Alkynes silaboration

The same type of bis-functionalization has been reported for the palladium-catalyzed borylstannylative carbocy-cyclization of 1,6-, 1,5-, 1,7-diynes, bis-propargylamine, and ether.377 It should be noted that even 1,2-dialkylidene cyclobutane can be obtained in reasonable yield. Ito has proposed the related silaborative reaction involving nickel(O) catalysis.378 This reaction has been performed in an intra- and intermolecular fashion. The intramolecular reaction allows the formation of cyclic dienes and the intermolecular process proceeds through a dimerization of alkynes to give acyclic dienes. 

Addition of silylpinacolborane to alkenes has been achieved with platinum catalysts (Equation (83)).225 The reaction proceeds regioselectively to provide products in which the silyl groups are attached to the terminal carbon atoms contrary to the silaboration of alkynes. Although no regioisomers are detected, 1-boryl-l-silylalkenes are formed as major byproducts. 

Suginome et al. discovered that the use of Ni(acac)2 and DIBAL-H as a reductant results in the double insertion of alkynes 280 across the B-Si bond in silaborane 281 to afford silaborated butadienes 282-284 (acac = acetylaceto-nate). NMR experiments revealed the stereoselective formation of head-to-head dimerization product 282 as the... 

With the palladium-isonitrile catalyst, terminal alkynes with a variety of substituents and internal alkynes were subjected to the silaboration with 2-silyl-l,3,2-dioxaborolane in refluxing toluene (Table 4). The reaction proceeded not only with high yields but also... 

The stereoselectivity in silaboration of terminal alkynes is switchable, when chlorine-substituted 4b is used in the reaction with subsequent treatment with isopropyl alcohol in the presence of p)uidine (eq 6). The normal Z-isomer is formed when the silaboration is carried out using a slight excess of alkyne (4b alkyne = 1.0 1.2), whereas the silaboration using an excess of 4b (4b alkyne = 1.2 1.0) results in stereoselective formation of the -isomer. This switch of stereoselectivity is ascribed to Z-to-E isomerization of initially formed Z isomer in the workup stage, in which a palladium catalyst for E-Z isomerization catalyst is generated from i-PrOH and 4b used in excess. 

The regioselectivity in silaboration of terminal alkynes is switched by the phosphine ligand on palladium catalysts (eq 7). In contrast to the normal regioselectivity (B-to-terminal) with PPh3, selective formation of the inverse (Si-to-terminal) regio-isomer 7, in which the sUyl groups are attached to the terminal positions, takes place when the reaction is carried out with P(t-Bu)2(biphenyl-2-yl) as a ligand. 

Silaborative C-C Bond Forming Reactions. In the presence of a nickel(O) catalyst generated in situ by the reaction of Ni(acac)2 with DIBAH, 1 reacts with 2 equiv of alkynes to give l-boryl-4-silyl-1,3-butadienes (eq 20). ... 

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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