Lead complexes stereochemistry

Lead, tris(ethyldithiocarbonato)-stereochemistry. 1,82 structure, 1, 82 Lead bromide, 3.194 Lead bromide hydrate, 3, 195 Lead carboxylatcs, 3,222 Lead complexes, 3,183-223 r 6-arene, 3,220 bivalent... 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The [Ni(NCS)f,]4 ion is almost perfectly octahedral, with Ni—N distances of around 209.5 pm and N—Ni—N angles around 89.5°. The Ni—N—C and N—C—S entities are practically linear.438,439 In [Ni(NCS)2L2] where L is a R-substituted pyridine, stereochemistry and spin state depend on the type and positions of R.431 While for 2-Me- and 2-Et-pyridine square planar complexes are observed, other pyridins lead to coordination polymers with pseudo-octahedral Ni11 due to N,S-bridging thiocyanate. Ni11 thiocyanato complexes have been studied quite intensively as hosts for inclusion compounds.440"442... 

The comparatively unreactive complex Mn(CO)3(C5H5BMe) (14) with MeCOCl/AlCl3 produces the 2-acetyl derivative 84 and small amounts of [Mn(CO)3(PhMe)]+ (27). The product ratio is rather insensitive to reaction conditions. It is reasonable to assume a common intermediate 85 (of unspecified stereochemistry at C-6) which under kinetic control may either irreversibly deprotonate to 84 or undergo a rearrangement ultimately leading to the ring-member substitution product (27). 

If one examines the minimal sequences of reaction steps for [2+2] cycloadditions, Eqs. 12—18, 32—35, one concludes that stereochemistry of addition, and perhaps relative reactivities might be calculable at several points. Oriented complexes could control regiospecificity, or the transition state leading to a biradical could be the important stage. Relative rates of product formation would be derived from relative perturbation stabilization energies for different configurations of the two reactants. 

This regio- and stereochemistry in these reactions can be accounted for as shown in Scheme 17.26 When coordinating electrophiles like ketones and aldehydes are used, the equilibrium between ij1- and 3-allyl complexes shifts to rj1, resulting in the formation of the least substituted -complex 52 preferentially. Carbon-carbon bond formation takes place via a six-membered ring transition state 53, leading to the formation of the branched homoallylic alcohols 54 with //-diastereoselectivity. 

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

Andrus et al. (109) proposed a stereochemical rationale for the observed selec-tivities in this reaction. The model is based on the Beckwith modification (97) of the Kochi mechanism, suggesting that the stereochemistry-determining event is the ally lie transposition from Cu(III) allyl benzoate intermediates 152 and 153, Fig. 13. Andrus suggests that the key Cu(III) intermediate assumes a distorted square-planar geometry. Steric interactions are decreased between the ligand substituent and the cyclohexenyl group in Complex 152 as opposed to Complex 153 leading to the observed absolute stereochemistry. 

Steric hindrance may hamper the correct stereochemistry required for 13-elimination, and perhaps this can be used to stabilise our metal alkyl complex. In the modem polymerisation catalysts for polypropene this feature has actually been observed, which leads to higher molecular weight polymers. This now forms part of the design of new catalysts. 

---