Alkene metathesis electron donation

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

Red ReOs, which can be obtained by CO reduction of the heptaoxide at 175 280 °C, is a good electrical conductor with a metal-like temperatme dependence the d electron is clearly delocalized. It has octahedral Re and easily forms nonstoichiometric bronzes in which an alkali metal donates an electron to the metal-0x0 lattice and enters the stmetme as a cation. Supported ReOs is important in alkene metathesis catalysis. No Tc analog is known. Blue Re20s, brown Re02, and black Rc203 are also known. For Tc, the only lower oxide is TCO2. 

These initial studies revealed that alkene metathesis activity was favored by the presence of (i) an electron-donating and bulky phosphine (PR3 PCy3 > P Pr3 PPh3) (ii) a weakly coordinating anion (TfO PFg BF ") (iii) an easily displaceable arene (p-cymene > CgMeg) and (iv) the addition of a small amount of acid [13,14, 22]. The resulting systems were shown to be efficient for the RCM of diallylamides, the formation of macrocycles [13, 14], and the preparation of cyclic, fluorinated amino acids [23]. 

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