Oxidative addition borane

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

The catalytic cycle for hydroboration is now widely accepted and direct examples of several intermediate species have been isolated and well characterized (Scheme 3).5-7 These now include (j-borane complexes, which have in some instances been found to be catalytic precursors for hydroboration.8-10 Oxidative addition of an H—B bond to a coordinatively unsaturated metal fragment... 

The proposed mechanism starts with a methyl group abstraction on platinum complex 416 with the borane reagent in the presence of diyne 414 (Scheme 105). The square-planar cationic diyne-platinum(n) complex 417 is converted to the octahedral platinum(rv) hydride intermediate 418 through oxidative addition of the hydrosilane. This complex decomposes rapidly with methane release to form another tetracoordinated platinum(n) species 419, followed by platinasilylation of the triple bond. The resulting vinylplatinum 420 undergoes an intramolecular carboplatination to... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

The result of theoretical investigations have suggested that cleavage of a B—H bond occurs to initiate ammonia borane dehydrogenation [38]. Alternatively, the oxidative N—H addition of ammonia to the dehydrogenated intermediate C may constitute a feasible reaction pathway due, in particular, to the fact that ammonia and aniline oxidative addition to la and related iridium-PCP systems has been reported experimentally [39]. 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

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. 

Another important subgroup of monohydrido complexes can be prepared by the oxidative addition of nido-heteroborane anions to [RhCl(PPh3)3] (Scheme 17). Further, the c/oTO-dicarborane complex is the starting material for the preparation of other rhodium(III) borane complexes (Scheme 18). They are also weakly active hydrogenation catalysts. 

Although oxidizing agents are not tolerated by most systems that activate C-H bonds through an oxidative addition pathway, they are compatible with boranes. In a series of elegant papers, Hartwig has demonstrated the selective formation of... 

Late-metal complexes of Pd, Pt, and Rh can also catalyze hydrosilylation, hy-drostannylation, hydroboration, and diborylation reactions of 7r bonds. Both C=C and C=0 bonds may be hydrosilylated or hydroborated, whereas hydrostannyla-tion is usually carried out only on C=C bonds. (Some boranes add to C=0 and C=C bonds in the absence of catalyst, but less reactive ones, such as catechol-borane ((C6H402)BH), require a catalyst. Moreover, the metal-catalyzed reactions sometimes display different selectivities from the uncatalyzed variants.) The mechanisms of all these reactions are the same as hydrogenation, except that oxidative addition of H-H is replaced by oxidative addition of a R3Si-H (R3Sn-H, R2B-H, R2B-BR2) bond. 

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