Hydroboration proposed mechanism

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

Scheme 1 A general proposed mechanism for the rhodium-catalyzed hydroboration of alkenes using HBcat...

Because syn addition to the double bond occurs and no carbocation rearrangements are observed, carbocations are not formed during hydroboration, as shown in Mechanism 10.5. The proposed mechanism involves a concerted addition of H and BH2 from the same side of the planar double bond the it bond and H-BH2 bond are broken as two new o bonds are formed. Because four atoms are involved, the transition state is said to be four-centered. 

Complexes of the 4 type catalyze the hydroboration of various olefins with catecholborane at ambient temperature [173], The proposed mechanism of the hydroboration reaction - although not within the scope of this book - parallels that of the hydrogenation and hydrosilylation reactions. The architecture of both olefins (terminal > terminal disubstituted > internal disubstituted > trisubstituted) and organolanthanides (TOF(La) 10 TOF(Sm) TOF(5) = 4 TOF(4) affects the rate of hydroboration, which for 4(La CH(SiMc3)2) and 1-hexene is TOF = 200 h , for example. The observed high regioselectivities are exclusively anti-Markovnikov. For smaller metal centers (Y, Zr, Ti) and other ligand systems (bis(cyclopentadienyl), bis(benzamidinato)) inactivation of the catalyst by catecholborane or Lewis base-metal complex induced disproportionation of catecholborane appeared to compete effectively with the catalytic conversion [174]. 

Proposed mechanisms of transition metal catalysed hydroboration of alkenes and diboration of alkynes... 

We will begin by focusing on the first step of hydroboration, in which borane is attacked by a Tt bond, triggering a simultaneous hydride shift. In other words, formation of the C—BH2 bond and formation of the C—H bond occur together in a concerted process. This step of the proposed mechanism explains both the regioselectivity (Markovnikov addition) as well as the stereospecificity (syn addition) for this process. Each of these features will now be discussed in more detail. 

Steric considerations. In the first step of the proposed mechanism, both H and BH2 are adding across the double bond simultaneously. Since BH2 is bigger than H, the transition state will be less crowded and lower in energy if the BH2 group is positioned at the less steri-cally hindered position (Figure 9.5). It is likely that both electronic and steric factors contribute to the observed regioselectivity for hydroboration-oxidation. 

The observed stereospecificity for hydroboration-oxidation is consistent with the first step of the proposed mechanism, in which H and BH2 are simultaneously added across the it bond of the alkene. The concerted nature of this step requires that both groups add across the same face of the alkene, giving a syn addition. In this way, the proposed mechanism explains not only the regiochemistry but also the stereochemistry. 

The proposed mechanism involves the regioselective c/s-hydroboration of the 4-bromo-l,3-enyne as observed by Zweifel [2] followed by addition of hydride, originating from t-BuLi [3], to initiate a 1,2-metalate rearrangement forging the C-C bond with inversion at the vinylic center (Scheme 2). This key step enables the stereospecific character of the whole process. To circumvent the low reactivity of (Z)-vinylborane toward aldehydes, the corresponding (Z)-vinylzinc was prepared by transmetalation with diethylzinc and reacted successfully with carboxaldehyde. The isolation of aUylic alcohols in high yields was subordinated to a careful selection of the... 

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

In order to account for the high regioselectivities observed in the rhodium-catalyzed hydroboration of styrenes, Hayashi proposed a modified mechanism which proceeds through 73-benzyl-rhodium complex 22 as a key intermediate (Scheme 7). Reductive elimination from this 73-benzyl-rhodium complex 22 produces the secondary alkylborane regioselectively.12 A related 73-benzyl-palladium complex was recently isolated by Hartwig in studies of hydroamination.75... 

Figure 2. Hydroboration reactions of olefin catalyzed by early transition metal complexes. The proposed reaction mechanism involves a o-bond metathesis step. (M = Lanthanide or other early transition metals.)...

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. 

A number of models have been proposed in order to account for the favored enantiomers produced during asymmetric hydroboration reactions. None of the early models were without criticism and were based on scanty theoretical background. Recently, however, much more has become known about the mechanism of the hydroboration reaction (see Section 3.10.6). From these studies it appears that monomeric dialkylboranes (and alkylboranes, etc.) are the active intermediates in hydroboration re-... 

Given the considerations mentioned above, the mechanism of Scheme 2 is proposed. This mechanism features a concerted addition of the Si-H bond to the C=C bond of the substrate, in a manner analogous to the B-C bond-forming step in the hydroboration of alkenes. Indeed,... 

In many catalytic processes and transition metal mediated reactions, a-bor-ane complexes have been shown to be intermediates. The bis(borane) complex Cp2Ti( 72-HBcat/)2 (HBcaT = HBcat-4-f-Bu) is a highly active catalyst for the hydroboration of vinylarenes [37]. A mechanism, shown in Scheme 3, has been proposed for the Ti-catalyzed hydroboration on the basis of a detailed mechanistic study [37]. Theoretical calculations provided further support to the proposed reaction mechanism and showed that the reductive elimination step, giving the product molecules, is rate-determining [38]. In the Cp2Ti(CO)2 catalyzed hydroboration of alkynes [36,37], the proposed reaction mechanism (Scheme 4) also involves a a-borane complex similar to 11 and 14. In the titanium-catalyzed decaborane-olefin hydroborations [47,48], a-borane complexes were also considered as intermediates. In the Cp2MH (M = Nb, Ta) mediated hydroboration reactions of olefins [39,41], Smith and his coworkers observed several interesting cr-borane complexes, such as 21-23 discussed above. 

While there was initial debate between Pasto5 and Brown6 about the mechanism of the hydroboration reaction, Brown s proposed mechanism6 is now generally accepted as the most likely pathway. Brown suggested that the reaction proceeds via an equilibrium between BH3 THF (5) and free BH3 (6). The free BH3 then rapidly adds B-H across a 71-system of the olefin (7) in an anti-Markovnikov fashion via an asynchronous, 4-centered transition state (8) to afford hydroboration product 9. 

The addition of boron and hydrogen across the carbon-carbon double bond appears to occur by stereospecific syn addition. The replacement of the C—B bond by a C—OH bond in the oxidation step is also stereospecific. The mechanism proposed for the oxidation of one of the carbon-boron bonds is shown in equations 9.47 through 9.51, so the configuration of the carbon-boron bond is retained in the product. As an example, hydroboration-oxidation of 1-methylcyclopentene (50, equation 9.52) followed by oxidation of the organoborane (51) gave only the trans-2-methylcyclohexanol (52). 

Hydroboration, as we have seen, can be classified as a concerted addition reaction in which no intermediate is formed. The mechanism is characteristic of a group of reactions called pericyclic (from the Greek, meaning around the circle) reactions, which involve a cyclic shift of electrons in and around the transition state.The mechanism proposed is further supported by the fact that rearrangements are not normally observed in hydroboration reactions, which implies that there are no carbocationic intermediates. 

Studies on the rate of hydroboration of 1-octene by several borane-Lewis base adducts indicate that the mechanism proceeds via prior dissociation of the adduct which is contrary to the direct attack mechanism proposed on the basis of ab initio calculations. Hydroboration of representative alkenes by borinane dimer in n-heptane proceeds through dissociation of the dimer followed by reaction of the monomer and alkene. Borinane shows similar behaviour to 9-BBN, both molecules differing markedly from other common monofunctional reagents. The factors that affect the stereoselectivities achieved by various hydroborating reagents have been investigated theoretically and the analysis developed into a predictive tool. The reduction of ketones and hydroboration of alkenes by catecholborane is catalysed by lithium bromide. 

Any mechanism that we propose for hydroboration-oxidation must explain both the regioselec-tivity ( inri-Markovnikov addition) as well as the stereospecificity (ry addition). We will soon propose a mechanism that explains both observations. But first, we must explore the nature of the reagents used for hydroboration-oxidation. 

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