Process of forming 2-(4-isobutyl-2-methylphenyl) propanal

Abstract

The regio-selective functionalization of a dialkyl benzene compound ##STR00001##
wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 70:30, more particularly at least 80:20, still more particularly at least 85:15, and still more particularly at least 90:10, characterised in that the substituent R is an isobutyl group.

Claims

1. A process of regio-selective functionalization of a dialkyl benzene compound according to the following formula ##STR00015## wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 70:30, and wherein the substituent R is an isobutyl group, wherein the process include a carbonylation reaction which introduces a benzaldehyde functionality into the position (a), and wherein the carbonylation reaction is carried out using HF/BF.sub.3 and carbon monoxide.

2. A process of forming a compound according to the following formula (I) ##STR00016## comprising the step of: regio-selectively functionalizing a dialkylbenzene compound ##STR00017## wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 70:30, and wherein the substituent R is an isobutyl group.

3. A process of forming a compound of the following formula (I) ##STR00018## comprising the steps of: ##STR00019## and subsequently, C) the hydrogenation of compound (VI) to provide compound (I).

4. The process of claim 3, wherein compound (III) is converted to the corresponding dialkylacetal for purification and is then further converted into compound (I).

5. A compound of the following formula IIIb ##STR00020## formed in a process of forming a compound of the following formula (I) ##STR00021##

6. The process of claim 4, wherein the corresponding dialkylacetal is compound IIIb.

7. The process of claim 1, wherein the wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 80:20.

8. The process of claim 7, wherein the wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 85:15.

9. The process of claim 8, wherein the wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 90:10.

10. The process of claim 2, wherein the wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 80:20.

11. The process of claim 10, wherein the wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 85:15.

12. The process of claim 11, wherein the wherein the ratio of the compound functionalized at position (a) to the compound functionalized at the position (b) is at least 90:10.

13. The process of claim 2, wherein the step of regio-selectively functionalizing a dialkylbenzene compound is a carbonylation, a chloromethylation, a bromination or a direct conversion into the compound of formula (I).

Description

(1) FIG. 1 depicts a comparison of the increasing differences of retention times in GC separation which are indicative of an improved separation by distillation.

(2) Accordingly, the invention provides in a first aspect a method of regio-selectively functionalizing an alkyl toluene compound (II)

(3) ##STR00003##
at position (a) on the ring, wherein the substituent R is an isobutyl group.

(4) In another aspect of the present invention there is provided a process of forming a compound according to the formula (I)

(5) ##STR00004##
comprising the step of regio-selectively functionalizing 3-methyl-1-isobutyl benzene (compound II)

(6) ##STR00005##
at the position (a) on the ring.

(7) By regio-selectively is meant that the functionalization is directed predominantly at the position (a) on the ring rather than the position (b). More particularly, regio-selectively means that the ratio (a):(b) is at least 70:30, more particularly at least 80:20, still more particularly at least 85:15, more particularly still at least 90:10.

(8) In yet another aspect of the present invention there is provided the use of 3-methyl-1-isobutyl benzene (compound II) as an intermediate in the synthesis of compound (I).

(9) As stated hereinabove, the applicant found that it is critical for an economically feasible and industrially scalable process into the compound (I) that the methyl group is already located on an available starting material. This finding determines that compound (II) (3-methyl-1-isobutyl benzene) is a key intermediate in any process, and that the functionalization of this compound must proceed with such high regio-selectivity.

(10) And yet, the idea of preparing the compound of formula (I) via the key intermediate, compound (II) is counter-intuitive because the prior art articulates a prejudice against doing precisely this. Specifically, Rao et al in the Indian Journal of Chemistry vol. 16B, May 1978 introduces aldehyde functionality into 3-methyl-1-isobutyl benzene (III e in that paper). The compound produced was 2-isobutyl-4-methylbenzaldehyde (III g), indicating that the isobutyl group directs the aldehyde functionality into the ortho-position and not the para-position relative to the iso-butyl group. In other words, the isobutyl group does not promote high regio-selectivity for the position para-relative to it.

(11) Applicant's own findings, which are summarized in Table 1 below, surprisingly are in disagreement with Rao et al. In the carbonylation of 1-butyl-3-methyl benzene compounds in the presence of carbon monoxide (approximately 40 bar) and triflic acid, the tertiary-butyl and sec-butyl ring substituents are found not to be at all effective at selectively directing the carbonylation to the ring position para- to the butyl substituent. Whereas, the n-butyl substituent, which is substantially more effective than the tertiary-butyl and sec-butyl substituents, is less effective when compared with the iso-butyl substituent.

(12) TABLE-US-00001 TABLE 1 Butyl substituent of 1- Rate of conversion after Aldehyde Ratio butyl-3-methyl benzene 1 h in % (a):(b) n-Butyl 100 2.58 sec-Butyl 90 1.17 iso-Butyl 100 3.00 tert-Butyl 80 no identifiable aldehyde

(13) In accordance with the present invention 3-methyl-1-isobutyl benzene (compound II) may be prepared from 3-isobutyl-1-methylcyclohex-1-ene (compound (Ia) according to known methods. In particular, (Ia) can be dehydrogenated under reduced pressure using a palladium catalyst, immobilized on aluminium oxide or carbon. The dehydrogenation can be carried out at room temperature or elevated temperatures, preferably at 200 C.

(14) ##STR00006##

(15) The choice of functional group that can be introduced into 3-methyl-1-isobutyl benzene (compound II) at the para position (relative to the isobutyl group) can be quite varied.

(16) Introduction of benzaldehyde functionality regio-selectively is a preferred embodiment of the invention. Introduction of this functionality can be achieved in several ways. One method is to first chloromethylate compound (II), before converting the 1-chloromethyl-2-methyl-4-isobutylbenzene into 2-methyl-4-isobutyl benzaldehyde. However, for reasons related to procedural efficiency, chloromethylation of compound (II) is not a preferred embodiment of the present invention.

(17) Direct introduction of benzaldehyde functionality into the ring by carbonylation is a preferred embodiment of the invention.

(18) ##STR00007##

(19) In a particular embodiment of the invention, this reaction may be carried out in 6.8 molar equivalents of triflic acid under about 40 to 60 atmospheres of carbon monoxide in an autoclave. The carbonylation will provide a mixture of regio-isomers, which may contain 70%, and more particularly 80% (or even higher amounts) of compound (III).

(20) Alternatively, the reaction can be carried out in HF/BF.sub.3 under conditions generally known in the art. This reaction is frequently referred to as Mitsubishi chemistry.

(21) A representative example of Mitsubishi chemistry is described in U.S. Pat. No. 3,962,343, which is incorporated herein by reference.

(22) In another embodiment of the invention, the compound of formula (II) can be brominated under reaction conditions generally known in the art

(23) ##STR00008##

(24) Operative reaction conditions are treatment of (II) neat with one molar equivalent bromine in the presence of 0.05 molar equivalents of iron powder at 10 C.

(25) In yet another embodiment of the invention, the compound of formula (II) can be directly converted into the compound of formula (I)

(26) ##STR00009##

(27) Reaction conditions for this transformation are generally known in the art, and may proceed with the reaction of compound (II) with titanium tetrachloride and a compound of the formula (V) in dichloro methane at 70 C. Hydrolysis of the intermediate enol acetate with dilute sulfuric acid affords (VI) to 40% in a mixture of regio isomers.

(28) ##STR00010##

(29) Compound of formula (III) can be converted into compound (VI) using Muller Conradi-Pieroh conditions, generally known in the art.

(30) ##STR00011##

(31) Particular reaction conditions include converting the benzaldehyde (III) to the dimethyl acetal by treatment with trimethyl orthoformate, followed by reaction with ethyl vinyl ether in the presence of catalytic boron trifluoride etherate at room temperature. The intermediate ethoxy methoxy acetals were hydrolyzed in the presence of 5% HCl to afford (VI).

(32) Another advantage attendant with the inclusion of the methyl substituent in the starting material is that, unlike the solid 4-isobutyl benzaldehyde used, for example, as a starting material in the synthesis of the well-known fragrance ingredient Bourgeonal, the presence of the methyl substituent in compound (III) renders the compound a liquid, which makes it easier to handle on an industrial scale.

(33) Thereafter, the compound of formula (VI) can be hydrogenated to provide the compound of formula (I). Hydrogenation conditions are generally well known in the art, and include catalytic hydrogenation of the double bond in the propenal side chain over palladium on carbon 5% at 500 mbar pressure.

(34) Compound (IV) described hereinabove can be further converted in accordance with the reaction schemes

(35) ##STR00012##

(36) Reaction scheme (a) describes the conversion of compound (IV) into compound (I). This reaction may be carried out by the Heck reaction of (IV) with allyl alcohol, catalysed by palladium in the presence of a secondary amine followed by oxidation of the resulting propanol side chain, using conditions generally known in the art, to afford compound (I).

(37) Reaction scheme (b) describes a similar process of reacting an allyl ether with a compound of the formula (IV) under Heck conditions to afford a mixture of compounds (VII) and (VIII). The skilled person would expect that acid hydrolysis of the mixture of compound (VII) and (VIII) would result in a mixture of compound (IX) and the desired compound (I), and as such, a synthetic route proceeding through this mixture would, at first sight, appear unpromising.

(38) ##STR00013##

(39) However, to applicant's surprise, both compounds (VII) and (VIII) were converted to compound (I) upon acidic hydrolysis suggesting that (VII) underwent acid mediated double bond migration to (VIII) before hydrolysing to give compound (I).

(40) Accordingly, the invention provides in another of its aspects a method of forming a compound of formula (I), comprising the step of forming the mixture of compound (VII) and (VIII).

(41) When compound (III) is obtained in a mixture with regio-isomers, in particular in mixture with 2-isobutyl-4-methyl benzaldehyde (X), it is difficult to separate them by distillation. However, the separation of compound (III) can be facilitated by distillation of the corresponding dialkylacetals, preferably the corresponding dimethyl acetals (Ma+Xa) or diethyl acetals (IIIb+Xb). This is demonstrated by the comparison of the increasing differences of retention times in GC separation which are indicative of an improved separation by distillation (FIG. 1).

(42) Therefore, the corresponding dialkylacetal of compound of formula (III), in particular the diethylacetal of compound of formula (III), which is compound (IIIb), is a useful intermediate for preparation of desired compound of formula (I), and constitutes a further aspect of the present invention.

(43) The mixture of regio-isomers (III) and (X) is converted in a first step a) to the corresponding mixture of diethyl acetals (IIIb+Xb) by treatment with triethyl orthoformate and catalytic amounts of BF.sub.3.Et.sub.2O. In a second step b), after neutralization of the crude reaction mixture, a distillation is performed to obtain the diethyl acetal (IIIb) in essentially pure form. The compound of formula (IIIb) can be either hydrolysed to the compound of formula (III), or converted directly to the compound of formula (VI) in a similar manner as described above.

(44) ##STR00014##

(45) Alternatively, BF.sub.3.Et.sub.2O can be replaced by p-Toluenesulfonic acid (pTSA), and after removal of the undesired isomer, the Mller-Cunradi reaction can be performed even in the presence of pTSA. Therefore the acetal formation, distillation and Mller-Cunradi reaction could be performed as a one-pot process.

(46) Accordingly, in a further aspect, the invention provides a method to purify or separate compound (III) and thereby to enhance the purity of the products obtained in further reactions, in particular the compound of formula (I).

(47) There now follows a series of examples that serve to further illustrate the invention.

EXAMPLE 1: SYNTHESIS OF 3-(4-ISOBUTYL-2-METHYLPHENYL)PROPANAL

A) 3-isobutyl toluene (II)

(48) A mixture of freshly distilled 3- and 5-isobutyl-1-methylcyclohex-1-ene (700 g, 4.6 mol) was passed vertically through a glass tube (250 cm) filled with 100 g of palladium on alumina pellets (Aldrich, art. 205745) and heated to 200 C. The cyclohexene was passed though the column at a rate of 2 ml/min at 32 mbar. The crude (II) was condensed and collected in a recipient at the bottom of the column. The product, containing 90% (II) and 10% 1-isobutyl-3-methylcyclohexane was purified by distillation (bp. 105 C., 88 mbar) over a 50 cm packed column to afford pure (II) (566 g, 83% yield).

(49) .sup.1H-NMR (400 MHz, CDCl3): =7.24 (dd, J=7.58 Hz, 1H), 7.05 (m, 3H), 2.52 (d, J=7.07 Hz, 2H), 2.41 (s, 3H), 1.94 (m, 1H), 0.99 (d, J=6.82 Hz, 6H) ppm. .sup.13C-NMR (400 MHz, CDCl3): =141.7 (s), 137.6 (s), 130.0 (d), 128.0 (d), 126.4 (d), 126.2 (d), 45.5 (t), 30.3 (d), 22.5 (2q), 21.5 (q) ppm. GC/MS (EI): 148 (M.sup.+, 26), 106 (42), 105 (100), 103 (8), 91 (18), 79 (7), 77 (11), 43 (8), 41 (8), 39 (8).

B) 2-methyl-4-isobutyl bromide (IV)

(50) The reactor was flushed with nitrogen and (II) (5440 g, 36.7 mol) was added. Iron powder (102 g, 1.8 mol) and iodine (1 g) was added while stirring. The mixture was cooled to 10 C. and dibromine (5860 g, 36.7 mol) was added drop wise over 6 hours at 10 C. During the addition one molar equivalent of hydro bromic acid is produced that must be absorbed by appropriate means. Following the addition the reaction was stirred for 1 h at room temperature and then washed with 101 of NaOH 2M. The mixture was extracted twice with hexane, then the organic layers were combined, washed with water and brine and concentrated in vacuo. Short path distillation (120 C., 8 mbar) afforded (IV) (4580 g, 55% yield).

(51) .sup.1H-NMR (400 MHz, CDCl3): =7.46 (d, J=8.07 Hz, 1H), 7.06 (s, 1H), 6.87 (d, J=8.07 Hz, 1H), 2.45 (d, J=7.09 Hz, 2H), 2.42 (s, 3H), 1.88 (m, 1H), 0.95 (d, J=6.60 Hz, 6H) ppm. .sup.13C-NMR (400 MHz, CDCl3): =141.0 (s), 137.2 (s), 132.0 (d), 131.7 (d), 128.3 (d), 122.0 (s), 44.7 (t), 30.1 (d), 22.9 (q), 22.3 (2q) ppm. GC/MS (EI): 228 (M.sup.+, 20), 226 (M.sup.+, 20), 186 (21), 185 (97), 184 (23), 183 (100), 105 (19), 104 (14), 103 (17), 77 (13).

C) 2-methyl-4-isobutyl benzaldehyde (III)

(52) Magnesium turnings (171 g, 7 mol) was placed in a reactor and covered with THF. A small amount (6 ml) of (IV) was added and the reaction initiated by gentle heating. The remaining (IV) (1589 g, 7 mol) was mixed with THF (3 l) and added drop wise while maintaining a gentle reflux (70-85 C.) without external heating. After the addition was complete, the mixture was stirred at reflux for an additional hour. The reaction mixture was cooled to 10 C. and dimethyl formamide (566 g, 7.7 mol) was added drop wise over 1 hour keeping the temperature below 30 C. The reaction mixture was stirred for 1 hour and then quenched with ice cold HCl (2M). The mixture was extracted with hexane, the organic layers were combined and washed with water and brine. The solution was dried over MgSO.sub.4 and concentrated in vacuo. Distillation over a 100 cm packed column (b.p. 105 C., 2.5 mbar) gave pure (III) (592 g, 48% yield).

(53) .sup.1H-NMR (400 MHz, CDCl3): =10.22 (s, 1H), 7.71 (d, J=7.82 Hz, 1H), 7.14 (d, J=7.58 Hz, 1H), 7.04 (s, 1H), 2.65 (s, 3H), 2.50 (d, J=7.34 Hz, 2H), 1.91 (m, 1H), 0.92 (d, J=6.85 Hz, 6H) ppm. .sup.13C-NMR (400 MHz, CDCl3): =192.3 (d), 148.2 (s), 140.5 (s), 132.6 (d), 132.3 (d), 132.2 (s), 127.1 (d), 45.4 (t), 30.1 (d), 22.4 (2q), 19.6 (q) ppm. GC/MS (EI): 176 (M.sup.+, 53), 134 (100), 133 (38), 106 (14), 105 (70), 103 (14), 91 (37), 77 (19), 43 (30), 41 (14).

D) 3-(4-isobutyl-2-methyl phenyl)propen-2-al (VI)

(54) A reactor was charged with (III) (1 kg, 5.68 mol), methanol (400 ml) and trimethyl orthoformate (900 g, 8.49 mol). The reaction mixture was cooled to 10 C. and hydrochloric acid (37%, 1 g) was added. The reaction was exothermal and the temperature was allowed to rise to 25 C., the mixture was stirred for 30 minutes. The reaction was quenched with sodium acetate (20 g) and the volatiles were removed by distillation under vacuum. The residual acetal was charged into a second reactor and boron trifluoride etherate (1 g) was added and ethyl vinyl ether (538 g, 7.5 mol) was added drop wise over 4 hours while maintaining the temperature at 25-30 C. The reaction mass was quenched with saturated sodium carbonate (500 ml). The resulting crude methoxy ethoxy acetals were hydrolysed with water (500 ml) containing hydrochloric acid (37%, 50 g) at 90 C. for 5 hours. The intermediate (VI) was short path distilled at 120 C.

(55) .sup.1H-NMR (400 MHz, CDCl3): =9.87 (s, 1H), 7.78 (d, J=15.89 Hz, 1H), 7.55 (d, J=8.31, 1H), 7.06 (m, 1H), 6.96 (m, 1H), 6.68 (m, 1H), 2.50 (s, 2H), 2.49 (s, 3H), 1.88 (m, 1H), 0.94 (d, J=6.60 Hz, 6H) ppm. .sup.13C-NMR (400 MHz, CDCl3): =194.0 (d), 150.4 (d), 145.5 (s), 137.8 (s), 131.9 (d), 130.3 (s), 128.7 (d), 127.5 (d), 126.7 (d), 45.3 (t), 30.1 (d), 22.40 (2q), 19.8 (q) ppm. GC/MS (EI): 202 (M.sup.+, 8), 187 (42), 159 (31), 145 (100), 141 (13), 131 (30), 129 (20), 128 (22), 116 (18), 115 (34).

E) 3-(4-Isobutyl-2-methyl phenyl)propanal (I)

(56) The distilled (VI) was charged into an autoclave and isopropanol (200 ml) was added. The unsaturated aldehyde was hydrogenated over palladium (5%) on carbon at 0.5 bar hydrogen pressure. The mixture was filtrated and concentrated in vacuo. The crude product was purified by distillation over a 50 cm packed column (b.p. 116 C., 0.05 mbar) to provide the product (I) (926 g, 80% yield based on (III)).

(57) Odor: floral, aldehydic, green, rubbery, Lilial, watery. .sup.1H-NMR (400 MHz, CDCl3): =9.88 (t, J=1.5 Hz, 1H), 7.07 (d, J=7.6 Hz, 1H), 7.0-6.95 (m, 2H), 2.98-2.93 (m, 2H), 2.79-2.74 (m, 2H), 2.46 (d, J=7.1 Hz, 2H), 2.33 (s, 3H), 1.95-1.82 (m, 1H), 0.95 (d, J=6.6 Hz, 6H) ppm. .sup.13C-NMR (400 MHz, CDCl3): =202.2 (d), 140.2 (s), 136 (s), 135.9 (s), 131.6 (d), 128.6 (d), 127.3 (d), 45.4 (t), 44.6 (t), 30.6 (d), 25.5 (t), 22.9 (q), 19.7 (q) ppm. GC/MS (EI): 204 (M.sup.+, 23), 161 (100), 147 (26), 143 (49), 119 (84), 118 (34), 117 (33), 115 (33), 105 (59), 91 (36).

EXAMPLE 2: SYNTHESIS OF 1-(DIETHOXYMETHYL)-4-ISOBUTYL-2-METHYLBENZENE (IIIB)

(58) A mixture 85:15 of (III) and (X) (200 g, 1.13 mol) was placed in a reactor and trifluoroborane THF complex (1 g, 0.01 mol) was added. Triethyl orthoformate (200 g, 1.35 mol) was added over 20 minutes at 25-30 C. while cooling with an ice bath. The dark red reaction mixture was stirred for 10 minutes and then triethylamine (2 ml, 0.01 mol) was added and the mixture containing (IIIb) and (Xb) was distilled over a 30 cm column filled with wire mesh cylinders (23 mm) to afford pure (IIIb) (b.p 100 C., 2.6 mbar, 197 g, 69% yield)

(59) .sup.1H-NMR (400 MHz, CDCl3): =7.46 (d, J=7.83 Hz, 1H), 6.96 (dd, J=7.58, 1.47 Hz, 1H), 6.93 (s, 1H), 5.54 (s, 1H), 3.56 (m, 4H), 2.42 (d, J=7.09 Hz, 2H), 2.35 (s, 3H), 1.84 (dt, J=13.39, 6.88(2) Hz, 1H), 1.22 (t, J=7.09(2) Hz, 6H), 0.89 (d, J=6.60 Hz, 6H) ppm. .sup.13C-NMR (400 MHz, CDCl3): =141.7 (s), 135.8 (s), 134.1 (s), 131.3 (d), 126.2 (2d), 100.2 (d), 61.3 (2t), 45.1 (t), 30.2 (d), 22.4 (2q), 18.9 (q), 15.25 (2q) ppm. GC/MS (EI): 250 (M.sup.+, 1), 206 (15), 205 (100), 177 (27), 162 (8), 134 (10), 105 (22), 103 (8), 91 (13), 57 (10), 29 (7).