Silylboration alkynes

Like alkenes, alkynes do not react readily with trialkylboranes. Under severe reaction conditions, only hydroboration products are obtained.116 Mikhailov113 showed that triallylboranes react with various alkynes (20 C) to afford syn addition products of type (78) which rapidly cyclize (40-60 C) to give the cyclic boranes of type (79 Scheme 42). In the case of trimethylsilylacetylene and ethoxyacetylene, the reaction affords compounds of type (78 R = MesSi or OEt) which do not cyclize further.113 Recently, a transition metal silylboration reaction has been described by Oshima and cowoikers.117 A formal syn carboboration reaction leading to a variety of alkenylboranes has been reported by Suzuki and cowoikers118 (Scheme 43). Hexamethyldistannylacetylene (80) reacts readily with various trialkylboranes119 to afford syn addition products of type (81 equation 25). 

Internal acetylenes, Ni-mediated reactions, 10, 546 Internal alkenes, ethylene co-polymers, 4, 1145 Internal alkynes in alder-ene reaction, 10, 567 intermolecular hydrosilylation with ruthenium, 10, 802 with yttrium, 10, 801 silylboration, 9, 163 silylformylation, 11, 483... 

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... 

Addition of Ge-B compound to alkynes which is analogous to silylboration was briefly studied.246... 

Table 16.4, which is adapted from a comprehensive review by Miyaura from 2001, illustrates the scope and types of catalysts that have been used for the diboration, silylboration, and stannylboration of alkynes. These data illustrate that the simple platinum(O) complex of PPhj is a suitable catalyst for diboration. ° - ° These data also show that both B rin and BjCat add to terminal alkynes, but that the tetraaminodiboron reagent does not. Finally, these data illustrate that palladium complexes of isonitrile ligands catalyze the silylboration of alkynes and that simple Pd(PPhj) catalyzes the stannylboration of terminal alkynes. ... 

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... 

In contrast, it is difficult to activate silicon-silicon bonds by palladium complexes because Si—Si bonds are nonpolarized, thermally stable (r-bonds with a dissociation energy of ca. 300 kJ/mol. However, very recently, the addition of Si—Si compounds to C—C unsaturated bonds have been extensively studied. The results are described in several reviews. In this section, bis-silylation, silylboration, and borylstannation of alkynes and alkenes will be reviewed. 

Silylboration and borylstannation of acetylenes have also been achieved easily in the presence of a catalyst of palladium complexes. In contrast, palladium complexes such as Pd(PPh3)4 or Pd(OAc)2-isocyanide were ineffective for diboration of alkynes. However, the development of another new ligand will solve this difficulty in the future. 

The Si-B bond in silylboranes are activated in the manner similar to disilanes by the group 10 transition metal catalysts to mediate silylboration of such unsaturated substrates including alkenes, alkynes, 1,2-, and 1,3-dienes. Catalytic enantioselective silylboration of 1,2-dienes has been achieved with a chiral palladium catalyst to give a-boryl-substituted chiral allylsilanes. The resulting C-B bonds are converted into various types of C-C, C-N, and C-O bonds by the rich chemistry of organoboron reagents (Scheme 3 1). ... 

The reaction of certain palladium-heteroatom complexes to alkenes and alkynes is a versatile tool for the synthesis of alkanes and alkene having heteroatoms attached. In particular, the various B-B, B-Si, and B-Sn compounds can be used for palladium-catalyzed borylation of alkenes and alkynes (Scheme 5-2). Borostannylation takes place at ambient temperature, whereas silylboration " only proceeds at a temperature above 80 °C due to the slow oxidative addition of a B-Si bond to a palladium(O) catalyst. Both reactions selectively provide cz j-products via addition of silicone or tin to the internal carbon and boron. The reactions are compatible with various functional groups for both terminal and internal alkynes. Cross-coupling reaction of boranes with organic halides selectively occurs at the terminal C-B bonds to provide regiodefined and stereodefined alkenylboron, alkenylsilicon, and alkenyltin compounds. 

Silaborative dimerization of two molecules of internal alkynes in the presence of a silylborate is catalyzed by Ni(0) generated in situ [155],... 

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