Butyllithium estimation

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Tertiary amides 9-1, 9-2, and 9-3 are lithiated at the (3-carbon, rather than the a-carbon by s-butyllithium-TMEDA. It is estimated that the intrinsic acidity of the a-position exceeds that of the (3-position by about 9 pK units. What causes the (3-deprotonation to be kinetically preferred ... 

Butyllithium initiation of methylmethacrylate has been studied by Korotkov (55) and by Wiles and Bywater (118). Korotkov s scheme involves four reactions 1) attack of butyllithium on the vinyl double bond to produce an active centre, 2) attack of butyllithium at the ester group of the monomer to give inactive products, 3) chain propagation, and 4) chain termination by attack of the polymer anion on the monomer ester function. On the basis of this reaction scheme an expression could be derived for the rate of monomer consumption which is unfortunately too complex for use directly and requires drastic simplification. The final expression derived is therefore only valid for low conversions and slow termination, and if propagation is rapid compared to initiation. The mechanism does not explain the initial rapid uptake of monomer observed, nor the period of anomalous propagation often observed with this initiator. The assumption that kv > kt is hardly likely to be true even after allowance is made for the fact that the concentration of active species is much smaller than that of the added initiator. Butyllithium disappears almost instantaneously but propagation proceeds over periods from tens to hundreds of minutes. The rate constants finally derived therefore cannot be taken seriously (the estimated A is 2 x 105 that of k ) nor can the mechanism be regarded as confirmed. 

An overcrowded PAH, 9,10,ll,20,21,22-hexaphenyltetrabenzo[a,c,I,n]pentacene (55), showed an interesting screw-type helicity (Fig. 15.21) [97]. An end-to-end twist of 144° was estimated from the X-ray structure of 55. Pentacene 55 was prepared by the reaction of l,3-diphenylphenanthro[9,10-c]furan 54 with the bisaryne equivalent generated from l,2,4,5-tetrabromo-3,6-diphenylbenzene in the presence of n-butyllithium, followed by deoxygenation of the double adduct with low-valent titanium. Pentacene 55 could be resolved by chromatography on a chiral support, but it racemized slowly at room temperature (t1/2 9 h at 25 °C). 

The kinetic order for. yec-butyllithium-initiated polymerization of styrene is close to 0.25 in benzene solution this result is also consistent with initiation by unassociated. yec-butyllithium, since yec-butyllithium is associated predominantly into tetramers in benzene solution [44]. The experimentally observed energy of activation (18 kcal/mol) for n-BuLi/styrene initiation [56] appears to be too low to include the enthalpy of complete dissociation of the aggregates, estimated to be 108 kcal/mol [82]. An alternative mechanism is the incomplete or stepwise dissociation of the aggregate [3]. 

The deprotonation of the secondary carbamate rac-254 by -butyllithium/(-)-sparteine (diethyl ether/hexane, -78°C) is combined with an efficient kinetic resolution [Eq. (72)] [155,168]. The lithium compound (S)-256 is configuration-ally stable under these conditions and is formed with greater than 80% ee as estimated from trapping experiments. The less reactive enantiomer (R)-254 is recovered with 41 % yield and 80% ee. 

A value for the enthalpy of reaction (7) in solution would be of perhaps greater utility than a gas-phase value. In order to estimate AH for Eq. (6), the heat of formation of n-butyllithium hexamer in the gas phase must be estimated. Fowell and Mortimer have estimated the heat of dissociation of the C—Li bond without taking into account the hexameric character of alkyllithium compounds either in solution or in the gas phase. An estimate of AH for the process... 

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