Multicenter Processes

In the seTOnd example, we make the stabilized allylic radical, and in the third one, the favored radical is at the tertiary center 

In the presence of peroxide, we expect anti-Markovnikov addition via the more stable tertiary radical, 11.17. 

In the presence of peroxides, HBr may be added to alkenes in an anti-Markovnikov fashion. The reaction involves a radical chain mechanism and proceeds via the most stabilized radical. 

Comparable reactions of other hydrogen halides are rarely successful. 

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P. B. Venuto and P. S. Landis On Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of a Complex Multicenter Processes G. N. SCHRAUZER... 

PbOj anode, 40 155-156 oxygen evolution, 40 109-110 PCE, catalytic synthesis of, l,l,l-trifluoro-2,2-dischloroethane, 39 341-343 7t complex multicenter processes of norboma-diene, 18 373-395 PdfllO), CO oxidation, 37 262-266 CO titration curves, 37 264—266 kinetic model, 37 266 kinetic oscillations, 37 262-263 subsurface oxygen phase, 37 264—265 work function and reaction rate, 37 263-264 Pd (CO) formation, 39 155 PdjCrjCp fCOljPMe, 38 350-351 (J-PdH phase, Pd transformation, 37 79-80 P-dimensional subspace, 32 280-281 Pdf 111) mica film, epitaxially oriented, 37 55-56... 

Many reactions occur in which one organic compound is convened into another. The molecular details of the intemiediate steps by which compounds are converted into new products are called reaction mechanisms. The four broad classes of reaction mechanisms are cationic, anionic, free radical, and multicenter processes in which neither charged species nor odd electron species is involved. Examples of each type will be given, but many variations can exist within each type. Also, varying degrees of sophistication exist in our knowledge of the exact reaction pathways that organic compounds follow. The examples discussed show only the major steps involved. 

Additionally, we speculated that the lowest OH internal excitations (i.e., v = 0, low N) might derive preferentially from higher-than-binary complexes and/or a mechanism that involves a five-atom HO(Br)CO intermediate produced by a multicenter process following photoexcitation (Hoffmann et al. 1990). 

Primary chlorides reacted predominantly by a direct mechanism (Sn2 or a multicentered process). Isobutyl or neopentyl halides led to contributions from electron transfer (free radicals) and halogen-metal exchange (anionoid) mechanisms. 

This remarkable catalytic reaction proceeds on a bimetallic catalyst e.g., Zn[Co(CO)4]2 and is believed to involve a concerted head-to-toe dimerization step incorporating the assistance of two metal centers 56). Since this reaction proceeds with high stereoselectivity, the authors proposed the n complex multicenter process in 56. The inherent forbiddenness... 

The trimerization reaction with benzene derivatives could sometimes follow a similar path but, since it also occurs with disubstituted acetylenes such as HOCH2C = CCH2OH, some other mechanism must also be operating. Schrauzer explained the cyclization of tolane with a catalyst based on bisacrylonitrilenickel and triphenylphosphine by the so-called V-complex, multicenter processes. With this process, three alkyne molecules would successively coordinate with nickel and then the ring closure would take place (See Figure 6). 

One can conclude that with some alkynes, if strong t acceptors are present in the catalyst, as CO or PX3, a catalytic multicenter process can take place in which as many as four alkyne molecules are bound to nickel (0). However, when a coordination site is blocked by a ligand difficult to replace, the four-center process changes to a three-center. 

On Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of TT Complex Multicenter Processes... 

Reppe s cyclooctatetraene synthesis also appears to be ir complex concerted (41). The formation of Binor-S of (XI), and of CaHg thus may be formally related, tt complex, multicenter processes. Accordingly, the reaction of acetylene with Zn[Co(CO)4]2 produced small amounts of cyclooctatetraene (52). This is the first reported case in which cyclooctatetraene was formed using a catalyst containing metals other than nickel, and on a binuclear catalyst center. 

The following arguments can be advanced for the stepwise bond formation via (24), which contradicts the concept of 7r-complex multicenter processes 404> ... 

The solvent used for most of polymerizations is THF or dioxane. However, it is also possible to use toluene, benzene, or hexane with an addition of polar substances (ether, amines). In those cases, it is necessary to use mild conditions and gentle catalysts because if the dissociation of the amides of phosphides is weakened, multicenter processes, for example, have an effect. These are favored by the free electron pair on the nitrogen or phosphorus, and they lead to chain-terminating reactions. 

Two limiting cases may be distinguished In certain simple cases, particularly if cyclic reaction products are formed it is possible to assume that the new —C bonds are closed simultaneously. Such reactions could be properly described as w-complex multicenter processes (79). The linear polymerization of alkynes in which the new C—C bonds cannot be formed simultaneously may be considered the other extreme possibility, but it must also be expected that there are intermediate cases between these two types of reaction. In view of the generally nonspecific action of the catalysts any generalizations must be treated with caution. The reactions of alkynes and of norbornadiene with bis(acrylonitrile)-nickel and related complexes, however, appear to be simple enough to allow some mechanistic conclusions. 

LI), 6-cyanotetracyclo(3.2 1.1.0)nonane, is isolated in 95% yield [with bis(acrylonitrile)-nickel as catalyst]. This is the first case of a transition metal-catalyzed transannular addition. It may be formulated according to Fig. 9 and provides a good example of a tr-complex multicenter process. 

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