CHAPTER 1 1
1.1.4. Transition Metals in Hydroboration
1.1.4.1. Rhodium Catalyzed Hydroboration
Rhodium is one of the rarest elements found on the earth’s crust and was one of the first elements used for homogeneous catalysis; consequently there is extensive literature on
19
this subject. The ease of oxidative addition to the four-coordinate square planar rhodium(I) to give the octahedral coordinate rhodium(III) species has attributed to the popularity of rhodium catalysts.61
As stated above Männig and Nöth were the first to report the first examples of rhodium- catalyzed hydroboration of olefins. These authors discovered that, under the influence of tris(triphenylphosphine)chlororhodium (I) (Wilkinson’s catalyst), the hydroboration of certain alkenes by catecholborane can be accomplished at room temperature, instead of the elevated temperature (100°C) required for the uncatalyzed hydroboration. They also observed that the catalyzed hydroboration showed noticeably different chemoselectivity and regioselectivity (Scheme 10).57
The mechanism of the rhodium complex catalyzed hydroboration reactions has been extensively studied; Männig and Nöth proposed the catalytic mechanism based on the hydroboration of an alkene with catecholborane (Scheme 11), suggesting a dissociative mechanistic pathway. The pathway was further supported by Evans and co-workers after conducting deuterium labeling experiments. 58
20 RhCl(PPh3)3
RhCl(PPh3)2 B
O O H
Ph3P Ph3P
Rh H
Cl Bcat
R
PPh3 Rh H
Cl Bcat
R
Rh Bcat Cl Ph3P Ph3P
R H
R
Bcat
1
2
3 4
5
-PPh3 +PPh3
PPh3 PPh3
Scheme 11
The dissociative mechanism proposed for the hydroboration of an olefin with a Wilkinson’s catalyst entails the loss of a phosphine ligand to provide the transient species, RhCl(PPh3)2 with a vacant coordination site. The transient species enters the catalytic cycle and is successively followed by (refer to Scheme 11):
The oxidative addition of the catecholborane (B–H bond) to the metal center (1→2)to form the rhodium boryl species.
An olefin coordination into the metal center results in the displacement of a second phosphine ligand to form a penta-coordinated rhodium intermediate complex (2→3).
Thereafter there is the subsequent insertion of the olefin into the Rh–H or Rh–B bond in conjunction with the phosphine ligand binding to the metal center (3→4).
21
And finally the reductive elimination of the borolane product (5) completes the catalytic cycle by regenerating the transient species (4→1).
Baker and co-workers62 rejected the mechanism suggested by Männing and Nöth, and they proposed an alternative dissociative mechanism for the hydroboration. The mechanistic pathway that they proposed was relatively similar to that of the associative mechanistic pathway but had a subtle difference (Scheme 12). 62
RhCl(PPh3)3
RhCl(PPh3)2 B
O O H
Ph3P Ph3P
Rh H
Cl Bcat
R
PPh3 Rh
H Cl Bcat
Ph3P
R Rh Bcat
Cl Ph3P Ph3P
R H
R
Bcat
1
2
3 4
5
-PPh3 +PPh3
Scheme 12
The first and second steps of the associative pathway are similar to that of the dissociative pathway, but the difference arises when the olefin has to coordinate to the rhodium centre, as seen in Scheme 12-3.The coordination of the olefin occurs without the loss of the phosphine ligand from the metal center, this generates a hexa-coordinate
22
intermediate. This step is followed by the reductive elimination that yields the borolane ester compound, similar to the dissociative pathway.62
There have been numerous reports about the investigation of both the dissociative and associative mechanistic pathways, and the results obtained conflict each other and support the pathways. The DFT study conducted by Ziegler and co-workers on the comparison of both the pathways is the most convincing study and it concluded that both pathways are feasible.63 In their study, Ziegler and co-workers were in agreement with the work proposed by Burgess and co-workers; that thermodynamically the associative pathway is favoured. However, they go on to state that the dissociative pathway is possible with bulky electron-withdrawing phosphine ligands. 63
1.1.4.1.1. Reaction Conditions
Ligand Effects: Pereira and Srebnik53 described the use of pinacolborane to hydroborate phenylacetylenein the presence of Wilkinson's catalyst gives two regio-isomers in the ratio of 48:52, in favor of the Markovnikov addition product. They further stated that when one of the phosphine ligands in the rhodium center is substituted by a carbon monoxide ligand, there is a drastic change in the regioselectivity of the catalyst towards phenylacetylene. This changes the ratio to 98:2 with an increased preference for the anti- Markovnikov addition product. A number of reports in the literature show that fine tuning of the ligand substituted on the rhodium metal center can have an effect on the regioselectivity of the hydroboration reaction, and the products formed.53,57,63,64
Substrate Effects: Fu and co-workers reported that the rate of the catalyzed hydroboration reaction is extremely sensitive to the olefin substitution pattern. They demonstrated using two rhodium-catalyst complexes that remarkably there is a quantitative hydroboration of terminal alkenes by catecholborane within minutes at room temperature. However, 1,1-disubstituted olefins took hours, 1,2-disubstituted olefins took even longer and trisubstituted were quite unreactive. They noted that the sensitivity of the catalyzed reaction to steric effects provides an opportunity for selective hydroboration of the less hindered of two olefins within a substrate.
23
Lata and Crudden65 have reported the effects of adding a Lewis acid to the catalyzed hydroboration reaction. They have illustrated that the addition of Lewis acids such as tris- pentafluoroboron as co-catalysts have an intense effect on the Rh-catalyzed hydroboration of olefins with HBPin. For instance aliphatic olefins do not react at all in non-coordinating solvents, but upon adding 2 % B(C6F5)3, the reaction goes to completion within minutes. Likewise, the reaction of aromatic olefins with HBPin occurs slowly, with no selectively in the absence of B(C6F5)3, but is accelerated and more selective upon the addition of B(C6F5)3. Mechanistic studies suggest that the Lewis acid needs to be present throughout the course of the reaction as it appears that the Lewis acid, along with THF, is involved in the heterolytic cleavage of the B–H bond of HBPin.65