Ligand apical

The activity in MMA polymerization can be dramatically affected by the apical ligands. Apical aquo or alcohol ligands are labile and rapidly exchange with the polymerization medium. Lewis base ligands (e.g. pyridine, triphenyl phosphine) are comparatively stable. In MMA polymeri/ation, it is found that activity increases with the basicity of the ligand. With alkyl Co " complexes, a different order is found possibly because the type of apical ligand also controls the rate of initial generation of the active Co" complex. 

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

The assumed transition state for this reaction is shown in Scheme 5.5. The two bulky t-butoxy groups are expected to locate at the two apical positions. One of the 3,3 -phenyl groups would effectively shield one face of an imine, and consequently, a diene attacks from the opposite side. Judging from this model, similar selectivities were expected in the Mannich-type reactions of imines with silyl eno-lates. Actually, when ligand 10 was used in the reaction of imine la with S-ethyl-thio-l-trimethylsiloxyethene, the corresponding / -amino thioester was obtained in 84% ee (Scheme 5.6). As expected, the sense of the chiral induction in this case was the reverse of that observed when using catalyst 6 [12, 25]. 

The structures of several adducts can be rationalized on the basis [128] that in a 5-coordinate low-spin d8 tbp system, the acceptor ligands prefer to occupy an equatorial site (IrCl(CO)2(PPh3)2) whereas a 7r-donor prefers an axial site. In a square pyramidal situation, it is weakly bonded acceptors that prefer the apical position, e.g. (IrCl(S02)(C0)(PPh3)2. 

The anionic pentafluorophenyls have square pyramidal structures but are evidently non-rigid in solution (19F NMR shows all ligands equivalent). The neutral adducts are also square pyramidal (apical C6F5, trans-L) [192],... 

Pi the Berry step is seen as a double bending of an equatorial and an apical angle. The two apical ligands become equatorial and two equatorial ones go to apical positions. One of the equatorial ligands, the so-called pivot, is on the fourfold axis of the tetragonal pyramidal intermediate state. The connectivity i.e. the number of isomers reached from a given one in one step, is three. 

Pj a twist permuting one apical and two equatorial ligands ( 2=6) P3 a twist permuting one apical and one equatorial ligand ( 3=6) F4, two simultaneous twists each involving one apical and one equatorial ligand ( 4 = 3)... 

The displacement of a ligand L from the phosphorus atom of a phosphoranyl radical can easily occur via a-scission of the L-P bond (Scheme 23). The fragmentation is a regiospecific process, i.e., the leaving group must be apical (Scheme 23) and it occurs via an intermediate o" structure (Fig. 5). 

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