Metal-activated alkenes

The metal-oxo molecular models outlined above have a quite remarkable potential for studying the metal activity in a quite unusual environment. Some of the possibilities could be (1) the generation and the chemistry of M—C, M=C, M=C functionalities (2) the interaction with alkenes, alkynes, hydrocarbons, and hydrogen (3) the activation of small molecules like N26 and CO (4) the support of metal-metal bonded functionalities and (5) the generation of highly reactive low-valent metals. 

A Cu(OAc)2-catalyzed intramolecular diamination of alkenes using sulfamide substrates such as compound 214 provides a route to fused thiadiazolidines 215 (Equation 48) <2005JA11250>. In this reaction, the transition metal activates the alkene toward nucleophilic attack by the first nitrogen, then becomes displaced by the second nitrogen nucleophile (a net M +z to M reduction). 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

Besides heterogeneous and homogeneous catalytic hydrogenations, chemical reductions can also transform alkynes to cis alkenes. Interestingly, activated zinc in the presence of a proton donor (alcohol), although a dissolving-metal reagent, reduces disubstituted alkynes to cis alkenes 199... 

Heteroatom Nucleophiles with Metal-activated Alkenes and Alkynes... 

Alcohols and carboxylic acids also readily add to metal-activated alkenes2 and industrial processes for the conversion of ethylene to vinyl acetate, vinyl ethers and acetals are well established. However, very little use of intermolecular versions of this chemistry with more complex alkenes has been developed. In... 

A similar hybrid type of radical/anionic reactions can be effected, when manganese metal, activated by catalytic amounts of lead dichloride and trimethylchlorosi-lane, is employed instead of zinc, which makes the original process synthetically more reliable and attractive by reducing the amounts of reagents (RX and ketone) needed to a 1.5 molar excess over the alkenes (Scheme 6.36) [57]. 

N-substituted porphyrins are formed during the metabolism of xeno-biotics that include terminal alkenes and alkynes, as well as activated organic molecules such as halocarbons, diazo compounds, and hydrazines. In the synthetic laboratory, N-substituted porphyrins are prepared easily via alkylation of a pyrrole nitrogen atom of the porphyrin, followed by metallation. Biomimetic reactions between iron porphyrins, oxidants, and alkenes (or activated carbon sources) may also be used to alkylate the pyrrole nitrogen. 

Nucleophilic addition to metal-activated alkenes as a synthetic method can be traced to the Wacker Process, the oxidation of ethylene to acetaldehyde with Pd and... 

Fig. 2.3. a) Chatt-Dewar-Duncanson model for alkene adsorption on a metal active site b) analogous representation for hydrogen adsorption. 

Hegedus, L. S. Heteroatom Nucleophiles with Metal-Activated Alkenes and Alkynes. in Comp. Org. Synth, (eds. Trost, B. M.,Fleming, I.), 4,... 

Step 1 is the oxidative addition of hydrogen the metal loses two electrons to form a bond to each of the two hydrogen atoms. Step 2 is the complexation of the alkene to the metal, activating the pi bond. Complexation usually occurs from the least hindered face of the pi bond. Step 3 is the insertion of the metal hydride bond into the pi bond, forming a metal-carbon bond. Step 4 is the reductive elimination (the reverse of oxidative addition), which returns the metal to its original oxidation state and releases the alkane. The reductive elimination is usually very fast. Both hydrogens are added to the same face of the pi bond, a syn addition. 

Complexation to a metal activates a double bond towards the addition of a nucleophilic species (Scheme 10.16). The metal has modified the behaviour of the alkene, which would normally undergo addition reactions with electrophiles. 

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