Methylene bases

At first, the dimeric nature of the base isolated from 3-ethyl-2-methyl-4-phenylthiazolium was postulated via a chemical route. Indeed the adduct of ICH, on a similar 2-ethylidene base is a 2-isopropylthiazolium salt in the case of methylene base it is an anilinovinyl compound identified by its absorption spectrum and chemical reactivity (45-47). This dimeric structure of the molecule has been definitively established by its NMR spectrum. It is very similar to the base issued from 2.3-dimethyl-benzo thiazolium (48). It corresponds to 2-(3 -ethyl-4 -phenyl-2 -methylenethiazolinilydene)2-methyl-3-ethyl-4-phenylthiazoline (13). There is only one methyl signal (62 = 2.59), and two series of signals (63= 1.36-3.90, 63= 1.12-3.78) correspond to ethyl groups. Three protons attributed to positions T,5,5 are shifted to a lower field 5.93, 6.58, and 8.36 ppm. The bulk of the ten phenyl protons is at 7.3 ppm (Scheme 22). 

The reason for this relative lack of reactivity of 2-methylthiazoIium is probably due to the too-weak nucleophilic character of its carbon-2. For example, any /S-alkoxyalcene (29) derivatives resulting from the condensation of o-ester could never have been isolated, whereas they constitute the essential intermediate step in trimethine syntheses for rings of acidic character (64). However, even if a negative 5-substituent such as ethoxy-carbonyl increases the yield (61) by promoting independently the possible formation of the methylene base, it may be stressed that the presence of this base is not the essential condition of the reaction, since the isolated anhydrobase itself is not reactive toward the o-ester (Scheme 41). 

If the dye contains no mobile substituents ia the chain, nucleophiles attack primarily the end carbon atoms (changing of terminal residues). Streptocyanines can be hydroly2ed ia aqueous alkaline solution to form the corresponding merocyanines and then the oxonoles (71,72). These processes are reversible. Nucleophilic reactions with the methylene bases of the corresponding heterocycles result ia polymethines containing new end groups (Fig. 

Fig. 5. Alteration of end groups by reaction with methylene bases.

In the next step, the polymethine chain can be lengthened by iateraction with the methylene bases of appropriate heterocycHc compounds (3). 

Fig. 5. Nonoxidative cyanine syntheses. Reactions of the methylene base from (14) with electrophilic reagents.

A methylene base formed from quinaldine ethiodide, l-ethyl-2-methyl-ene-1,2-dihydroisoquinoline (129), exhibits a number of reactions characteristic of enamines (207,209). On treatment with benzoylchloride a dialkylated product (130) is produced by C and subsequent O benzoylation (210). 

The key intermediates for the synthesis of unsymmetrical, heterocyclic oxo-squaraines are the mono-squaraines (semi-squaraines) shown in Fig. 5. These intermediates can be synthesized via condensation of dialkylsquarate with an equimolar amount of methylene base [51]. The obtained alkoxy-mono-squaraines are then reacted with the second methylene base to yield unsymmetrical oxo-squaraines. These mono-squaraine intermediates display a higher reactivity compared to squaric acid or its esters they allow the synthesis of the corresponding... 

Imino-amino methylene base intermediates, 15 781 Iminobispropylamine, 3 485... 

Alkylazines reactions involving traces of reactive anions or traces of methylene bases... 

In acidic media, loss of a proton can give traces of methylene forms of type (609). Alternatively, a Lewis acid catalyst such as acetic anhydride may be used which involves formation of complexes of type (618) from which proton loss is facile. Such methylene bases can also react with electrophiles, gradually causing complete conversion of the heterocycle. 

Diazotization, coupling, and methylation may be combined in a one-pot process by dissolving the aromatic amine and the heterocyclic methylene base in an organic acid and effecting the simultaneous diazotization and coupling by addition of sodium nitrite. The dye base is released by addition of alkali and is methylated with dimethyl sulfate to form the alkylarylhydrazone dye (e g., 11) [27]. 

Violet and blue hydrazone dyes are obtained with a methylene base derived from naphtholactam (13) [33],... 

A thiazole 56 (0.05 mol) or a methylene base 57 (0.05 mol), a hydroxypyridone (60) (0.05 mol) and dimethylformamide (0.075 mol, 5.5 g) were heated at 90°C in acetic anhydride (20-30 mL) for about 3 h. The solid that precipitated upon cooling to room temperature was filtered off, washed thoroughly with 2 -propanol and/or aqueous ethanol until the color of the filtrate changed from violet to red and subsequently dried in a vacuum-drying cabinet at 50 °C. For physical characterization, the dyes were recrystallized from acetic anhydride, toluene or toluene-hexane mixtures. 

The first method is perhaps the most common, and is exemplified by the condensation shown in Scheme 3 of a 2-alkyl heterocyclic quaternary salt or the corresponding methylene base with a 2-hydroxy unsaturated aldehyde grouping (which usually is part of an aromatic ring, as in salicylaldehyde). These intermediates have given a broad assortment of spiropyran classes. The ready availability of 1.2.3.3.-tetraalkyl-3/f-indoleninium salts and salicylaldehydes has led to a large number of spiro-(2i/-l-benzopyran-2,2 -indolines) [this name will be used in preference to the correct T,3 -dihydrospiro(2//-l-bcnzopyran-2,2 -(2 //(-indole)]. A common acronym for this class, BIPS, will be used in this chapter as both singular and plural. 

The second method corresponds to moving the formyl group from the aldehyde intermediate to the methylene base, and is a standard method for preparing a merocyanine (the open form of a spiropyran). This method is useful for the reaction of the easily obtained and stable Fischer s aldehydes (2-formylmethyleneindolines) (1) with ketomethylene compounds such as 2-hydroxybenzofuran (2) or 2-hydroxybenzothiophene where the corresponding hydroxyaldehyde is difficult to obtain (Scheme 4). These two methods include the several routes to various symmetrical and unsymmetrical spiro(dipyrans).6... 

Not all heterocyclic methylene bases give spiropyrans (at least under the usual conditions). l,3,4,5,5-Pentamethyl-2-methylene-2,5-dihydropyrrole (69)77 did not condense with 5-nitrosalicylaldehyde to give any photochromic material.7 However, l-methyl-2-methylene-3,3,4,5-tetraphenyl-2,3-dihydropyrrole (70), prepared by reaction of methyllithium with 1 -methyl-3,3,4,5-tetraphenylcrotonolactam, did condense with 5-nitrosalicylaldehyde to give a product exhibiting reverse photo-chromism the unexposed form was red and the exposed form yellow. This behavior probably represents a trans to cis isomerization of the open form 7... 

The behavior of the 1 -methiodide (179) is more difficult to explain as its spectrum in alcohol is nearly identical to that in alkaline solution, and these both show the same pattern as the 2-methylene indoline, i.e., with an intensified band II and a band III shifted to the red as compared to its methiodide. It seems possible that these compounds exist as the carbinolamine, or pseudobase, in hydroxylic solvents and are in tautomeric equilibrium (179 181). The direction of this equilibrium has been attributed by Beke to the basicity of the nitrogen, as well as other factors. B14ha and Cervinka discuss the importance of steric and polar structural factors, citing examples of stable pyrrolidine pseudobases. Indeed, Ficken and Kendall isolated the 4-azaindoline pseudobase (123), characterizing it by analysis and infrared spectral evidence. They similarly characterized the 2-methylene base (121), and it is unfortunate that their ultraviolet spectra were not determined. The 2-methylene-7-azaindoline... 

Thermally induced rearrangement of quinazoline derivative 8 affords ethyl A -[(6-methyl-l,3-dioxolo[4,5-g]quinazolin-8-yl)methyl]carbamate (9a) in 5 /o yield. Similarly, the thermal and photochemical rearrangement of ethyl A-(6-methyl-8-methylene-l,3-dioxolo[4,5-g]quinazolin-7-yl)-A-phenylcarbamate (7b) produces ethyl A -[(6-methyl-l,3-dioxolo[4,5-g]quinazolin-8-yl)-methyl]-Ai -phenylcarbamate (9b) in 45% yield. Thermal rearrangement of 8, therefore, probably involves the tautomeric methylene base 7a (R = H) as an intermediate. 

Photochemical reactivation of methacrylate oligomers has been observed.292 For instance, MMA dimer was metalated with a cobaloxime. The preformed methylene-bound product was photolyzed in the presence of styrene yielding 85 rather than the expected 86. The poor yield of 85 is not unexpected, considering the high reactivity of the methylene-based radical species that would be generated. 

Under proton or Lewis acid catalysis, 2- and 4-alkylpyridines of the above type are in equilibrium with the tautomeric methylene bases 57 and 58, which can function as enamines ... 

Imino-amino methylene base intermediates obtained by the decomposition of hexamethylenetetramine (hexamine) stabilized by the presence of strong anions (hexamine sulfate) have been shown to markedly improve the water and weather resistance of hardened MUF resins used as wood adhesives (101-105). Even with only small additions between 1% and 5% the use of MUF resins of much lower melamine content with constant performance of the boards is possible. 

Then in 1925 Konig used N,2-dimethylbenzothiazolium perchlorate (52) for the preparation of methylene base (53, scheme-15) which was used in the synthesis of thiocyanine dyes (Konig Meier 1925). The quaternary salts of benzothiazoles also found recognition in 1928 by Clark (Clark 1928) while studying the absorption sensitivity of the methylene bases from the quaternary salts of 2-methyl benzothiazole. 

Scheme 15. Konig preparation of methylene bases from benzothiazolium salts...

Clark, L. M. (1928). CCCllL-The methylene bases from 1-methylbenzthiazole and 1-methylbenzselenazole methiodides with a note on the preparation of 1-substituted benzthiazoles. /, Chem. Soc. pp. 2313-2320, ISSN. 0368-1769 Davis, J. H., Jr. Forrester, K. J. (1999). Thiazolium-ion based organic ionic liquids (OILs). 

---