Borane bonds activation

The Addition of Boranes to Activated Double Bonds Hydro-alkyl-addition (overall transformation)... 

Arene ruthenium and osmium complexes play an increasingly important role in organometallic chemistry. They appear to be good starting materials for access to reactive arene metal hydrides or 16-electron metal(O) intermediates that have been used recently for carbon-hydrogen bond activation. Various methods of access to cyclopentadienyl, borane, and carborane arene ruthenium and osmium complexes have been reported. 

This result represents the first example of a synthetic cycle for arene borylation facihtated by an f-element. The authors investigated the reaction mechanism and proposed a concerted direct B—H attack on an aromatic C—H bond, which resulted in the formation of the B—C bond and the release of one molecule of H2. This type of reactivity resembles a-bond metathesis if boron is considered a metal center. Albeit uranium was not directly involved in the borylation of the arene C—H bond, the formation of the diuranium arene inverse-sandwich complex plays an important role. The arene, i.e., benzene or naphthalene, is partially reduced upon coordination to uranium,which makes it more susceptible to attack by an electrophile such as borane. Therefore, although uranium is not direcdy involved in the C—H bond activation step, this example illustrates that f-elements can render arenes reactive in nonmetal-mediated transformations by forming activated arene metal complexes. 

Figure 7 Varying degrees of B—H bond activation in group 9 borane complexes. Counterions omitted for clarity. ...

In 2003, Rawal reported the use of TADDOLs 177 as chiral H-bonding catalysts to facilitate highly enantioselec-tive hetero-Diels-Alder reactions between dienes 181 and different aldehydes 86 (Scheme 6.29A) [82], and also BINOL-based catalysts 178 were found to facilitate this reaction with excellent selectivities [83]. TADDOLs were also successfully used as organocatalysts for other asymmetric transformations like Mukaiyama aldol reactions, nitroso aldol reactions, or Strecker reactions to mention a few examples only [84]. In addition, also BINOL derivatives have been employed as efficient chiral H-bonding activators as exemplified in the Morita-Baylis-Hilhnan reaction of enone 184 with different carbaldehydes 86 [85]. The use of chiral squaramides for asymmetric reactions dates back to 2005 when Xie et al. first used camphor-derived squaric amino alcohols as ligands in borane reductions [86]. The first truly organocatalytic application was described by Rawal et al. in 2008 who found that minute amounts of the bifunctional cinchona alkaloid-based squaramide 180 are... 

Bourumeau K, Gaumont A-C, Denis J-M (1997) P-H bond activation of primary phosphine-boranes access to a-hydroxy and a,a -dihydroxyphosphine-borane adducts by uncatalyzed hydrophosphination of carbonyl derivatives. J Organomet Chem 529 205-213... 

Borane 2 adds to carbon-carbon double bonds without the need of catalytic activation. This reaction has been discovered and thoroughly investigated by H. C. Browm, and is called hydroboration It permits a regioselective and stereospecific conversion of alkenes to a variety of functionalized products. 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

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