Complexes steric compression

Based on the discussed acylpalladium 7i-allylic complex (Scheme 5.22) and the reported X-ray structure of the (R)-MOP—Pd 7i-allylic complex [31], the acylpalladium (R)-MOP Ti-allylic complex C (Scheme 5.24) is proposed for the formation of the (R)-product. Complex D, which would give the (S)-product, suffers from steric compression between the MeO-naphthyl ring and the acyl group, while there is no such steric interaction in complex C. Thus, reductive elimination of Pd(0) from C would preferentially yield the... 

Other clear-cut evidence that the dichotomy between primary and secondary amines cannot be due to differential steric compression in the tr-complexes formed in these reactions has been afforded by Nudelman and Cerdeira82 in their study of the reactions of o-and p- lluomn i Irobenzenes with two primary amines n- and iso-propylamine in... 

A dissociation constant of about 1.5 X 10-3 (25°C) was assessed from the intensity ratio. The complex does not show intramolecular fluxional behavior up to +50°C. Of the three methyl resonances the chemical shift of that at position b occurs at highest field owing to the influence of the phenyl ring current. Consistent with a corollary of the X-ray study (132) on Pt(PPh3)2(CH2=C=CMe2), we may infer the existence of a steric compression between the methyl and the phenyl groups which may be primarily responsible for the ready dissociation of TMA. 

A C n.m.r. study of diamagnetic complexes of propane-l,3-diamine-NNiV N -tetra-acetate ion (pdta ), including [CoL] , detected no complexes in which L was quinquedentate, due to steric compression of the glycinate chelate arms and not to decreased ring strain in the six-co-ordinate complexes as previously reported. ... 

The same pattern was observed for the fumarate complex but the rates were about 1000-fold slower. The increase in specificity is the interesting and surprising feature. For analogous amino systems, the ratio of the diastereoisomers (5) at equilibrium is about 2 1 or less with the same structural relationships as the analogous malate products. It appears, therefore, that the product ratio of the pH independent path resembles the equilibrium situation whereas the path dependent on base is far from that condition. An explanation for the increase could arise if Co-0" addition was rate-determining. In the transition states for the generation of the two isomers configuration (a) will be more compressed than that of (b) by virtue of the non-bonded interactions between the substituents on the olefin and adjacent Co(en) chelate. This steric compression will be much more evident in the transition states than in the product malates. 

The origin of the enantioselectivity has been examined using semiempirical (AMI) computations. " The main differences in stability arise at the stage of formation of the borane-ketone complex, where the boron changes from sp to sp hybridization. The boron substituents introduce additional steric compressions. Table 2.6 gives some typical results for enantioselective reduction of ketones. 

Apparently the rate of radical recombination is high so that under anaerobic conditions the compound is stable, but in air, radical trapping by oxygen promotes net decomposition. The unusual lability of neopentylcobalamin towards spontaneous mode II cleavage was attributed to steric compression around the coordinated carbon in 6 coordinate complexes as evidenced by the fact that the compound is stable in acid (even in the presence of O2) where the pendant ligand is dissociated and protonated (see Section 5.3). 

It is reasonable to assume that high steric compression in the phospho-nium cation would assist its dissociation into PB- and triphenylphosphinium radical cation (Hellwinkel, 1966 Tomaschewski, 1966). Circumstantial evidence for mechanism (51) is the detection of PB- as a transient species by esr spectroscopy. Since, after hydrolysis, no ai/,a//-hexachlorotoluene has ever been detected, it is concluded that the Wittig complex (CgH5)3P= CClCgClj is not formed, possibly because of the high reactivity of PB-. 

The addition of metalated sulfones to aldehydes is reversible and in simple cases the reaction displays modest selectivity for the erythro isomer (eq 6). The reverse reaction is favored when the adducts are sterically compressed (e.g. ketone adducts) or when the sulfone anion is stabilized by conjugation (i.e. allylic or ben-zylic sulfones) or proximate heteroatoms. However, in unfavorable cases the position of the equilibrium can be tuned by varying the metal. For example, the lithio sulfone (2) did not give a stable adduct with aldehyde (4) but the ate complex derived from the lithio derivative and BF3 gave the desired adduct (5) (eq 1) ... 

By replacing cyclopentadienyl by pentamethylcyclopentadienyl, steric compression at the metal centre is greatly increased, especially at lutetium, the smallest lanthanide. The complex Cp LuCHdOEt ) polymerizes ethene and oligomerizes propene. (Cp = / -C-MeJ. Its behaviour thus provides a good model for Ziegler-Natta catalysis. The insertion of propene into the Lu—CH bond and some further propagation steps have been followed by n.m.r. spectroscopy. Clear examples of such insertions are rare in d-block chemistry (p. 380). Termination can occur either by j -H transfer or by -methyl transfer the latter is a novel process which has yet to be observed for d-block elements. 

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