are additionally stabilized by an electron-withdrawing substituent, R = NO2 (98MRC343). Both hydroxy and 0x0 tautomers are capable of forming stable dimers owing to the in-termolecular hydrogen bonds (126 and 127 [76AHC(S1), pp. 377,379). 

When methyl 2-(indol-2-yl)acrylate derivative (22a) reacted with A-methoxy-carbonyl-l,2-dihydropyridine (8a) in refluxing toluene, in addition to the dimer of 22a (25%), a mixture of the expected isoquinculidine 23a and the product 24a (two isomers) was obtained in 7% and 45% yields, respectively (81CC37). The formation of 24a indicates the involvement of the 3,4-double bond of dihydropyridine. Similarly, Diels-Alder reaction of methyl l-methyl-2-(indol-2-yl)acrylate (22b) with 8a gave, in addition to dimer of 22b, a mixture of adducts 23b and 24b. However, in this case, product 23b was obtained as a major product in a 3 2 mixture of two isomers (with a- and (3-COOMe). The major isomer shows an a-conhguration. The yields of the dimer, 23b, and 24b were 25%, 30%, and 6%, respectively. Thus, a substituent on the nitrogen atom or at the 3-position of indole favors the formation of the isoquinuclidine adduct 23. 

Dimerization is markedly subject to steric hindrance, thus, whereas 3-n-propylindole dimerizes readily, neither 3-isopropyl- nor Z-tert-butyl-indole dimerizes. This failure is most probably the result of steric hindrance of approach of the electrophilic reagent to position 2 by the bulky 3-substituent in the unprotonated molecule. On the other hand, models show that approach of a nucleophilic reagent to position 2 of a 3-protonated molecule is quite open, it should, there-... 

Dimerization is also prevented by the presence of a formal positive charge on a j8-substituent, thus tryptamine cannot be dimerized. The formation of the trimer can be rationalized by analogy with the reactions of gramine and its derivatives. Thus, Thesing and Mayer found that methylphenylskatylamine (49) reacts with... 

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