Pentakis cyclopentadiene

The increasing order of acidity, CF3H < CF3CF2H < (CF3)2CFH < (CF3)3CH, and the fact that pentakis(tnfluoromethyl)cyclopentadiene (pK < -2 [69]) is about 10 times more acidic than cyclopentadiene, whereas pentafluorocyclopentadiene IS onlv sliffhtlv more acidic 1701. illustrate that a trifluoromethvl eroup has a much... 

The pentakis(methoxycarbonyl)cyclopentadiene is a strong acid that displaces weaker organic acids upon interaction with their metal salts (Equation (32)) 2124... 

The template synthesis for the intermediate tris- and tetrakis(meth-ylthio)-Cp s as the chloro modification is described in Scheme 21. Synthesis of pentakis(methylthio)-Cp from unsubstituted cyclopentadiene (method 2) is shown in Scheme 13. 

Pentakis(methoxycarbonyl)cyclopentadiene, which is a strong protic acid, reacts with bistributyltin oxide in benzene at room temperature to give a salt between the diaquotri-butyltin cation and the cyclopentadienide anion as shown in equation 9-48.109 The cation has a trigonal bipyramidal structure, with the water molecules in the apical positions, and these can be replaced by other ligating molecules such as HMPT and DMF. 

The strong destabilization of sp centers by fluorine is impressively demonstrated by the acidities of different fluorine-containing cyclopentadiene derivatives (Scheme 2.38) - despite the strong, cumulative negative inductive (—4) effect of five fluorine atoms, the acidity of pentafluorocyclopentadiene [90] is similar to that of cyclopentadiene. The reason for this unexpected behavior is that deprotonation would create one more fluorinated sp center. In contrast, the expected behavior is observed for the pentakis (trifluoromethyl) derivative, which is more acidic by 16 orders of magnitude [91]. 

Fluorine bonded directly to a carbanionic center is generally destabilizing by 1 repulsive interactions but when the carbanion ic atom is surrounded by trifluoromethyl groups, it is stabilized. In the case of pentakis-(trifluoromethyl)cyclopentadiene (3), stabilization of the carbanion accounts for a surprisingly high acidity (pKa<-2), comparable to nitric acid (62). 

Hoffmann and co-workers have made extensive kinetic studies of the [l,5]sigmatropic shift in pentakis(methoxycarbonyl)cyclopentadiene (88). First-order rate constants for the process (88) (89) were determined by n.m.r. at 373 K... 

---