Silaboration

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]. 

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. 

Allylplatinum intermediates generated in the course of silaboration of dienes readily react with aldehydes to give homoallylic alcohols (Equation (125)).484 The observed high diastereoselectivity is explained by a chair-like cyclic transition state. 

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. 

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. 

Platinum catalysts are also effective for the silaboration of 1,3-dienes.234 Although almost no stereoselectivitiy is observed in the silaboration of acyclic 1,3-dienes, 1,3-cyclohexadiene undergoes the stereoselective silaboration in fair yields (Equation (87)). Enantioselective silaboration of 1,3-cyclohexadiene has been achieved with 70% ee by using a platinum catalyst bearing a binol-based optically active phosphoramidite ligand.235... 

Silaboration of bicyclopropylidene proceeds via proximal G-G bond cleavage at 130 °C in the presence of the palladium// r/-alkyl isocyanide catalyst (Equation (88)). In contrast to the aforementioned silaboration of methylene cyclopropanes, vinylsilane products are obtained with high regioselectivity.102... 

The reaction of silylborane with 1-halo-l-lithio-l-alkenes yields 1-boryl-l-silyl-l-alkenes via borate formation followed by 1,2-migration of silyl group (Equation (90)).76,240 The mechanism seems to be closely related to that proposed for the silaboration of isocyanide (Figure 2). Vinyl-substituted carbenoids, l-chloro-l-lithio-2-alkenes, react with silylpinacolborane to give l-boryl-l-silyl-2-alkanes in good yield (Equation (91)).241 This methodology is applied to the synthesis of l-boryl-l-silyl-2-cyclobutene.2 2 Similar reactions are carried out with other carbenoid... 

The 12-vertex closo-borate [MeSiBnHn]- 10 was synthesized in a typical cluster closure reaction with the amineborane adduct Et3NBH3 either from the ft>-silaborate [MeSiB10H12] or from the arachno-silaborane [MeSiB9H12(NHMe2)].34,46... 

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]. 

This doso-silaborate could also be synthesized by the two atom cage expansion of the siladecaborane, MeSiB9Hi2(NHMe2) depicted in Scheme 3.3-4 [15, 16]. 

Apart from incorporation of a silicon vertex into the decaborane skeleton by salt elimination reactions, the controlled nucleophilic degradation of the disilaborane Me2Si2BioHio with NaOH is another versatile method for the preparation of the nido-silaborate, [MeSiBioHi2], which is depicted in Scheme 3.3-9 [29]. 

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... 

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