Preparation of indolenines

A process for the preparation of an indolenine of the general formula I ##STR1## where R, R.sup.1 and R.sup.2 are aliphatic radicals and R.sup.1 and R.sup.2 may also be linked and the ring A may be additionally substituted, wherein a compound of the formula II ##STR2## is heated in the presence of a Lewis acid and of a halide. The products are valuable starting compounds for the preparation of dyes.

The present invention relates to a process for the preparation of a 
compound of the general formula I 
##STR3## 
where R, R.sup.1 and R.sup.2 are aliphatic radicals and R.sup.1 and 
R.sup.2 may also be linked and the ring A may be additionally substituted, 
wherein a compound of the formula II 
##STR4## 
is heated in the presence of a Lewis acid and of a halide. Examples of 
aliphatic radicals R, R.sup.1 and R.sup.2 are C.sub.1 -C.sub.9 -alkyl or 
-alkenyl, more specifically ethyl, propyl, butyl, hexyl, octyl, nonyl and 
allyl, and especially methyl. 
Examples of suitable substituents of the ring A are chlorine, bromine, 
nitro, cyano, C.sub.1 -C.sub.4 -alkoxycarbonyl, eg. methoxycarbonyl, 
ethoxycarbonyl or butoxycarbonyl, alkylsulfonyl, eg. methylsulfonyl or 
ethylsulfonyl, alkyl, eg. methyl, ethyl, propyl or butyl, and alkoxy, eg. 
methoxy or ethoxy. The ring A may also carry a fused benzene ring. 
Examples of suitable Lewis acids for the reaction are tin-II chloride, 
tin-IV chloride, titanium tetrachloride, boron trifluoride, aluminum 
chloride, iron-III chloride and especially zinc chloride. The reaction 
apparently requires the presence of a halide; other examples of suitable 
halide additives are NaCl, KCl, LiCl, MgCl.sub.2, CaCl.sub.2, BaCl.sub.2, 
NaBr, NaI, LiI and KI. The use of a halogen-containing Lewis acid, 
especially of zinc chloride, is preferred. A combination of, for example, 
a zinc salt not containing halogen, and a halide, can at times also be 
advantageous. The combination of zinc chloride and lithium chloride in a 
molar ratio of from about 1:0.5 to 1:4, preferably 1:2, is particularly 
preferred. 
The process according to the invention is advantageously carried out by 
heating a compound of the formula II, in the presence or absence of a 
solvent, with the Lewis acid and the halide at from 150.degree. to 
250.degree. C., preferably from 180.degree. to 220.degree. C. The reaction 
to give the compound of the formula I takes place with elimination of 
CO.sub.2 and is as a rule complete after from 2 to 20 hours. 
Per mole of compound II, it is advantageous to use from 1 to 50 mole 
percent, preferably from 5 to 15 mole percent, of Lewis acid plus halide, 
if any. 
Suitable solvents for the reaction are, in particular, high-boiling 
compounds which are inert under the reaction conditions: examples are 
silicone oils and, preferably, mineral oils, which can of course be 
recycled, together with the catalyst, after isolating the reaction 
product. 
A compound of the formula II is obtained from a compound of the formula III 
##STR5## 
by reaction with an amine of the formula IV 
##STR6## 
at an elevated temperature, with removal of water. The amine of the 
formula IV is advantageously employed in excess, a molar ratio of from 
1:1.5 to 1:3 being preferred. 
In a particularly preferred variant of the process according to the 
invention, the latter is preceded by the preparation of the compound of 
the formula II, and this compound is converted, without isolation, to a 
compound of the formula I in a one-vessel process. 
The process according to the invention is of particular importance for the 
preparation of a compound of the formula Ia 
##STR7## 
where X.sup.1 is hydrogen, chlorine, methyl or methoxy and X.sup.2 is 
hydrogen, chlorine or methyl.