1. Field of the Invention
This invention relates to a process for producing 2-hydroxy-4-methyltetrahydropyran. More particularly, the invention relates to an improved process for producing 2-hydroxy-4-methyltetrahydropyran (hereinafter referred to briefly as MHP)which comprises hydroformylating 3-methyl-3-buten-1-ol under specific conditions.
2. Description of the Prior Art
The reaction of an olefin with hydrogen and carbon monoxide in the presence of a rhodium catalyst, which gives an aldehyde, is well known generally as hydroformylation.
However, it has been found that even if the hydroformylation reaction is conducted using the same catalyst under the same conditions, there is a marked difference between different olefins or olefinic compounds. By way of illustration, compared with a straight-chain olefin, a branched olefin is tens of times slower to react (Chem. Ber., 1969, 102). It is also known that functionally substituted olefinic compounds such as allyl alcohol and vinyl acetate show quite different reactivities than olefins having no functional group because the double bond and functional group in the former may coordinate to the rhodium atom in the manner of chelation [J. Molecular Cat., 18 (1983), 381 and ibid., 16 (1982), 195]. It has also been found that even when a given olefin is subjected to hydroformylation under a given set of conditions, the difference in the kind of ligand in the catalyst results in a marked difference in reactivity. If, for example, in the hydroformylation of an .alpha.-olefin such as 1-octene, in the presence of a rhodium catalyst, tris(2-phenylphenyl) phosphite or tris(2,6-dimethylphenyl) phosphite is employed as the ligand, both the reaction rate and the selectivity to the straight-chain aldehyde are remarkably lower than it is the case when triphenylphosphine or triphenyl phosphite is used. [J. Org. Chem., 34 (1969), 327]. On the other hand, in the hydroformylation of a branched olefin such as 2-methyl-1-hexene in the presence of a rhodium catalyst, the use of tris(2,6-dimethylphenyl) phosphite as the ligand tends to give similar results and rather, in this case of 1-octene, the reactivity is further depressed and virtually no reaction takes place. However, it is known that when tris(2-phenylphenyl) phosphite is used as the ligand, the result is the reverse of that found for 1-octene, i.e. a higher reactivity is obtained than it is the case when triphenylphosphine is employed (Japanese Patent Application Laid-Open No. 123134/1982). Thus, it is known that the hydroformylation differs markedly in reactivity according to the kind of olefin or olefinic compound and the kind of ligand. When a hydroformylation reaction is conducted advantageously on a commercial scale in the presence of a rhodium catalyst, it is difficult to predetermine the optimum combination of olefin or olefinic compound with ligand.
In regard to the hydroformylation of 3-methyl-3-buten-1-ol (hereinafter referred to briefly as IPEA) which is an exemplary olefinic compound and is used in the practice of the present invention, all that is known is that the hydroformylation is carried out using a rhodium catalyst having as the ligand an organic tertiary phosphine, typically triphenyl phosphine (Japanese Patent Application Laid-Open No. 106910/1975).
The above laid open patent application No. 106910/1975 so states that in order to assure a satisfactory reaction rate in the hydroformylation reaction of IPEA using triphenylphosphine as the ligand, the rhodium catalyst should be used in a high concentration and, moreover, in this literature the hydroformylation is conducted at a high reaction pressure of 250 atms.
The hydroformylation of IPEA, when discussed from the industrial viewpoint, can hardly be said economical if the expensive rhodium catalyst cannot be recovered for repeated use for a long period. The higher the concentration of the rhodium catalyst is, the more important this aspect becomes. Since, however, the hydroformylation product from IPEA is a compound very readily acetalizable, the rhodium catalyst is deteriorated by heat or by accumulation of high-boiling products even when attempts are made to recycle the catalyst for reuse while maintaining the catalyst activity for a prolonged period of time. Such problems in commercial production have not been solved yet. As regards the reaction pressure, the higher the reaction pressure is, the greater the number of problems is which are encountered relative to reaction apparatus. This is a matter of course but, for decreasing the reaction pressure in the above reaction system, it is necessary to use triphenylphosphine in large amounts relative to rhodium so that the rhodium catalyst can be stabilized. However, when triphenylphosphine is used in large excess relative to rhodium at low pressures, the rate of reaction falls drastically and, in extreme cases, the reaction hardly proceeds. Thus, another problem is encountered.
On the other hand, it is described in Japanese Patent Application Laid-Open No. 123134/1982 that in the case of those olefins that have no functional groups, for example 2-methyl-1-hexene, the use of tris(2-t-butylphenyl) phosphite as the ligand leads to higher reactivity as compared with the use of triphenylphosphine.
However, it was found that when IPEA, which is the starting material in the process according to the present invention and contains a double bond and a hydroxyl group which can coordinate to the rhodium atom, is subjected to hydroformylation under the conditions described in the above-cited Japanese Patent Application Laid-Open No. 123134/1982, not only was the reaction rate low but the selectivity to the desired product compound was extremely low. It was evident, therefore, that such a process cannot be practiced advantageously on a commercial scale.