Organometallic Reactions in Water

Water is particularly suitable for use in biphasic catalysis. It readily separates from organic solvents because of its polarity, density and because of the hydrophobic effect. Water will form biphasic systems with fluorous solvents, some ionic liquids, many volatile organic solvents, and also with scCC 2 [18], 

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At first, the idea of performing organometallic reactions in water might seem ridiculous, since it goes against the traditional belief that most organometallics are extremely sensitive to traces of air and moisture and rapidly decompose in water. ... 

This concept meshes with another important environmental issue solvents for organic reactions. The use of chlorinated hydrocarbon solvents, traditionally the solvent of choice for a wide variety of organic reactions, has been severely curtailed. In fact, so many of the solvents favoured by organic chemists have been blacklisted that the whole question of solvents requires rethinking. The best solvent is no solvent and if a solvent (diluent) is needed then water is preferred. Water is non-toxic, non-inflammable, abundantly available, and inexpensive. Moreover, owing to its highly polar character, one can expect novel reactivities and selectivities for organometallic catalysis in water. 

S. Kobayashi, T. Wakabayashi, H. Oyamada, Use of an Organometallic Reagent in Water Sc(OTf)3-Catalyzed Allylation Reactions of Aldehydes in Micellar Systems Chem. Lett. 1997, 831-832. 

Research into homogeneous hydrogenation and its applications prior to 1973 are comprehensively described in the now classic book of James [3]. More recent books on hydrogenation [1] and on aqueous organometallic catalysis [2] contain special chapters on hydrogenation reactions in water. In adition, all reviews on aqueous organometalhc catalysis devote considerable space to this topic, see e.g. references [9-12]. 

Equation 7.30 demonstrates the increasing use of the green solvent H20 (see Section 9-1-5 for more discussion on green chemistry) as a reaction medium for organometallic reactions in this case the product is the result of syn addition of H2, which is exactly the same result observed in organic solvents.62 The TPPMS (triphenylphosphine m-monosulfonate) group attached to phosphorus makes both the phosphine and the resulting Ir complex water soluble. 

Reactions that otherwise would be carried out in more than one phase (heterogeneous reactions) can be transformed to homogeneous ones, with the aid of supercritical fluids, where interphase transport limitations are eliminated. This is realized due to enhanced solubilities of the supercritical fluids. Typical examples are reactions in water (supercritical water can solubilize organic compounds), homogeneous catalytic reactions, reactions of organometallic compounds. Homogenizing one compound more than the other may also affect relative rates in complex reactions and enhance the selectivity. 

These examples confirm that water as a solvent offers a number of variations to the possibilities offered by organic solvents in organometallic reactions and underline the importance of the analytical methods to characterization of organometallic compounds in water. There is still a huge potential for the use of water as the solvent of choice for homogeneous or biphasic catalysis. 

In conclusion, it can be shown that asymmetric catalysis, and particularly asymmetric hydrogenation, occurs in a two-phase system, allowing the very easy recycling of the catalyst without loss of enantioselectivity. Different techniques have been used in order to solubilize the catalyst and the products in the aqueous phase. It is obvious that in the future other asymmetric organometallic-catalyzed reactions in water or in a two-phase system will appear in the literature. 

As exemplified in the previous sections, organometallic catalysis in water and bi-phasic water-containing systems has not only been intensively examined during recent years but its scope has also considerably broadened. Between 1945 and 1997, the publishing date of the first edition of this book, about 1650 articles dealing with catalysis in aqueous media appeared in scientific journals. Approximately the same number of articles has been published between 1998 and the end of 2002. In the first half of 2003 nearly 200 new articles appeared [1], Most of the research dedicated to reactions in aqueous-phase and aqueous biphasic or multiphase systems refer to the reactions described in the previous sections. In this chapter some other new results, not described above in more detail, on reactions in aqueous media will be presented. 

It is probably true that activation of zinc by means of modern inventions in the 1980s has had the most dramatic results in changes in its reactivity. Organometallic reactions with water as the solvent were never dreamt of before with zinc as the metal. The modern organometallic chemist must come under the impression that a better understanding of the mechanistic aspects of one-step processes is coming nearer once technical problems as the initiation of such reactions is mastered by simple techniques as the use of a sonicator or of Zn and Zn/Gr. 

The first example of dihalocarbene insertion into a saturated C—H bond of an organometallic complex has been reported using a phase-transfer catalyst The first example of phase-transfer catalysts for electrophilic substitution involved an azo-coupling reaction in water-dichloromethane using sodium 4-dodecylbenzene-sulphonate giving a rate acceleration of ca. 200-fold. ... 

The Monsanto acetic acid synthesis is a classic example of an industrially useful transformation based upon organometallic reactions. In this synthetic procedure, carbon monoxide and methanol are coupled to form acetic acid in water. Two catalysts are used. The first is HI, playing the role of a strong acid with a nucleophilic counterion, and the second is the... 

Tucker, C.J., Welker, M.E., Day, C.S. and Wright, M.W., Preparation of an aquocobaloxime 1,3-dienyi complex and its Diels-Alder reactions in water and organic solvents, Organometallics, 2004,23,2257-2262. 

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