Boranes metal—carbon bonds

The synthesis of various new chiral (o-hydroxyaryl)oxazaphospholidine oxides (139), derived from (S)-proline derivatives, from precursors (140) have been elaborated. This two-step reaction involves an unstable metallated intermediate that undergoes a fast 1,3-rearrangement with the formation of phosphorus-carbon bond. These catalysts have been successfully applied to the catalytic asymmetric borane reduction of numerous prochiral ketones with enantiomeric excess up to 84% ee (Scheme 35). ... 

The diastereoselective synthesis of a 7 -chirogenic p-aminophosphine ligand 300 by carbon-carbon bond formation of the ethano bridge in a 3 1 ratio via reaction of an a-metallated f -chiral phosphine borane (5)-297 with a benzaldimine was described. The major diastereoisomeric p-aminophosphine borane (Sp)-298 was separated and decomplexed into the corresponding p-aminophosphine (5p)-300 under neutral conditions and without epimerization by heating at reflux in EtOH (Scheme 100) [198]. 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

The C—C and C—B interatomic distances in carboranes can also be related to the coordination numbers of the skeletal atoms. Two factors tend to make these distances shorter than the B —B distances in comparable boranes the preference of the carbon atoms for sites of low coordination number and the greater electronegativity of carbon than boron, which increases the electron density in the region of the carbon atoms and so strengthens the bonds that they form. Table IX lists some C—C distances for closo- and wido-carboranes 13, 20, 21, 26, 98,121,168) and metal-acetylene 50, 58,112) complexes, relating them... 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

In the first of these reactions (Equation 11-2), a hydrocarbon is produced by the cleavage of a borane, R3B, with aqueous acid, or better, with anhydrous propanoic acid, CH3CH2C02H. The overall sequence of hydroboration-acid hydrolysis achieves the reduction of a carbon-carbon multiple bond without using hydrogen and a metal catalyst or diimide (Table 11-3) ... 

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. 

The transition metal activates the C-X bond in the oxidative addition step and normally the substrates have sp or sp carbons at or immediately adjacent to an electrophilic centre. The reactivity of aliphatic C-X bond towards the oxidative addition with a transition metal is somewhat low. However, in 1992, Suzuki and co-workers discovered that Pd(PPh3)4 can catalyze couplings of alkyl iodides with alkyl boranes at 60°C in moderate yields (50-71%). These conditions tolerated a wide variety of functional groups such as esters, ketals and cyanides. 

Treatment of the bis(propynyl)zirconocene 759 with B(C6Fs)3 results in a linear G-G coupling of the alkynyl ligands to form the zwitterionic complex 76 0582,583 (Scheme 186). Complex 760 reacts with nitriles R CN to form initially the 1 1 adduct 761 that concurrently equilibrates with 760 and the metallacyclocumulene 762 (and the nitrile-borane adduct) subsequently, irreversible reaction in the presence of excess nitrile yields the methylene-cyclopropene derivative 763.584,585 Calculations have shown that the conversion 760 —> 763 is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate that features coordination of the three-membered carbocycle through one of its G-G cr-bonds. 

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