Deprotonation base/temperature effects

In the group of van Koten, dilithiated precursors for the peripheral functionalization of carbosilane dendrimers were generated by deprotonation of compounds 32, 34a and 34b using f-butyllithium. The reaction was effected in n-pentane at room temperature, using the appropriate amount of the alkyllithium base. Dilithiated compounds 33, 35a and 35b were almost quantitatively obtained, the para positions of the aromatic ring systems... 

Korneev and Kaufmann successfully lithiated 2-bromo-l,l-diphenylethylene (46) by bromide-lithium exchange to form 2-lithio-l,l-diphenylethylene (47). A second lithia-tion could be effected in four hours at room temperature by deprotonation of the aromatic ring with w-butyllithium in the presence of TMEDA (Scheme 17). Like in the synthesis of compound 23, the first lithiation activates the ortho-hydrogen atom of the Z-phenyl substituent to give 1,4-dilithium compound 48. In total, three equivalents of the alkyl-lithium base are required the third equivalent is consumed in the trapping reaction of w-bromobutane with generation of octane. 

The condensation of silanols in solution or with surfaces has not been as extensively studied and therefore is less well understood. The limitation until recently has been the lack of suitable analytical methods necessary to monitor in real time the many condensation products that form when di- or trifunctional silanols are used as substrates. With the advent of high-field wSi-NMR techniques, this limitation has been overcome and recent studies have provided insights into the effects of silanol structure, catalysts, solvent, pH, and temperature on the reaction rates and mechanisms. Analysis of the available data has indicated that the base catalyzed condensation of silanols proceeds by a rapid deprotonation of the silanol, followed by slow attack of the resulting silanolate on another silanol molecule. By analogy with the base catalyzed hydrolysis mechanism, this probably occurs by an SN2 -Si or SN2 -Si type mechanism with a pentavalent intermediate. The acid catalyzed condensation of silanols most likely proceeds by rapid protonation of the silanol followed by slow attack on a neutral molecule by an SN2-Si type mechanism. 

In 1999, Carreira identified Zn(II) as a metal that, like Ag(I) and Cu(I), is capable of effecting the metalation of terminal acetylenes under mild conditions. Thus, treatment of terminal alkynes with Zn(OTf)2 and NEt3 at room temperature led to the formation of zinc alkynylides (Eq. 4). The zinc salt and the amine base work in synergy to weaken the acetylenic proton, with the acetylene undergoing complexation to the Zn(II) center and the base effecting subsequent deprotonation (Fig. 1) [11]. 

Nolan s heterocyclic carbene-based system (7 + KOf-Bu/Pd2(dba)3) was effective in the coupling of secondary amines with aryl chlorides at elevated temperatures, Eq. (47) [52]. This protocol could be used for the room-temperature ami-nation of aryl bromides as well. Hartwig reported that the saturated heterocyclic carbene ligand prepared by deprotonation of 16 forms a catalyst that is considerably more reactive than the system reported by Nolan. The resulting complex formed was capable of coupling aryl chlorides with cyclic amines at room temperature [76]. 

In addition to the generation of phosphonium ylides from phosphonium salts by deprotonation with bases in some instances ylides may result from pyrolysis of phosphonium salts, especially silylated salts (equation 17). Similar fluoride ion induced desilylation (equation 18) of phosphonium salts proved to be a very useful alternative for the synthesis of ylides which are difficult to synthesize by the conventional salt method (as in the case of R, R = alkyl). - The most effective fluoride source is cesium fluoride and the reaction proceeds at room temperature. 

Both of these centres were selectively carboxymetliylated (hard reaction, Scheme 13). Classical metal catalysts such as Pb(0Ac)2 or Sn[O2CCH(Et)Bu]2 effected N-2 activation under refluxing conditions (Table 10, entries 4-6) whilst potassium /-butoxide at room temperature activated N-1 (Table 10, entiy 1). It was reasoned that the reaction with a strong base was due to the deprotonation of N-1 creating an anion with hard nucleophilicity... 

The a-metallation of dialkyl chloromethylphosphonates with strong bases such as n-BuLi, x-BuLi, LDA, or LiTMP quantitatively gives dialkyl 1-lithio-l-chloromethylphosphonates. In contrast, the use of t-BuLi at low temperature leads to a mixture of H/Li and Cl/Li exchange products in a 9 1 ratio. In these conditions, the I-lithio-I-chloromethylphosphonate is stable only at low temperature. The same carbanion prepared using 2 eq of hindered amides such as LDA or LiTMP is stable at 0°C for 30 min without apparent degradation. The first equivalent of base effects the deprotonation, and the second probably stabilizes the resulting carbanion. ... 

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