Catalysis of Organic..

Falling et al. 331salt, TOP18 has virtually no vapor pressure and it is reasonably low melting. It iseasily synthesized by reaction of tri(n-octyl)phosphine with n-octadecyl iodide at areasonable cost. Toxicity studies on TOP18 showed it to be of low toxicity: OralLD-50 (rat), >2000 mg/kg; Dermal LD-50 (rat), >2000 mg/kg.Lewis Acid Catalyst ComponentAs in the case of the iodide salt component, a high concentration of Lewis acid isnecessary for adequate reaction rate. Inorganic Lewis acids (such as zinc iodide) arethe obvious first choice based on availability and cost. However these catalysts alsohave very poor solubility in the reaction mixture without the use of a polar, aproticsolvent. Fortunately a family of Lewis acids was found with improved reactionselectivity and good solubility—tri(organo)tin iodides. These Lewis acids wereinvestigated due to their known activity in the formation of cyclic carbonates fromepoxides and carbon dioxide (13). Tri(organo)tin iodides were found to beconsiderably better than di(organo)tin diiodides and mono(organo)tin triiodides forthe desired 1 rearrangement. Tri(organo)tin bromides and chlorides (in conjunctionwith an onium bromide or chloride) were less active and less selective than the alliodidesystems.Organotin compounds are produced commercially for use in PVC stabilizationand as agricultural chemicals (14). Tri(organo)tin iodides are appropriate candidatesfor consideration due to their excellent catalytic activity, high selectivity, availabilityof starting materials and ease of preparation. Triphenyltin iodide is a crystallinesolid (mp 121ºC) and is soluble in hot alkanes. It is easily prepared fromcommercially-available triphenyltin chloride by reaction with sodium iodide.Although triphenyltin iodide had the highest activity for rearrangement with lowoligomerization, it had less than acceptable stability. In extended continuouslaboratory runs, the breakdown of triphenyltin iodide to diphenyltin diiodide andbenzene was observed. This reaction can occur by reaction with low levels ofhydrogen iodide that are present in the system.Ph 3 SnI + HI Ph 2 SnI 2 + PhHThis loss of catalyst and contamination of product with benzene caused us toselect the more stable tri(n-alkyl)tin iodides. These catalysts are not as active as thetriaryltin iodides but exhibit very good stability under normal reaction conditions.They are also readily prepared from industrially-available bulk starting materials.Tri(n-octyl)tin iodide [(n-Oct) 3 SnI, referred to as "TOT"] can be prepared fromthree different starting materials although the iodide displacement is preferred:(n-Oct) 3 SnCl + NaI (n-Oct) 3 SnI + NaClTOTTri(n-butyl)tin iodide is also an effective catalyst and its starting materials areavailable but TOT was selected due to its lower volatility. The octyltin compoundsare also generally much less toxic than their butyltin counterparts (15).