Hydroboration transition-metal catalyzed

Scheme 25 Transition metal catalyzed hydroboration of 1,3-dienes...

The field of transition metal-catalyzed hydroboration has developed enormously over the last 20 years and is now one of the most powerful techniques for the transformation of C=C and C=C bonds.1-3 While hydroboration is possible in the absence of a metal catalyst, some of the more common borane reagents attached to heteroatom groups (e.g., catecholborane or HBcat, (1)) react only very slowly at room temperature (Scheme 1) addition of a metal catalyst M] accelerates the reaction. In addition, the ability to manipulate [M] through the judicious choice of ligands (both achiral and chiral) allows the regio-, chemo-, and enantioselectivity to be directed. 

Burgess, K. Ohlmeyer, M. J. Stereocontrol in Catalysed and Uncatalysed Hydroborations. In Homogeneous Transition Metal Catalyzed Reactions Moser, W. R. Slocum, D. W., Eds. Advances in Chemistry Series 230 American Chemical Society Washington DC, 1992 pp 163-177. 

This chapter has been organized into three sections. The first section deals with transition metal-catalyzed hydroboration in organic synthesis and this is divided into three subsections - mechanism, chemoselectivity, and stereoselectivity. The second section deals with the application of transition metal-catalyzed hydroalumination reactions in organic synthesis, and this is also divided into three subsections - mechanism, chemoselectivity, and stereoselectivity. The third section examines the application of both hydroborations and hydroaluminations in total synthesis. 

This landmark discovery paved the way for the development of transition metal-catalyzed hydroboration. The conversion of an alkene into an organoborane intermediate has made this a valuable synthetic technique, particularly since the development of enantioselective variants.9,10 They serve as synthons for numerous functional groups11 and are often subjected to a consecutive carbon-oxygen, carbon-carbon, boron-carbon, boron-chlorine, or carbon-nitrogen24 bond-forming reaction (Scheme 3). 

Among recent examples that highlight the synthetic utility of transition metal-catalyzed hydroborations are its direction toward a formal syntheses of the non-steroidal anti-inflammatory agents Ibuprofen 131 and Naproxen 13214 15 139 as well as the anti-depressant Sertraline 133 (Figure 14).140 In the majority of cases, rhodium-catalyzed hydroboration is utilized and the rhodium(i) source generally is Wilkinson s catalyst RhCl(PPh3)3. 

While transition metal-catalyzed hydroboration is a well-established reaction, the same cannot be said for the transition metal-catalyzed hydroalumination. The synthetic utility of this reaction is only just beginning to emerge. Lautens has led the way in the use of hydroaluminations as the key step in the total synthesis of complex natural products. The synthesis of the anti-depressant sertraline130 involved the formation of the tetrahydronaphthalene core, and this is best achieved using the nickel-catalyzed hydroalumination of oxabicyclic alkenes (Table 16). 

In conclusion it is evident from the foregoing examples that transition metal-catalyzed hydroborations and hydroaluminations occupy an important role in organic synthesis. While rhodium-catalyzed hydroboration has been extensively developed, the hydroalumination is just starting to emerge as a useful reaction in organic synthesis. 

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. 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

Transition-metal-catalyzed hydroboration, a reduction using B H bonds, of styrene derivatives has been demonstrated to occur in supercritical carbon dioxide using tunable Rh(I) complexes as catalysts. In the case of vinyl anisole (eq. 2.5), significantly higher regioselectivities were reported for the reaction in this medium than in THF or perfluoro(methylcyclohexane) (Carter et ah, 2000). 

Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

Herein, we report on a novel process for the synthesis of organomodlfied polydimethylsiloxanes employing ionic liquids for the heterogenization and/or immobilization of the precious metal catalyst [13]. The advantage of this novel hydrosilylation process is that standard hydrosilylation catalysts can be used without the need for prior modification to prevent catalyst leaching. To the best of our knowledge, this is the first example of a hydrosilylation of olefmic compounds using ionic liquids (Scheme 1). However, a method for the transition metal-catalyzed hydroboration and hydrosilylation of alkynes in ionic liquids has recently been described [14]. 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

Transition metal-catalyzed hydroboration of carbon-carbon multiple bonds using organoborane derivatives and coupling with alkanes to form linear alkyl-... 

Thus, detailed experimental and theoretical studies are highly desirable on the mechanism of the transition-metal-catalyzed olefin hydroboration reactions, as well as on the role of the transition-metal center, substrates, and electronic and steric factors in the mechanism. MMM [67] have presented the first detailed ab initio molecular orbital (MO) study of possible reaction pathways illustrated in Fig. 22 for the reaction of C2H4 with the boranes HB(0H)2 and HB02(CH2)3 catalyzed by the model Wilkinson catalyst RhCl(PH3)2. The reaction of BH3 with C2H4 catalyzed by the Rh(PH3)2Cl have been studied by MMM [68] and DS [69]. 

Transition metal-catalyzed hydroboration of carbon-carbon multiple bonds using organoborane derivatives and coupling with alkanes to form linear alkyl-boranes has provided valuable applications in organic synthesis.Titanocene catecholborane complexes recently synthesized and characterized by Hartwig also appear to be the best examples of genuine / -BH a complexes (Figure 13.3). ... 

Transition-metal-catalyzed borylation of organic halides with hydrobor-anes 12H(85)1795. 

In 1985, Manning and N6th first reported the hydroboration of alkenes catalyzed by Wilkinson s catalystJ Since this pioneering work, the development of transition-metal-catalyzed hydroboration has been investigated extensively. Burgess and Ohlmeyer demonstrated asymmetric catalysis with the use of BINAP and Diop-derived Rh-catalysts. Hayashi et al. later reported improvement of the enantioselectivity for the hydroboration of styrenes using Rh-BINAP complexes (up to 96% ee at -78 Other catalyst systems have also been shown to be effective... 

Boryl complexes participate in a variety of catalytic processes, including transition-metal-catalyzed hydroboration and diboration of olefin and acetylene C-C ir-bonds (Chapter 16) and tihe functionalization of alkane and arene C-H bonds (Chapter 17). The chemistry of these complexes has been developed primarily since 1990. A body of literature on metal-boiyl complexes was published in the 1960s, but the structures of these compounds were apparently misassigned in the absence of modem X-ray crystallographic and NMR methods. Boryl complexes of all transition metals except for group 3 (Sc, Y, and La) and group 4 (Ti, Zr, Hf) metals - have been isolated. A majority of transition-metal boryl-complexes contain late metals. 

More often in organometallic chemistry, the catalytic reaction occurs by a mechanism that is completely different from the mechanism of the uncatalyzed process. In this case, the reaction typically occurs by more steps, but the activation energy of each of the individual steps is lower than the activation energy of the imcatalyzed process. The overall barrier is then lower than that of the uncatalyzed reaction. A comparison of the uncatalyzed and catalyzed hydroboration of alkenes with a dialkoxyborane (ROl BH, such as cat-echolborane (see Chapter 16), illustrates this scenario. Qualitative reaction coordinates for tihe uncatalyzed and rhodium-catalyzed process are shown in Figure 14.4. In the absence of a catalyst, the B-H bond adds across the alkene through a concerted four-center transition state, albeit at elevated temperatures in neat alkene. hi contrast, late transition metal-catalyzed hydroborations first cleave the B-H bond by oxidative addition. Coordination... 

Transition-Metal-Catalyzed Hydroboration, Diboration, Silylboration, and Stannylboration... 

This interest in catalytic hydroboration led to the development of the transition-metal-catalyzed diboration of alkenes and alk5mes. The diboration of alkenes and alkynes generates bifunctional products, and additions to alkenes have now been conducted with high enantioselectivity. The following sections describe the t es of catalysts used for catalytic hydroboration and diboration of alkenes, alkynes, and dienes, as well as catalytic cycles that accoimt for selectivities and side products formed during these processes. 

Transition metal-catalyzed cis-hydroboration of alkynes HBX2... 

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