## 3 Results and Discussion
### Rationalization of the Formation of Monoterpene Isomers and Dimeric Hydrocarbons  
There have been a few papers on the SnCl4.5H2O-catalyzed organic reactions. Based on the crystal structure of SnCl4.5H2O that consists of [SnCl4(H2O)2] linked by hydrogen bonding with additional water molecules [26], the mechanism of those reactions has been explained in the manner of both Lewis and Brensted acid-base interaction. When Lewis basic heteroatom-containing reactants are used, their coordination to stannic (Sn(IV)) center is considered to be an important step of the reaction. Such examples involve \(\beta\)-amino alcohol synthesis by epoxide ring-opening [27] and \(\beta\)-amino carbonyl compound synthesis by the Mannich-type reaction [28]. On the other hand, in the absence of heteroatom in reactants, the Bronsted acidity of the coordinated water ligands is considered to be important. For example, the hydration of alkyne has been explained by the initial protonation of reactants by water, which is activated by the coordination to the stannic center [29]. Such Lewis acid-assisted Bronsted acids were also developed for enantioselective polyene cyclization [19; 20] and enantioselective protonation of silyl enol [21; 22]. Given \(\alpha\)-pinene has no heteroatom that can directly interact with the stannic center in a Lewis acid-base manner, the catalytic conversion of \(\alpha\)-pinene by SnCl4.5H2O should follow the second explanation above.  
The possible mechanisms for the conversion of \(\alpha\)-pinene to isomerized products (tricyclene, camphene, limonene, terpinolene, \(\alpha\)-terpinene, and \(\gamma\)-terpinene), disproportionated products (menthenes and \(p\)-cymene), and dimerized products by various acid catalysts have been proposed in the previous papers [8; 23]. In this work, we focused on a plausible mechanism involving interaction between \(\alpha\)-pinene and the catalyst, which is shown in Scheme 1.  
First, the protonation of \(\alpha\)-pinene (1) is thought to be promoted by the Lewis acid-assisted Bronsted acid activity of the coordinated water ligands in the Sn(IV) complex (A) via TS1 (Scheme 1). Since there is no polar solvent that can solvate the generated ions, an intimate ion-pair between carbocation and Sn(IV) are complex should be a reaction intermediate (Int1) [19; 30]. To form limonene (2), the main isomerized product from compound 1, the pinanyl moiety in the intimate ion-pair Int1 should suffer  
Figure 1: A plausible catalytic cycle of SnCl4.5H2O (**A**) proposed for the synthesis of limonene (**2**) and dimeric hydrocarbons (C20H32) from \(\alpha\)-pinene (**1**).  
from a ring-opening rearrangement, which generates a carbocation/Sn(IV) at complex ion-pair (Int2). In this intimate ion-pair Int2, the abstraction of a proton from the carbocation moiety by the hydroxo ligand via TS2 regenerates stannic complex A and furnishes compound 2 as the product of a catalytic cycle. If the insertion of another compound 1 into the intimate ion-pair Int2 is prior to the proton abstraction step, a carbocation/Sn(IV) ate complex ion-pair (Int3) appears for dimeric hydrocarbons (C\({}_{20}\)H\({}_{32}\)) as the product of the catalytic cycle. Along with the case of compound 2, we can construct distinct routes to complete the catalytic cycle involving the possible combination of carbocation/Sn(IV) ate complex ion-pairs, which are able to explain the various isomers and dimeric hydrocarbons from compound 1.