Isomerization carbon complexes

By heating 2,4-( 2Br,H7 at 260° in the presence of C5H5Co(CO)2, two isomeric trimetallic complexes formulated as (CTDsCosC BsHv were isolated (76). One isomer has one Co-Co interaction with a high-coordinate carbon atom, whereas the proposed structure of the other isomer has the 3 cobalt atoms positioned on the same equatorial belt of the bicapped square antiprism, bound to each other, and the carbon atoms occupying the apical vertices. 

Harvey has examined these issues by density fimctional calculations, and has concluded that (1) "primary alkyl complexes are usually more stable than secondary and tertiary ones," that (2) "this is an electronic effect, due to the partial carbanionic character of the alkyl group," and that (3) "steric effects,... usually invoked in the literature. .. play only a minor role in many cases." The electronic effect is proposed to parallel that in alkyl lithium reagents. Because of the partial negative charge on the a-carbon in alkyl lithium and transition metal complexes, the stability of the alkyl complexes parallels the stability of the carbanions. When the charge on the a-carbon is small, as in neutral, late transition metal complexes, other factors, such as steric effects and agostic interactions, can dominate the stability of the isomeric alkyl complexes. 

The selectivity of the "ring-closure" was tested employing [(rj5-C5H4Me)-(CO)2Mn=C-Ph]+BF4. Two isomeric carbene complexes (3 and 4) were obtained (Scheme 2). Complex 4 resulting from attack of the NCPh+ functionality at the slightly more nucleophilic 13-carbon atom constituted the major product (ratio 3 4 = 1 5). 

The effect of substituents on the equilibrium between a carbonium ion and an isomeric n complex can easily be predicted. In the carbonium ion, one atom carries a full unit of formal charge while the others are neutral. In the n complex, the charge is shared between the apical carbon atom in the acceptor and the two basal carbon atoms in the olefin moiety. The apical carbon atom and one basal carbon atom are therefore more positive in the 71 complex than in the carbonium ion, while the other basal atom is much more positive in the carbonium ion. If then we replace one of both hydrogen atoms at a basal position in (49) by — / groups, e.g., alkyl, we will stabilize one of the classical ions very strongly relative to the n complex. There is little doubt that in the rearrangement of neopentyl cation [equation (5.195)], the intermediate n complex immediately rearranges to the more stable r-amyl... 

The proton has no p electrons, so back-coordination cannot occur. Hydrogen is also approximately equal in electronegativity to carbon. However, the CH bond is much stronger than C—C. Since simple carbonium ions are more stable in solution than are isomeric n complexes, the same should be even more true for n complexes with an apical proton. Addition of acids to olefins should therefore take place by the carbonium ion mechanism—as indeed it does. 

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

Plasticizer Range Alcohols. Commercial products from the family of 6—11 carbon alcohols that make up the plasticizer range are available both as commercially pure single carbon chain materials and as complex isomeric mixtures. Commercial descriptions of plasticizer range alcohols are rather confusing, but in general a commercially pure material is called "-anol," and the mixtures are called "-yl alcohol" or "iso...yl alcohol." For example, 2-ethyIhexanol [104-76-7] and 4-methyl-2-pentanol [108-11-2] are single materials whereas isooctyl alcohol [68526-83-0] is a complex mixture of branched hexanols and heptanols. Another commercial product contains linear alcohols of mixed 6-, 8-, and 10-carbon chains. 

Shell Higher Olefin Process) plant (16,17). C -C alcohols are also produced by this process. Ethylene is first oligomerized to linear, even carbon—number alpha olefins using a nickel complex catalyst. After separation of portions of the a-olefins for sale, others, particularly C g and higher, are catalyticaHy isomerized to internal olefins, which are then disproportionated over a catalyst to a broad mixture of linear internal olefins. The desired fraction is... 

C-19 dicarboxyhc acid can be made from oleic acid or derivatives and carbon monoxide by hydroformylation, hydrocarboxylation, or carbonylation. In hydroformylation, ie, the Oxo reaction or Roelen reaction, the catalyst is usually cobalt carbonyl or a rhodium complex (see Oxo process). When using a cobalt catalyst a mixture of isomeric C-19 compounds results due to isomerization of the double bond prior to carbon monoxide addition (80). 

Although lupinine is thus a comparatively simple alkaloid its detailed chemistry has been difficult to unravel owing (a) to the presence in its molecule of two asymmetric carbon atoms as asterisked in (XI), and (6) the possibility of cis-trans isomerism in certain of its proximate (ieriva-tives. Winterfeld and Holschneider have pointed out that a further complexity arises from the presence in natural Z-lupinine of a structural isomeride, aZZolupinine for which formula (XII) is suggested. They also quote Kreig s observation that by the action of sodium on a benzene solution of Z-lupinine (m.p. 68-9° [ajo — 23-52°), the latter is converted... 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

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