Silylboration mechanism

The palladium-catalyzed stannylboration (90) [124] or silylboration (87) [109, 114] succeeds in the intramolecular carbocyclization of diynes and enynes (Scheme 1-27). It is interesting that a very strained four-membered cycUzation of hexa-l,5-diyne proceeds without any difficulties, similarly to five- or six-membered cycUzation. The boryl group is selectively introduced into the more reactive C=CH rather than C=C for enynes and into the terminal C=CH rather than the internal C=CR for diynes, again suggesting a mechanism proceeding through the first insertion into the Pd-B bond in preference to the Pd-Sn or Pd-Si bond. 

Both amido and pinacol derivatives of B-Si compounds 125 and 126 added to terminal and internal alkynes in the presence of a palladium244-246 or platinum(O) catalyst247 by a mechanism involving an oxidative addition-insertion process (Equation (39)).248 On the other hand, phosphine-free nickel(O) catalyst resulted in the dimerization of alkynes giving a Z,Z-isomer of l-silyl-4-borylbutadiene derivatives.249 Since the palladium-catalyzed cross-coupling at the C-B bond is faster than the G-Si bond of 137, a silylboration-cross-coupling sequence provided a method for the synthesis of 1-alkenylsilanes.246... 

The diborations of alkenes and alkynes are thought to occur by the general pathway shown in Scheme 16.15 and analogous silylborations and stannylborations would occur by related mechanisms. By this mechanism, the oxidative addition of the diborane(4), silylborane, or stannylborane reagent leads to a bisboryl, a silylboryl, or a stannylboryl complex. These oxidative addition processes are described in Chapter 6. In brief, Miyaura and Ishiyama, Marder, and Smith all published examples of the oxidative addition of diboron compounds to and Marder has published examples of the oxidative... 

---