A-Pinenes

C, b.p. 156 C. The most important of the terpene hydrocarbons. It is found in most essential oils derived from the Coniferae, and is the main constituent of turpentine oil. Contains two asymmetric carbon atoms. The (- -)-form is easily obtained in a pure state by fractionation of Greek turpentine oil, of which it constitutes 95%. Pinene may be separated from turpentine oil in the form of its crystalline nitrosochloride, CioHigClNO, from which the ( + )-form may be recovered by boiling with aniline in alcoholic solution. When heated under pressure at 250-270 C, a-pinene is converted into dipentene. It can be reduced by hydrogen in the presence of a catalyst to form... 

In practice hydrogenation of a pinene is observed to be 100% stereoselective The only product obtained is cis pinane No trans pinane is formed... 

Hydroboration-oxidation of a pinene (page 235) like catalytic IS stereoselective Addition takes place at the less hindered face and a single alcohol is produced in high yield (89%) Sug... 

Monoisopinocampheylborane [64234-27-17, IpcBH2 (6) is an important asymmetric hydroborating agent. It is prepared from a-pinene (7) either directiy or better by indirect methods. The product obtained by hydroboration of a-pinene [80-56-8] is an equiUbrium mixture. Optically pure monoisopinocampheylborane is best prepared from a-pinene via diisopinocampheylborane [1091-56-1] IPC2BH (8) (64,65). Both enantiomers are readily available. 

The method does not require optically pure a-pinene because 100% enantiomeric excess (ee) is achieved by crystallisation of the intermediate TMEDA-2IpcBH2 adduct, where TMEDA = (CHg )2NCH2CH2N(CH3 )2 (tetramethylethylenediamine). Other chiral monoalkylboranes derived from 2-alkyl- and 2-phenylapopinene are slightly more selective reagents as compared to monoisopinocampheylborane (66—68). 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Terpene-based hydrocarbon resins are typically based on natural products such as a-pinene, P-pinene, and ti-limonene [5989-27-5] which are obtained from the wood and citms industries, respectively. These resins, which were originally the preferred tackifiers for natural mbber appHcations, possess similar properties to aHphatic petroleum resins, which were developed later. Terpene-based resins have been available since the mid-1930s and are primarily used in the adhesives industry. 

Terpenes, specifically monoterpenes, are naturally occurring monomers that are usually obtained as by-products of the paper and citms industries. Monoterpenes that are typically employed in hydrocarbon resins are shown in Figure 2. Optically active tf-limonene is obtained from various natural oils, particularly citms oils (81). a and P-pinenes are obtained from sulfate turpentine produced in the kraft (sulfate) pulping process. Southeastern U.S. sulfate turpentine contains approximately 60—70 wt % a-pinene and 20—25 wt % P-pinene (see Terpenoids). Dipentene, which is a complex mixture of if,/-Hmonene, a- and P-pheUandrene, a- and y-terpinene, and terpinolene, is also obtained from the processing of sulfate Hquor (82). 

A process of polymerization of isomerized a-pinene or turpentine with vinylbenzenes has been disclosed (105). a-Pinene or turpentine is isomerized by flash pyrolysis at 518 5° C in a hot tube reactor to yield a mixture of predominantly dipentene and i7t-alloocimene... 

---