Benzyl complexes

A further dramatic comparison of the comparative reactivities of chromium alkyls in diverse oxidation states was furnished by another set of benzyl complexes. [7] Shown below, these three compound are isomers, yet they range in oxidation state from Cr to Cr °. Of the three, only the mixed-valent complex Cp G0i-ti n3-Bz)Cr(Bz)Cp, containing a Bivalent chromium bound to an T)3-benzyl and a ni-benzyl ligand, catalyzed the polymerization of ethylene. 

A fourth focus of catalytic chemistry in our laboratory has been iridium-catalyzed asymmetric allylic substitution. Dr. Toshimichi Ohmura had been studying additions to rhodium and iridium allyl and benzyl complexes in hopes of developing... 

Oxidative addition of a silyl-protected 4-(bromomethyl)phenol precursor to (tme-da)Pd(II)Me2 (tmeda = tetramethylethylenediamine), followed by ethane reductive elimination, resulted in formation of the benzylic complex 16 (Scheme 3.10). Exchange of tmeda for a diphosphine ligand (which is better suited for stabilizing the ultimate Pd(0) QM complex), followed by removal of the protecting silyl group with fluoride anion, resulted in the expected p-QM Pd(0) complex, 17, via intermediacy of the zwitterionic Pd(II) benzyl complex. In this way a stable complex of p-BHT-QM, 17b, the very important metabolite of the widely used food antioxidant BHT20 (BHT = butylated hydroxytoluene) was prepared. Similarly, a Pd(0) complex of the elusive, simplest /)-QM, 17a, was obtained (Scheme 3.10). 

Attempts to prepare this and the corresponding 5-benzyl complex gave highly... 

Figure 62 The structure of the benzyl complex frans-[(p-ButBz)2Ca(THF)4] 121.

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Under CO, (10) or (11) gave, together with the termination ji-benzyl complex (15), the poly(styrene-alt-CO)-attached P-chelates (12) and (13), respectively. At the end of the reaction (7 h), all of the initial 1,2 complex (10) was consumed, while some initial 2,1 complex (11) was still present (Figure 7.15, trace b). There... 

Analytical data on the soluble products isolated from chloroform are in excellent agreement with the composition 1 Ni+2 1 monoalkylated ligand 1 I or Br. The magnetic moment of this methylated complex was found to be 1.89 Bohr magnetons per nickel (II). The molar conductivities of the methylated and benzylated complexes in methanol at 25° C. are 75.4 and 68.4 ohm-1, respectively. These values approximate those expected for uni-univalent electrolytes in this solvent. The formulation of these alkylated compounds as dimeric electrolytes (structure VII) does not appear to be totally consistent with their physical properties. One or both halide ions may be bound to the metal ion. These results lead to the easily understood generalization that terminal sulfur atoms alkylate more readily than bridged mercaptide groups. 

Reactions with Acyl Halides. The palladium-carbon bond in alkyl VII and acyl VIII complexes also can be broken by acyl and alkyl halides, respectively, to give ketones (55). The reaction of the benzyl complex Vila with acetyl chloride in the presence of 2,6-lutidine provided good yield of methyl benzyl ketone, XVII. In the absence of the base, lower yields (26% ) were obtained. Surprisingly, a similar reaction between the benzyl complex and an excess of phenylacetyl chloride afforded a 156% yield of dibenzyl ketone, XVIII, based on the starting benzyl complex (see bottom of p. 105). 

The decomposition of the phenyl complexes Ilia to chlorobenzenes and IV is first order and irreversible. Triphenylphosphine retards the rate of this decomposition, indicating that the decomposition pathway involves prior dissociation of triphenylphosphine from Ilia. The benzyl complexes Illb decompose both by rearrangement to lib and to the benzyl chlorides plus IV. The conversion of Illb to IV is irreversible, but it is not retarded by added triphenylphosphine. The p values for the decomposition of the phenyl complexes Ilia and the benzyl complexes Illb are also opposite in sign but low enough to fall in the range observed for radical reactions. 

Although optically active chloride is not racemized under the reaction conditions, and it will not add to IV (reverse reaction) to afford a complex of the type Illb, under forcing conditions (high temperature and/or long reaction times) the 7r-benzyl complex (VII in Reaction 5) is obtained which results in a loss of chirality. 

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