Olefin hydroboration

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

The catalysis of olefin hydroboration by early transition metal complexes, e.g., titanium- and lanthanide-complexes, has also attracted considerable interest in recent years [14-17]. These catalytic systems show different... 

The three theoretical studies presented above give somehow different conclusions regarding the detailed reaction mechanisms. The difference is apparently related to the different models used and different theoretical approaches employed in the calculations. In any case, the emerging overall picture is that the reaction mechanism of olefin hydroborations must be complicated. We would like to see more experimental work done in the future so that the theoretical results can be tested. In particular, it would be nice to evaluate experimentally the utility of the suggestion made by Ziegler and co-workers in the choice of the phosphine ligands in order to produce more pure product. 

Sm(III)-catalyzed olefin hydroboration reaction by catecholborane has recently been theoretically investigated [28], The stationary structures on the model reaction path considering ethylene as the olefin, Cp2SmH as the active catalyst, and HB(OH)2 as the model borane were obtained at the RHF and MP2 levels. MP4SDQ energy calculations were also carried out at the MP2 structures. 

Figure 8. The Sm(III)-catalyzed olefin hydroboration reaction mechanism calculated by Koga and Kulkami.

Alcohols 3-8, obtained by the reduction of the corresponding ketones with equimolar amounts of BMS and are obtained with high ees (ee values given are obtained using 0.1 equiv of (R)-1). Enantioselectivity is excellent (often similar or only slightly lower than those reported in the CBS reduction) for aromatic and hindered methyl ketones, (e.g. 3-5) and is also good for linear and a-monobranched enones (e.g. 7 and 8), but lower for linear methyl ketones like 2-octanone (6). In should be noted that in the reduction of unsaturated ketones, the time of addition is critical (the optimum being around 15-20 min) in order to avoid concomitant olefin hydroboration. In sharp contrast to the CBS process, the use of catecholborane (instead of BMS) or alternative solvents proved to be detrimental. 

Although numerous advances in syntheses occurred during this period, two warrant special mention. First is the addition of boron-hydrogsn bonds to olefins (hydroboration) 26-28) ... 

Kulkarni, S. A., Koga, N. Ab initio mechanistic investigation of samarium(lll)-catalyzed olefin hydroboration reaction. THEOCHEM1999, 461-462, 297-310. 

Widauer, C., Gruetzmacher, H., Ziegler, T. Comparative Density Functional Study of Associative and Dissociative Mechanisms in the Rhodium(l)-Catalyzed Olefin Hydroboration Reactions. Organometattics 2000,19, 2097-2107. 

Starting Olefin Hydroborating Agent Addition in Position (%) Ref... 

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. 

Hydroboration of olefins with 9-BBN is normally complete after 1 hr. in refluxing THF or after 8 hr. in refluxing benzene or hexane. The regioselectivity in olefin hydroborations is higher with 9-BBN than with disiamylborane or dicy clohexylborane. ... 

Kulkarni and Koga217 performed a similar study to the previous one from the point of view of AIM, i.e. the characterisation of bonds in reaction schemes. This time the ( , V2pb and eb) triplet was used in the mechanistic investigation of samarium(III)-catalyzed olefin hydroboration reaction via ab initio calcula-... 

Figure 22. Possible mechanisms for olefin hydroborations mediated by RhCKPPHslj [67],...

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]. 

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