Rearrangement of 4,4,5,8-Tetramethyl-l-oxaspiro- 2,5 octane

Ring opening of (16) can be performed with Lewis acid homogeneous catalysts such as Bp3.Et20 [8], Mgl2 [41], or SnC [41] with almost quantitative yield at room temperature in toluene as solvent. 

The performance of zeolitic catalysts is moderate in liquid-phase slurry reactors, e. g. with H-US-Y (Si/Al = 35) and H-US-Y (Si/Al = 35) modified by HCl treatment (Table 3). 

Two main reactions take place, rearrangement of (16) to the aldehyde (17) then oxidative decarbonylation to the olefin 1,3,3,4-tetramethylcyclohex-l-ene (18), presumably as a result of elimination of formaldehyde. Decarbonylation can also occur, leading to 1,1,2,5-tetramethylcyclohexane (19). Besides these main products many other side-products can be formed. 

The highest selectivities are obtained at low temperatures (ca 0 °C). To obtain high conversion, long residence times are necessary. This means that the space-time yield (STY) in the liquid phase is inferior to that in the gas phase. The catalytic performance of HCl-treated H-US-Y zeolite is better than that of the untreated catalyst, possibly because of the high Lewis acidity and high mesoporosity of this material [29]. 

Examination of gas-phase reactions revealed that the conversion of (16) decreases rapidly in the presence of acidic zeolites such as H-B-ZSM-5, H-ZSM-5, and H-US-Y [43]. This behavior is even more apparent over BPO4 and Nb205- The best performance was that of the silica catalyst Dll-10 (BASF AG), which is mildly acidic-selectivity for 2 was ca 40 % with no decrease in conversion after 8 h on stream, even at 230 °C. The starting material seems to be too bulky to enter the pore system of the medium-sized weakly acid boron-pentasil zeolites [43]. By optimization of reaction temperature and the residence time with respect to the WHSV, and reducing the pressure to 100 mbar the selectivity could be increased to 53 % with complete conversion and no deactivation occurred during 8 h on stream. 

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