Introduction
One of the classes of chemical complexes whose coordination chemistry has generated a lot of interest is the bidentate phosphines. This can be attributed to the fact that this class of ligands facilitates the combination of phosphorus atoms that have strong metal bonds with atoms from donor groups that are more labile in nature. Examples include Nitrogen atom from the aminophosphines donor group or an Oxygen atom from etherphosphines. This is unlike hard donors that are weak and basic like MeOH and H2O that do not have good binding with transitional metals including Rhenium. Due to this fact, complexes formed from these donors show very interesting catalytic properties and reactivity.
The formation of this type of heteroditopic ligands is facilitated by the ability of the oxygen and nitrogen atoms to freely disassociate in a reversible manner from the soft metal in which it is attached therefore generating a free and unused coordination site. This behavior has particularly found wise usage in homogenous catalysis processes because in these processes, the production unsaturated intermediate chemical species is hugely favored. Other closely related ligands of the hemilabile class are the diphosphine monoxides (Crabtree 52). These have also been used extensively in some catalytic transformations.
Another donor phosphine group is the iminophosphorane (containing Ph2PCH2P(=NR)Ph2) that is a very important hemilabile ligand group that also contain the nitrogen-phosphorus donor groups. This class of ligands has been used extensively to produce several ruthenium (II), rhenium (II) and palladium (II) complexes. Their catalytic activity on some organic reactions, for example, the ketone hydrogenation transfer and the cycloisomerization of some organic compounds into others has also been studied (Nguyen 94). This is therefore one class of ligands that is very interesting in deed.
An example of rhenium (I) derivatives is the tricarbonyl complexes. This complex contains an iminophosphorane- phosphine. These are very active catalysts that are used in propargylic alcohols isomerization into the α, β-unsaturated carbonyl chemical compounds.Treating the [ReBr(CO)5] complex with an almost equal amount of an iminophosphorane like Ph2PCH2P(=O)Ph2 or Ph2PCH2P(=NR)Ph2 by using a refluxing THF yields a mixture of several neutral complexes. Therefore, carbonated iminophosphorane ligands are not very effective in the synthesis of the desired new rhenium (I) complexes.
The production of the desired complexes is however achieved by using iminophosphorane ligands that are N-thiophosphorylate. Similar reaction conditions are used. This class of iminophosphorane facilitates the diphenylphosphino (PPh2) and thiophosphoryl ((RO) 2 P=S) unit selective coordination to the centre of the rhenium (I) centre. This advertently leads to the formation of selective complexes unlike in the previous situation where a mixture of complexes was formed. These complexes are in referred to as κ2-P, N complexes and example include [ReBr (κ2-P, N-Ph2PCH2P{NRF}Ph2) (CO)3] (3e−g) complexes .An alternative mode of synthesis of these complexes is the use of fluorinated ligands.
Scientists have been able to isolate some of these complexes as white solids that are air stable. This has been done in a yield range of 82-96%. Scientists have also been able to characterize these complexes using standard spectroscopic methods like Infrared radiation and Nuclear Magnetic Resonance together with elemental analysis. For instance, IR spectroscopy has shown that there are three absorptions of ν (CO) at 1887-2026 cm-1. This is in accordance to the stipulated fac-arrangement of carbonyl ligands. Nuclear Magnetic Resonance provides an allowance for the distinguishing of the adopted coordination mode of the ligands of the iminophosphorane-phosphine nature in 2 and 3a, b.
Since Rhenium (I) and Ruthenium (II) are species that belong to the isoelectronics d6 series and exhibit coordination chemistry that is analogous in nature, they can be used to synthesize new Rhenium derivatives and their catalytic activity can consequently be explored.In the synthesis of these new complexes of rhenium (I), the molecular structure has been explored or determined using diffraction studies of the X-ray single crystal (Nguyen 94). This analysis has shown that most of the new complexes formed have an octahedral geometry that is distorted (Nguyen 99). Those that contain carbon atoms for example [Re(κ2-P,NPh2PCH2P{=N(4C6F4CHO)}Ph2)(CO)3] are bonded to three molecules of carbon monoxide , a phosphorus atom form the diphnylphosphino group, a nitrogen atom form the iminophosphoranyl unit and finally a bromine atom. Studies have also found that bond distance between the different complexes is almost similar. The thermal, electrochemical, electroluminescent and the photophysical properties of the complexes have also been done.
There has been intense research aiming at finding new catalytic approaches based on nonconventional solvents as media for reaction. In relation to this, many organomettalic catalysts have been utilized with success in a number of organic transformations involving supercritical CO2, water, ionic liquids and perflourinated compounds as substitutes to VOCs (Crabtree 19). Despite the widespread studies that have concentrated on use of metal as catalysts in the isomerization of propargylic acids to various unsaturated carbonyl compounds, there are few efforts focused to coming up with catalytic systems that can work in non conventional solvents such as ionic liquids. It is only a few catalysts that are active in the Meyer-Schuster- propargylic alcohols rearrangements have been explained in the literature, utilizing ionic liquids as solvents (Lebedev 77). To asses the catalytic potential of e-g,4c,d,2a-d,b and 3a complexes, the conversion of 1,1-dephenyl-2-propyn-1-ol to 3,3 –diphenylpropenal (polymer) was used. In the reaction, ionic liquids were used.
When the catalytic activity of 4c,d,2a-d,b and 3a complexes was checked, it was found that selective isomerization took place leading to a formation of enal 8a as the only end product. The complex which produced best results was the 4d complex which led to the formation of enal 8a (99%) in just 10minutes.Apart from the 1,1-dephenyl-2-propyn-1-ol ,the 4d complex proved to be an equally efficient catalyst in the selective isomerization of other secondary and tertiary propargylic acids to their corresponding enals. It is also of worth to note that all the reactions will proceed to the end if there are no cocatalysts are involved. The effect of the electronic features of aryl rings was apparent. This explains why alkynols having electron with –drawing parts showed less reactivity when compared to the alkynols with electron donating parts. 1-phenyl-1-propyn-1-ol (7f) –a secondary alcohol showed uniqueness in that the enal 8f obtained was obtained as a thermodynamically more stable E isomer.
What makes the use of ionic liquids advantageous is the ability to recycle the catalytic scheme or system by separating the product of the catalytic conversion with simple and easy processes such as the use of organic solvents during extraction. Reusability and ability to recycle is one of the most important factors in any catalytic system.
The catalytic activity of the 4d complex was also tested in isomerization involving propargylic alcohols containing a C-H at the β-location with respect to the 9a, b-alsohol group. Isomerization of these alcohols leads to a selective formation α, β-methyl ketones (unsaturated).These reactions are attributed to the formal alkynols Rupe-type rearrangement .Quantitative transformation is also attained, although at relatively higher temperature with regard to the Meyer-Schuster rearrangement. The 4d catalyst is also recyclable but is more active through lower number of successive runs. These catalytic conversions can be used successfully with more complex substrate like the one mestranol (a hormonal steroid).Mestranol can be catalytically converted to the corresponding 10c-enone.
There was no transformation observed when internal propargylic alcohols were used. The catalytic inactivity observed with the internal alkynols is attributable to the proposed mechanism which appears to be based on the major transitional hydroxyvinylidene complex – (Re) +=C=C (H) C (OH) R2. It is only the terminal alkynols, which can undergo tauomerization to produce vinilydene-type species (Lebedev 71). To determine the formation of the intended intermediate rhenium (1) hydroxyvinylidene, stoichiometric reaction of the 4d catalyst were performed in (BMIM) (PF6). However, all attempts to characterize or isolate the reaction product were not successful Contrastingly, when the 4d complex was reacted with 3-butyn-1-ol in refluxing THF as a substitute to the ionic liquid, [Re{=C(CH2)3O}(κ2-P,S-Ph2PCH2P{=NP==S)- (OPh)2}Ph2)(CO)3][SbF6] (5e) complex was obtained. The formation of this carbene was because of intermolecular attack on the hydroxyl group in the α-carbon in the vinilydene intermediate. The multinuclear NMR(1H, 31P{1H}, and 13C{1H} characterized rhenium(I)−oxacyclocarbene 5e.The cationic structure of the tricarbonyl-rhenium (1) oxacyclic complex was unconfirmed due ton the ambiguity of the structure.
The fact that there was no reaction with the internal alkynols and the inability to produce vinilydene species and ,the fact that; catalytic transformations do not continue in the absence of the 4d catalyst in ionic liquids at 353K ,can be attributed to the classic mechanism. The classic mechanism originates from the propargylic alcohol π-coordination, as well as, the consequent production of vinilydene complex through the [1,2]-shift.
Works Cited
Crabtree, Robert H. The Organometallic Chemistry of the Transition Metals. New York: Wiley, 2010. Print
Lebedev, K B. The Chemistry of Rhenium. London: Butterworths, 2009. Print
Nguyen Hung Huy, Ulrich Abram. Synthesis and Reactivity of Structurally Analogous Phenylimido and Oxo Complexes of Rhenium (V) with N,N-Dialkyl-N′-benzoylthioureas . Zeitschrift Fur Anorganische Und Allgemeine Chemie 2008 DOI: 10.1002/zaac.200800120 Catalytic transformation/isomerization of propargylic alcohols to α,β-carbonyl compounds (saturated) in ionic liquids
