Electron-deficient vinylarenes

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

Electron-deficient vinylarenes were also converted via in situ formation of [Cu (NHBn)(NHC)], but the functional group tolerance remained limited. When electron-withdrawing groups (EWG) (such as CF3 and Br) were present in the / ra-position, no reaction was observed even after 48 h. The mechanism involves the formation of [Cu(NHBn)(NHC)] in situ from benzylamine and [Cu (NHPh)(IPr)]. This complex is sufficiently reactive to enter the catalytic cycle with less activated substrates. 

Munro-Leighton C, Delp SA, Alsop NM, Blue ED, Gunnoe TB. A ti-Markovnikov hydroamination and hydrothiolation of electron-deficient vinylarenes catalyzed by well-defined monomeric copper(I) amido and thiolate complexes. Chem. Commun. 2008 111-113. 

Compared to the hydroamination of alkynes, the protocols for more readily available olefins were rather limited. Ackermann presented a TiCU-catalyzed hydroamination reaction of norbornene using a variety of functionalized amines. Regioselective hydroarylation occurs as a competing reaction and the chemose-lectivity for hydroamination products ranges from poor to excellent [312]. This protocol can also be utilized in the hydroamination of vinylarenes with some electron-deficient amines [313] and the hydroamination of enyne compound. In the latter reaction, a subsequent rearrangement gives rise to a tetrahydroisoquinoline derivative (Scheme 14.134). 

In the absence of transition metal catalysts, InBr3 was found by Peppe et al. to catalyze the dimerization of vinylarenes [281]. A variety of arenes, electron deficient to electron rich, was studied. The extent of reaction of the proposed benzylic carbocation, and hence the dominant product, depended on reaction temperature and time (Figure 8.127). 

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