Catalysis of Organic..

330Rearrangement of 3,4-Epoxy-1-Butene to 2,5-Dihydrofurananalysis (TGA) of tetra(n-heptyl)ammonium iodide shows decomposition beginningat 180ºC and complete loss of weight by 285ºC.Quaternary phosphonium iodides are also good choices for the iodide saltcatalyst component because they are highly active and, in some cases, soluble in thereaction and recovery processes. The simple quaternization of tri(n-alkyl)phosphinesor triarylphosphines with n-alkyl iodides produces a wide variety of low costphosphonium iodide salts:R 3 P + R’I R 3 R’P + I -Although the number of available tri(alkyl/aryl)phosphine starting materials maybe somewhat lower than with the tri(n-alkyl)amines, the large number ofcommercially available n-alkyl iodides allows for the synthesis of manyphosphonium iodide catalyst candidates. Triphenylphosphine is an attractive startingmaterial due to its industrial availability and low cost. However it was found that theresulting (n-alkyl)(triphenyl)phosphonium iodides had poor solubility in alkanesolvents. They also tended to have high melting points which is a disadvantage inprocess operation. Fortunately a number of trialkylphosphines are also produced inbulk industrially. Of particular interest is tri(n-octyl)phosphine which ismanufactured in large volume for production of tri(n-octyl)phosphine oxide (TOPO)– a mining extraction solvent.The critical advantage of the quaternary phosphonium iodides is their excellenttemperature stability. For example, the selected tetra(alkyl)phosphonium iodide salt,tri(n-octyl)(n-octadecyl)phosphonium iodide [(n-Oct) 3 (n-Octadecyl)P + I - , referred toas “TOP18”], shows no TGA weight loss up to 300ºC.Table 1. Tetra(n-alkyl)phosphonium iodides tested.Tetra(n-alkyl)phosphonium iodideCarbonCount M.W. M.P. (°C)Solubilityin Octane(n-Oct) (n-Bu)PP+ 3 I - 28 554.63 55-57 insol.(n-Oct) 3 (n-Hexyl)P + I - 30 582.68 semisolid insol.(n-Oct) 3 (n-Heptyl)P + I - 31 596.71 77-79 insol.(n-Oct) 4 P + I - 32 610.73 86-88 sol. hot(n-Oct) (n-Nonyl)PP+ 3 I - 33 624.76 79-81 sol. hot(n-Oct) 3 (n-Decyl)P + I - 34 635.79 64-65 sol. hot(n-Oct) 3 (n-Dodecyl)P + I - 36 666.84 50-52 sol. warm(n-Oct) 3 (n-Hexadecyl)P + I - 40 722.94 56-58 sol. warm(n-Oct) 3 (n-Octadecyl)P + I - , TOP18 42 750.99 62-65 sol. warm(n-Dodecyl) 4 P + I - 48 835.15 77-79 sol. hot(n-Hexadecyl) 4 P + I - 64 1059.57 87-88 sol. hotThus the selected iodide catalyst, TOP18, has good catalytic activity, selectivity,and stability. It is readily soluble in 2,5-DHF and in warm alkane solvents. As a