Лако-красочные материалы — производство

In the 1950s, a new class of musks was discovered, the polycyclic musks. These materials mostly have indane, tetralin or isochroman structures, heavily substituted by methyl or other small alkyl groups. Typical members of this family are Traseolide® (42), Phantolide® (43) and Celestolide®/Crysolide® (44).

Currently, the most important polycyclic musks, in commercial terms, are 6-acetyl-l, l,2,4,4,7-hexamethyltetralin and 4,6,6,7,8,8-hexa- methyl-1,3,4,6,7,8-hexahydrocyclopenta[g]lisochromene. These two compounds are each known by a variety of tradenames, depending on the manufacturer. The commonest tradenames for the tetralin are Fixolide®, Tetralide® and Tonalid® and for the isochroman, Abba — lide® and Galaxolide®. Both fragrance materials have been of major importance to the perfume industry throughout the second half of the twentieth century and so it is worthwhile looking in more detail at the chemistry of their syntheses. The synthesis of Galaxolide® is shown in Scheme 4.44.

Cumene is prepared on a large scale from benzene and propylene as an intermediate in the synthesis of acetone and phenol. This makes it an inexpensive and readily available starting material for the production of Galaxolide®. Three further electrophilic addition reactions complete the synthesis. Firstly, isoamylene is added, to form pentamethylindane, to which propylene oxide is added. Finally, treatment with formalde­hyde leads, via the hemiacetal, to the isochroman.

The synthesis of Tonalid® is shown in Sheme 4.45. The starting materials, as for Galaxolide®, are all inexpensive and readily available chemicals. /?-Cymene is a terpene and is found as a by-product in many processes that involve heating of terpenes, since it lies at the bottom of a potential energy well. Further details are given in the section on terpenes. Dimerization of butylene gives di-isobutylene, which under­goes olefin metathesis with ethylene to give neohexene. The Friedel — Crafts reaction between />-cymene and neohexene gives hexamethyl — tetralin, which can then be acetylated to produce Tonalid®. The cycloalkylation stage in this sequence is interesting and more detail of the mechanism is shown in Scheme 4.46. As written in Scheme 4.45, the reaction requires two moles of neohexene per mole of /?-cymene. Half of the olefin serves as a hydride abstraction agent (i. e. an oxidant) and the other half as the alkylating species. Protonation of the olefin generates a carbocation. This abstracts a hydride ion from the /?-cymene to give the more thermodynamically stable /?-cymyl cation. The latter then adds to a second molecule of neohexene. This addition occurs with a concomitant methyl shift, so that the next cation to be formed is tertiary rather than secondary and hence somewhat more stable. This product cation adds to the aromatic ring to give the hexamethyltetralin.

Neohexene is by far the more expensive of the two starting materials, and so the process shown in Scheme 4.45 is unsatisfactory, since it consumes twice as much of this reagent as is desirable. One way of overcoming this problem is to add a sacrificial oxidant that is less expensive than neohexene. For example, /-butyl chloride, under the influence of aluminium chloride, loses its chlorine to produce the /- butyl cation, which can abstract the hydride ion from p-cymene and thus save one molar equivalent of neohexene.

An alternative solution is shown in Scheme 4.47, the Quest synthesis of a musk mixture known under the trade name Extralide®. In this synthesis, no oxidation is necessary in the cycloalkylation stage because the appropriate carbon atom of the /?-cymenyl structure is already at the correct oxidation level, since it carries a hydroxyl group. This is achieved by dehydrogenation of a-terpineol to give /ьсутепоі. Reac­tion of this with 2,3-dimethylbut-2-ene, as an alternative to neohexene, gives a mixture of hexamethyltetralin and isopropyltetramethylindane, acylation of which gives a mixture of two musks. This mixture performs almost identically with pure Tonalid® in perfumes and has the advantage of a lower melting point, which makes dissolution in perfumes easier.