Lanthanides: Chemistry and Use in Organic Synthesis

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Issue doi: Kumar, T. The selective activation of one carbon-fluorine bond in polyfluorinated aromatic molecules or in trifluoromethyl-containing substrates offers the possibility of accessing unique fluorine-containing molecules, which are difficult to obtain by other synthetic pathways. Among various metals, which can undergo C-F activation, lanthanides Ln are good candidates as they form strong Ln-F bonds. In addition, lanthanide metals display a promising functional group tolerance and their reactivity can vary along the lanthanide series, making them suitable reagents for fine-tuning reaction conditions in organic and organometallic transformations.

However, due to their oxophilicity, lanthanides react readily with oxygen and water and therefore require special conditions for storage, handling, preparation, and activation.

Original Research ARTICLE

These factors have limited a more widespread use in organic synthesis. We herein present how dysprosium metal - and by analogy all lanthanide metals - can be freshly prepared under anhydrous conditions using glovebox and Schlenk techniques. The freshly filed metal, in combination with aluminum chloride, initiates the selective C-F activation in trifluoromethylated benzofulvenes.

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The resulting reaction intermediates react with nitroalkenes to obtain a new family of difluoroalkenes. Lanthanide metals have been sporadically used in organic synthesis since the late s 1. An increased availability and purity of lanthanide metals from the s on, as well as the development of methodologies and equipment to handle air and moisture sensitive compounds led to new applications of lanthanide metals. The preparation of the widely used SmI 2 directly from Sm metal and diiodoethane or iodine was a breakthrough in lanthanide chemistry 2.

Lanthanides: Chemistry and Use in Organic Synthesis - PDF Free Download

In recent years, new reactivity patterns of lanthanide metals have been described, for example, the Barbier-type reaction of allylic halides with carbonyl compounds 3 , the reductive coupling reactions involving diaryl ketones 4 or acyl chlorides 5 , selective cyclopropanation reactions 6 , and the combination of lanthanide metals with group 4 metallocenes 7 , 8. These studies showed that lanthanide metals display a promising functional group tolerance and that their reactivity can vary along the lanthanide series, making them suitable reagents for fine-tuning reaction conditions in organic transformations.

Organolanthanide complexes and inorganic lanthanide salts have been studied in C-F activation reactions with aromatic and aliphatic carbon-fluorine bonds for over 40 years 9 , 10 , In , the first report on C-F activation using elemental ytterbium metal appeared It showed the regioselective reaction of Yb with pentafluorobenzene to afford p -tetrafluorobenzene and YbF 2. It turned out that the combination of dysprosium metal and aluminum chloride gave the highest yields and best selectivities.

We herein present an extension of this work using nitroalkenes as electrophiles 14 , leading regioselectively to a new class of difluoroalkenes Subscription Required. Please recommend JoVE to your librarian. This lanthanide-mediated C-F activation procedure followed by reaction with nitroalkenes provides readily access to new difluoroalkenes containing a nitro group.

A plausible reaction mechanism is depicted in Figure 2. In contrast to our previous work using aldehydes as electrophiles Figure 1 13 , nitroalkenes afford the 1,3-disubstituted indene products regioselectively.

This may be explained by the greater steric bulk of the nitroalkene group. In order to find the best reaction conditions we investigated the combination of two different lanthanide metals La, Dy with AlCl 3 in this reaction with nitroalkenes. It turned out that in both cases, the reaction proceeded with comparable regioselectivity and with the same poor diastereoselectivity. Under optimal conditions, the crude 19 F NMR spectrum of the new difluoroalkene products in CDCl 3 shows two sets of two doublets, one at As an excess of starting benzofulvene is used, one will also observe another set of two doublets at In case moisture entered the reaction, e.

If the first step of the reaction did not go to completion, e. Analysis of the infrared spectrum of the final product also nicely shows the incorporation of the nitro group and cm -1 and the transformation of the CF 3 group into the CF 2 alkene cm This reaction works smoothly with a range of 2-arylnitroalkenes leading to the corresponding difluoroalkenes as mixtures of diastereoisomers in good yields Figure 5 , Table 1.

The aryl groups can contain an electron-donating group -OMe or electron-withdrawing groups -Cl, Br in the para-position. The reaction also proceeds with phenyl- or 1-naphthyl-containing nitroalkenes. We have previously shown that benzofulvenes substituted with aryl groups other than phenyl on the exocyclic carbon also afford a C-F activation reaction Here we include another example of 2-thienyl-substituted benzofulvene, which after C-F activation and reaction with p -Cl-phenylnitroalkene yields the corresponding difluoroalkene entry 6. It should be noted that with the electron-rich p -dimethylaminophenyl group on the benzofulvene, no reaction occurs under these conditions Figure 1: Lanthanide-mediated C-F activation in trifluoromethylated benzofulvenes.

Please click here to view a larger version of this figure. Figure 2: Plausible reaction mechanism for the formation of nitro-group containing difluoroalkenes via C-F activation.

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Below, red: reaction worked well insert shows the mixture of two diastereoisomers in a ratio. Above, blue: reaction did not work well, with considerable amounts of starting benzofulvene and hydrolysed product present. Figure 5: Scope of reaction with various 2-arylnitroalkenes Please click here to view a larger version of this figure.

Table 1: Scope of reaction with various 2-arylnitroalkenes. This protocol involves work with highly reactive, air and moisture sensitive lanthanide metals. Therefore, the whole reaction procedure must be carried out under dry inert gas, and all starting materials, including solvents, must be very clean and dried before use. There are two advantages for the preparation of freshly filed metals over the purchase of already filed metals: i the purchase of pieces or ingots is considerably more economic and ii freshly filed metal is more reactive because of less surface passivation caused by the storage of the filed metal.

The described procedure has two main critical steps: i the preparation of the lanthanide metal by filing a shiny piece of metal and ii the initiation of the C-F activation process by adding a small amount of iodine and carrying out the reaction with small volumes of THF solvent. The advantage of this reaction is that it involves two characteristic color changes, which will directly indicate whether the reaction is performing well or not and where to start looking for modifications. The first one is the disappearance of the deep brown iodine color, leading slowly to a dark-green turbid solution at the end of the C-F activation process.

The second is the change to yellow-green after addition of the nitroalkene.

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If these color changes do not occur, there may be a problem with the starting materials or the solvent. It is important to note, that this procedure describes how the reaction can be performed in "dry" gloveboxes, i. A possible modification for "wet" gloveboxes is to carry out the solvent addition steps in the glovebox, thereby avoiding possible contaminations with air or moisture during the transfer of the AlCl 3 -THF solution. Among the drawbacks of working with elemental lanthanides, is the fact that reactions often only work in ethereal solvents, mainly THF.

Other solvents could be interesting with respect to variation of selectivity outcomes. The described C-F activation process is the first one concerning CF 3 groups attached to diene systems, leading to unprecedented difluoroalkene systems. It opens up many possible extensions with respect to substrates and electrophiles.