Reviews and Accounts
ARKIVOC 2014 (i) 109-126
3.1.2 The influence of the vessel material. When we attempted to carry out the fluorodesilylation reactions (Equation 2) in a PTFE flask no reaction was observed. In a series of experiments using 4-tolyltrimethylsilane 10 (Ar = 4-MeC6H4) and two equivalents of XeF2 in C6F6 in reaction flasks made of PTFE, quartz or soda glass no reaction was observed. However, in a Pyrex flask a ~70% yield of 4-fluorotoluene was obtained. When the Pyrex flask was pre-washed with 2M NaOH, rinsed with acetone and dried, there was no reaction of the silane substrate but the flask could be reactivated by washing with chromic acid. We concluded from these studies that the Pyrex surface acts as a heterogeneous acid catalyst.23,25 Pyrex is a borosilicate glass containing ~13% B2O3 and ~2% Al2O3 and we propose that XeF2 can bind to the Lewis acid sites residing in the Pyrex surface to give a polarised form [FXe+---F→Pyrex-] that can act as an electrophilic agent leading to the intermediate 13 as shown in Equation 3. In our earlier studies our enthusiasm for making direct comparisons with hypervalent iodine chemistry led us to suggest that electrophilic reactions occur by reaction with FXe+ (cf (Ar2I+). However, FXe+ is far too unstable to be formed under these reaction conditions and it is more reasonable to propose that it reacts as an FXe+ equivalent, as for instance, in FXe+---F→Pyrex-. 3.1.3 The influence of solvent. We established that freon (CFCl3) is an alternative solvent to C6F6 but when acetonitrile was used as solvent aryl fluoride formation (Equation 2) did not take place for a range of substrates 10 (yields 0-4%). Acetonitrile is a weak base and we assume that it blocks the Lewis acid sites on the Pyrex surface [MeCN→Pyrex] preventing activation of the XeF2, as shown in Equation 3. These observations prompted us to make a 19F NMR study of the stability of XeF2 in C6F6, CFCl3, MeCN, CHCl3, CH2Cl2 and H2O solution, and in the corresponding deuterated solvents. These studies were carried out using nmr tubes made of Pyrex or quartz, or lined with PTFEFEP. Table 1 shows the observed half-lives (t½) and lifetimes (t1) of XeF2 in the various vesselsolvent combinations.25 Table 1. Approximate half-lives (t½) and lifetimes (t1) of XeF2 in vessel/solvent systems26 Solvent CH2Cl2 CHCl3 CFCl3 CH3CN H2O C6F6
PTFE-FEP t½ t1 >7 days »7 days ~4 days 6 days >7 days »7 days >7 days »7 days <1 h < 24 h >7 days »7 days
Pyrex t½ 0.6 h 0.25 h 1h 48 h 2h 4h
t1 2h 0.6 h 2.5 h 72 h 7h 10 h
Quartz t½ 2.5 h 2.5 h 1.5 h >168 h 1.5 h -
t1 4h 4h 3h »168 h 4h -
With the exception of H2O as solvent, XeF2 has good stability in PTFE-FEP with half-lives of several days in all the solvents studied (Table 1). In contrast, solutions in Pyrex tubes have half-lives of hours, with the interesting exception of MeCN as solvent in which the half-life is Page 114
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