## 3 Results and Discussion
### Rationalization of the Formation of Heteroatom-Containing Dimeric Products  
As previously mentioned, from \(\alpha\)-pinene, stannic chloride molten salt hydrates (SnCl\({}_{4}\cdot\)5H\({}_{2}\)O) can produce dimerized products that are high-density fuel candidates. There is one limitation of this reaction system, however, which involves forming not only the dimeric hydrocarbons that show molecular ion peaks at \(m\)/z = 272 [C\({}_{20}\)H\({}_{32}\)]\({}^{+}\) but also the oxygen-containing dimeric products, such as possibly dimeric alcohols or dimeric ethers, that show molecular ion peaks at \(m\)/z = 290 [C\({}_{20}\)H\({}_{34}\)O]\({}^{+}\). Although the inclusion of oxygen-containing species increases the overall density of the fuel mixture [5], which matches the purpose of augmenting the energy density of renewable fuel, this is not suitable considering the synthesis of hydrocarbons for drop-in fuel. Therefore, to exclude the production of oxygen-containing species from the reaction system catalyzed by SnCl\({}_{4}\cdot\)5H\({}_{2}\)O, we have started the rationalization of how such species are formed.  
From the fact that the formation of borryl chloride (3), one particular chlorine-containing side-product, the decomposition of Sn(IV) complex (A) during the reaction can be postulated (Scheme 2, left circle). In other words, to form compound 3, the protonation of \(\alpha\)-pinene (1) by stannic complex A and successive ring enlargement by Wagner-Meerwein rearrangement occur first, furnishing a carbocation/Sn(IV) ate complex ion-pair (Int4). This intimate ion-pair Int4 then suffers from the dissociation of chloride ligand, producing both compound 3 and decomposed Sn(IV) complex (B).  
**Scheme 2.** A plausible mechanism of catalyst (**A**) decomposition suggested based on the production of borryl chloride (**3**, left circle) and oxygen-containing dimeric products (C\({}_{12}\)H\({}_{32}\)O, right circle).  
As previously mentioned, for the conversion of \(\alpha\)-pinene (1) to occur, carbocation/Sn(IV) are complex ion-pairs (Int1-4) should be reaction intermediates (Schemes 1 and 2). We considered that these intermediates are essential clues for the explanation of how the oxygen-containing species are formed. While bornyl chloride (3) is formed when the dissociation of chloride ligand (Sn-Cl bond) occurs in the intimate ion-pair Int4, monoterepene alcohols can be formed by the dissociation of hydroxo ligand (Sn-O bond). At this point, the monoterepene alcohols can occupy an empty coordinate site of the stannic center as an alcohol ligand, which generates ligand exchanged Sn(IV) complex (C) (Scheme 2, right circle). Since the same Lewis acid-assisted Brensted acidic water ligands work as both catalysts and reactants, this alcohol ligand can also protonate another compound 1, furnishing a carbocation/Sn(IV) are complex ion-pair (Int5) via TS3. From this intimate ion-pair Int5, the dissociation of the remained alkoxo ligand can cause the formation of dimeric ethers (C\({}_{20}\)H\({}_{32}\)O) and decomposed Sn(IV) complex (D).  
Based on the above rationalization, we have concluded that, although the reaction system catalyzed by the Lewis acid-assisted Brensted acid activity of SnCl\({}_{4}\)5H\({}_{2}\)O can produce high-density fuel candidates from \(\alpha\)-pinene, to synthesize dimeric products as only hydrocarbons, the water ligands must be switched. Instead of them, monodentate alcohols or phenol ligands, which will form alkoxo or aryloxo ligand after the protonation of \(\alpha\)-pinene, respectively, also should be avoided since these ligands can furnish undesirable ether products. Therefore, we have considered bidentate 2-alkoxyphenol ligands such as 2-methoxyphenol (guaiacol) and 2,6-dimethoxyophenol (syringol), given they can be tightly bonded to the metallic center with their methoxy group even after the protonation of \(\alpha\)-pinene [19; 20; 21; 22]. To prove this hypothesis, further study should be followed.