Alkoxylation of limonene

The liquid phase alkoxylation of limonene (3) with C4-C4 alcohols to 1-methyl-4-[a-alkoxy-isopropyl]-l-cyclohexene (5) was carried out both in batch and continuous fixed-bed reactor at 60 °C on various acidic catalysts (Scheme 3.1) [16]. The best yields were obtained in batch (85%) or continuous reactor (81%) using a /1-type zeolite with Si02/Al203 = 25. 

The zeolite-catalyzed alkoxylation of limonene (53, 54) and alpha-pinene (55, 56) over acid-treated mordenite, clinoptilolite and ferrierite as catalysts has already been reported in the literature. The best results were obtained for methoxylation of limonene in the presence of a clinoptilolite-type zeolite (60% yield). The alkoxylation of alpha-pinene with methanol in the presence of mordenite also achieved the highest yields of 66% for l-methyl-4-[alpha-methoxy-isopropyl]-l-cyclohexene. Syntheses of l-methyl-4-[alpha-alkoxy-isopropyl]-l-cyclohexenes via zeolite-catalyzed alkoxylation of other terpenes were reported in a review paper (57). 

The reaction pathway is identical to that of alpha-pinene alkoxylation except no bicyclic compounds are formed. As mentioned in the literature (53, 54), alkoxylation of limonene to l-methyl-4-[alpha-alkoxy-isopropyl]-l-cyclohexene can be carried out only in the presence of acidic catalysts. After a catalyst screening using various zeolitic and non-zeolitic acid heterogeneous catalysts, we found that beta zeolite is the best candidate. 

Addition of methanol to limonene in the presence of a beta zeolite produces the highest selectivity to l-methyl-4-[alpha-methoxy-isopropyl]-l-cyclohexene of about 93% at 91% conversion. Surprisingly, the highest yield of about 85% has been obtained at room temperature. The other zeolites and solid acids applied for the alkoxylation of limonene reveal considerably lower conversion and selectivity. 

The most likely reason for the high activity of zeolite BEA is the relatively high BET surface area of the catalyst (750 m2/g). Furthermore there are hints by temperature-programmed desorption (TPD) of ammonia that a large amount of acid sites are present. We assume that the alkoxylation of limonene takes place inside the pore structure of the beta zeolite. The high selectivity of zeolite BEA might originate from suitable acid sites in pores of its defined size and shape. 

We assume that the alkoxylation of limonene takes place inside the pore structure of the beta zeolite. We also assume that the high selectivity of beta zeolite originates from suitable acid sites in pores of its defined size and shape. 

Several ways to produce such a-terpinyl alkyl ethers are known, but limonene and a-pinene are the most used feedstock molecules (Figure 8). Alkoxylation of limonene and a-pinene over homogeneous or heterogeneous catalysts as, for example, strong acids FICl, H2SO4 or p-toluenesulfonic acid, aluminium trichloride and boron trifluoride etherate and acidic cation exchange resin has... 

Reactions in the Batch Reactor - Methanol reacts with limonene over acidic catalysts in a batch reactor to l-methyl-4-[a-methoxyisopropyl]-l-cyclohexene (a-terpinyl methyl ether) as the main reaction product (see Figure 8 R = Me). Besides the desired methoxylation, isomerization reactions leading to terpinolene and traces of a- and y-terpinene can be observed. Furthermore, the addition of methanol to the terpinyl methyl ether leads to the undesired cis- or fra/w-1,8-dimethoxy-p-menthane. The amount of unidentified products does not exceed 1%. At high temperatures and long reaction times the reverse reaction of the a-terpinyl methyl ether and the other addition products to limonene and its isomers can be observed. The reaction scheme of the alkoxylation of limonene is illustrated in Figure 10... 

The heterogeneously catalyzed alkoxylation of alpha-pinene and limonene over beta zeolite provides excellent results in both a discontinuous batch reactor and a continuous flow-type apparatus with a fixed bed reactor. In both reactors, the use of methanol as addition compound and limonene as feedstock gives l-methyl-4-[alpha-methoxy-isopropyl]-l-cyclohexene with the yield of 85% (conversion 93%, selectivity 92%). By means of variation of the reaction parameters, the limonene conversion can be adjusted within the range 40 - 90%. The selectivity to 1-methyl-4-[alpha-methoxy-isopropyl]-l-cyclohexene always remains at about 95%. 

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

It is interesting to note that the destruction of the structure of beta zeolite by treatment with strong acids or high temperature leads to a complete deactivation of the catalyst for limonene alkoxylation. By using a higher reaction temperature only isomerisation and polymerisation products have been obtained. 1-methyl-4-[alpha-methoxy-isopropyl]-1 -cyclohexene or other addition products cannot be found. 

Interestingly, a cascade alkoxyl radical fragmentation-peroxidation-hydrogen abstraction reaction occurs in some cases when a hemiacetal is treated with DIB/I2 under oxygen pressure. This reaction may have interesting applications in synthetic organic chemistry. We have used it in a one-step synthesis of A and A rings of the tetranortriterpene limonene and related compounds (Eq. 19, Scheme 6) [48]. 

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