Silaboration allenes

In 2009, Ritter and coworkers reported a selective Fe-catalyzed hydroboration of 1,3-dienes to produce linear ( )-y-disubstituted allylboranes under mild conditions when a combination of Ll-FeCl2 and magnesium metal as a catalyst was used. Tbe branched ( )-allylboranes were obtained by using L2-FeCl2 instead of Ll-FeCl2 (Scheme 26) [88]. For the synthesis of 2-borylallylsilanes, this method was superior to the previously reported silaboration of allenes [89]. 

Allenes undergo silaboration with silylpinacolborane in the presence of palladium catalysts (Scheme 47). Although... 

On the other hand, platinum-catalyzed silaboration of allenes results in an opposite regiochemical preference. Silaboration of a 1,1-disubstituted allene affords terminal addition product regioselectively in the presence of a platinum catalyst, whereas the internal addition product is obtained with a Pd2(dba)3-PPh3 catalyst (Scheme 49).227... 

A unique system for catalytic silaboration of allenes, in which a catalytic amount of organic halide is used as a crucial additive, has been reported (Equation (86)).232 In the presence of Pd2(dba)3 (5 mol%) with 3-iodo-2-methyl-2-cyclohexen-l-one (10mol%), reactions of terminal allenes with a silylborane afford /3-silylallylboranes in good yields with excellent regioselectivity. It is worth noting that the addition takes place at the terminal C=C bond in contrast to the above-mentioned palladium-catalyzed silaboration. The alkenyl iodide can be replaced with iodine or trimethylsilyl iodide. The key reaction intermediate seems to be silylpalladium(n) iodide, which promotes the insertion of allenes with Si-C bond formation at the central -carbon. 

Silaboration of 3-substituted 1,2-dienes takes place smoothly at the internal double bond in the presence of the catalytic Pd(acac)2-2,6-xylyl isocyanide complex and the boryl group is regioselectively introduced to the central carbon atom of an allene (Scheme 16.55) [59, 60]. The same regioselectivity is observed with the catalytic system Pd2(dba)3-P(OCH2)3CEt [59]. 

A highly enantioface-selective silaboration of chiral allenes (30) and (31) has been developed using a combination of a chiral silylborane, (—)-(34), and a chiral Pd-(/ )-(35) catalyst (Scheme 2). The chiral reagent system efficiently controlled the stereochemistry of the new stereogenic centres even in the case of mismatched combinations.47... 

Under catalysis by a Pd complex with homochiral phosphine ligand 19, silaboration of terminal allenes with homochiral silylborane 18 shows high enantioface-selectivity by double asymmetric induction (Equation (25)).lls The resultant /3-borylated allylsilanes can be used for asymmetric carbonyl allylation. 

Silaboration. Regioselective silaboration of monosubstituted allenes at the internal double bond is observed with this catalyst system. However, perfluoroalkylallenes show a different reaction profile. 

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