Process for preparing phthalides

Details Process for preparing phthalides Process for preparing phthalides

1998-12-16

2000-02-22

Owens, Amelia

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

549305, C07D30787

Patent

active

060282045

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

The present invention relates to a process for preparing phthalides by catalytic hydrogenation of phthalic anhydrides in the phthalide as solvent.
DE-C-28 03 319 discloses a process for preparing phthalide by catalytic hydrogenation of phthalic anhydride in the gas phase, but this process is prohibitively costly at the product isolation stage, involving as it does multistage condensation and an attendant waste gas scrub.
It is further known to conduct the catalytic hydrogenation of phthalic anhydride to phthalide in inert solvents such as tetrahydrofuran (EP-A-0 542 037), benzoic esters optionally mixed with alcohols (EP-B-0 089 417), esters of lower monobasic alcohols with lower fatty acids (U.S. Pat. No. 2,079,325), lower monobasic aliphatic alcohols (U.S. Pat. No. 2,114,696).
Existing processes for the catalytic hydrogenation of phthalic anhydrides to phthalide in a solvent have the disadvantage of affording, as reaction products, mixtures of phthalide and solvent, which have to be separated by costly separating operations such as distillation and/or extraction to free the desired phthalides from the solvents used. In industry, where ecological and economic pressures dictate that the solvents be re-used, it is generally necessary to carry out additional process steps to recover the solvent(s).
It is an object of the present invention to remedy the aforementioned disadvantage of using solvents in the catalytic hydrogenation of phthalic anhydrides.
We have found that this object is achieved by a novel and improved process for preparing a phthalide of the general formula I ##STR3## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each independently of the others hydrogen, C.sub.1 -C.sub.4 -alkyl or C.sub.1 -C.sub.4 -alkoxy, by catalytic hydrogenation of a phthalic anhydride of the general formula II ##STR4## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each as defined above, in a solvent, which comprises using said phthalide I, the reaction product of the catalytic hydrogenation, as solvent.
It is surprising that the phthalide formed in the catalytic hydrogenation of phthalic anhydride can be used as solvent instead of an inert organic solvent, since it is known that phthalide can be further hydrogenated to form troublesome by-products such as o-tolylic acid and products which are hydrogenated in the aromatic nucleus and which are troublesome particularly in relation to the purification of the crude phthalide and which are very difficult to remove.
Compared with the use of the known organic solvents, the process of the invention has the advantage that the reaction mixtures obtained consist essentially of the phthalide product with small amounts of by-products. The phthalide can thus be isolated without a need for a solvent removal step and without additional cleaning steps for a solvent which is to be re-used. The novel process is thus distinguished from existing processes by simpler handling and by improved economy.
A further surprising advantage of the use of phthalide as solvent compared with the use of prior art solvents, for example compared with the use of butyrolactone as solvent, described in EP-A-0 542 037, is that the hydrogenation of phthalic anhydride to phthalide proceeds more rapidly. For instance, a comparison of the hydrogenation rates in butyrolactone and in phthalide as solvent shows that the hydrogenation rate in phthalide is about 3 times higher than that in butyrolactone.
The process of this invention can be carried out either by adding the phthalide solvent as such to the starting phthalic anhydride for the reaction, or by initially starting from phthalic anhydride and utilizing as reaction medium the phthalide formed in the course of the hydrogenation of the phthalic anhydride. If high melting phthalic anhydrides are used, it can be advantageous to use the phthalide as solvent from the start, since phthalide/phthalic anhydride mixtures have a lower melting point than the pure compounds. In addition, the heat of reaction is easier to remove from such mixtures from the s

REFERENCES:
patent: 2079325 (1937-05-01), Larchar
patent: 2114696 (1938-04-01), Rolland
patent: 4528385 (1985-07-01), aus der Funten
patent: 5296614 (1994-03-01), Henkelmann et al.
J. Org. Chem. 51, 3439-3466, 1986.
Synthesis 223-224, 1985.

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