Geometries, carbanion

Given that fluorine has a destabilizing influence upon planar carbanions, geometry considerations favor the formation of pyramidal anions.69 71 The influence of /(-fluorination on carbanion stability has been described in a variety of ways. In early work the role of negative hyperconjugation was proposed to account for these effects.72... 

An X-ray diffraction structure analysis of cyclopropylidenetriphenylphosphorane (5) provided unambiguous proof of a pyramidal carbanion geometry in the crystalline solid.The phos-... 

Contrary to predictions from the CH acidities, deprotonation of phosphonium cations (i-Pr)n(cyciopropyl)4 P , which contain both isopropyl and cyclopropyl substituents, leads exclusively to ylides with the open-ehain isopropylide group.Evidently, the pyramidal carbanion geometry of cyclopropyhdes is much less favored than the planar carbanion geometry of iso-propylides. 

There have been numerous attempts to elucidate both the nature of the ion-pair interactions and the delocalized carbanion geometries of chelated alkali metal organic systems in solution (18). There is now apparent agreement that this equilibrium ... 

Structural features of the ir carbanion organometallic complexes are discussed here in three sections (1) 7r-carbanion geometry, (2) metal-base coordination, and (3) metal—7r-carbanion geometry. The primary tool used in these studies was single-crystal x-ray crystallography. Se-... 

Metal—Base Coordination and Metal—w-Carbanion Geometry... 

Obtain the energies of the different possible carbanions alleyne-H+). Which one is most stable Does it correspond to removal of the most electron-poor proton Examine the geometry and atomic charges of the favored carbanion. Where is the negative charge Draw the Lewis structure of this ion. Predict the structure of the Sn2 product. 

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

Carbanionic additions usually do not provide significant simple synjanti diastereoselectivities. The EjZ geometry is determined by the configuration of the reacting ally lie carbanion. The attack at position 3 conserves the configuration of the 1,2-partial double bond and vice versa. 

The kinetics, products, and stereochemistry of the addition of HC1, HBr, and HI to propiolic acid in water have been studied.28 The addition is predominantly trans to give the cz s-3-haloacrylic acid. Both the rate of addition and the selectivity giving trans-addition increase with the nucleophilicity of the halide in water (i.e., I- > Br > Cl-). The order of reactivity is also consistent with the order of the softness of the nucleophiles. The reaction is first order in propiolic acid and the halide anion. It was proposed that the addition involves two mechanistic pathways a major /ra/z.v-addition via a transient carbanion formed with specific geometry and a minor cO-addition process (Scheme 10.2). 

A certain dualism is observable in carbonium ion-carbanion chemistry, a dualism rather like that of lines and points in projective geometry. The reader may recall that interchanging the words "line and "point in a theorem of projective geometry converts it into a statement that is also a theorem, sometimes the same one. For most carbonium ion reactions a corresponding carbanion reaction is known. The dualism can be used as a method for the invention of new, or at least unobserved, carbanion reactions. The carbanionic reaction corresponding to the carbonium ion rearrangement is of course the internal nucleophilic... 

This Li-phenyl interaction is similar to the r/ -Ph contact observed in the silicon-stabilized carbanion complex [Li C(SiMe2Ph)3 (THF)] (140). Addition of tmeda to 49 yields the monomeric compound [Li C(SiMe2Ph)(PMe2)2 (tmeda)l in which the lithium is bound by the two P atoms of the phosphinomethanide ligand and the two N atoms of the tmeda in a distorted tetrahedral geometry (139). There... 

Focusing our attention on anionic molecules, we turn first to the problem of conformational isomerism in the 2-propyl carbanion, CH3CHCH3. The two geometries which we will compare are again the staggered and eclipsed conformations shown below ... 

Eor an sp hybridized carbanion, one expects that it will prefer a structure where the central carbon adopts a trigonal-pyramidal geometry with a lone-pair occupying... 

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