Transmetalation reaction with metal halides

In the transmetalation of tetrakis(tertbutyl)imidosulfate (1) by means of metal halides, driving force is to react [M2(NtBu)4S] and the corresponding lithium halide. However, the transmetalation can also proceed without the salt elimination. This was confirmed by the results of my diploma thesis,^[34] wherein the cadmium iodide complex [{(thf)4Li}2{I4Cd2(NtBu)4S}] was synthesized. Based on this work, different metal halides were tested for the transmetalation reaction. The metal halides had been stored in the glove box. It had been assumed that these metal halides were dry and usable for transmetalation reactions. The metal halide and [(thf)4Li2(NtBu)4S] (1) in a mixture of THF/toluene were stirred over two hours at 0°C and then two days at room temperature. After removing the solvent in vacuo, pentane was added and the reaction mixture was stirred for one hour. In the next step, the precipitated lithium halide was filtered off and the resulting solution was then stored at –24°C. Scheme 3.2 shows the expected reactions.

Scheme 3.2: Expected reactions of the transmetalation of 1 with metal halides.

In the reaction of aluminum-, gold-, and nickel chloride colorless crystals were obtained, which were analyzed by X-ray structure determination. All the crystals turned out to consist of lithium bromide coordinated by THF molecules. The bromide originates from a previous step, the formation of S(NtBu)3 byoxidation.In the reaction of iron- and copper bromide, as well as copper-, iron-, ruthenium- and samarium chloride and iron

iodide an amorphous solid lithium halide could be filtered off. The desired product did not crystallize. The ¹H and ¹³C NMR showed several signals, which could not be assigned.

Table 3.1 shows the applied substances.

Table 3.1: Metal halides, which were used for the transmetalation reaction of [(thf)4Li2(NtBu)4S] (1).

metal halides solution color

(toluene/THF) solution color

(pentane) crystals FeBr2 green-blue brown colorless O2S(NtBu)2

CuBr2 blue colorless colorless [(thf)2LiBr]2

ZnBr2 blue

MnBr2 blue blue colorless [(thf)2LiBr]2

FeCl2 green

CuCl2 black

NiCl2 brown colorless [(thf)2LiBr]2

SmCl2 yellow

AlCl3 colorless colorless [(thf)2LiBr]2

AuCl3 colorless colorless [(thf)2LiBr]2

RuCl3 brown

FeI2 black

The experimental proceedings were changed because the reaction product did not crystallize at 0°C, –24°C, –35°C and –78°C. Also the analytic control mostly gave too many signals for unambiguous assignment of signals of the desired compounds. Only the

1H NMR spectrum of the reaction mixture with CuBr2 showed a signal at 1.37 ppm for the tertbutyl groups, which might be assigned to the desired product. Instead of toluene and THF, the reactions of iron-, copper-, zinc- and manganese bromide were carried out in pentane to realize the crystallization. Like before, only lithium bromide and the hydrolyzed product O2S(NtBu)2 could be obtained, which were analyzed by X-ray diffraction.

Summarizing, the reactions with metal halides as transmetalation reagents were not successful. Instead, coordinated metal halides may yield the desired products because these substances should be more stable and due to the coordination of ally-, alkyl- and other CH-groups the new transmetalation products might crystallize easier.

For the coordinated metal halides, RnMX2, there are two possible products. The elimination of lithium halide to synthesize [(R2M)2(NtBu)4S] and the elimination of LiR to synthesize [(X2M)2(NtBu)4S] are possible (Scheme 3.3).

Scheme 3.3: Expected reactions of the transmetalation with coordinated metal halides.

The lithiated starting material is extremely water sensitive so that the utilized metal halides need to be dried again in vacuo applying heat before they can be used for the metalation of the sulfur-nitrogen ligand. In the reaction with coordinated metal halides, THF was added to a mixture of [(thf)4Li2(NtBu)4S] and followed by the metal halide at – 30°C. After the filtration of lithium salt, pentane or toluene was added. After two weeks half of the solvent was removed in vacuo. It had been assumed that used metal halides and donor stabilized metals were dry and usable for transmetalation reactions. These metal halides and donor stabilized metals, their color in solution and whether crystals were obtained are summarized in Table 3.2.

In the reaction of the rhodium compound (PPh3)3RhCl and [(thf)4Li2(NtBu)4S] crystals were obtained. Unfortunately, these crystals turned out to consist of hydrolyzed PPh3 -ligand and lithium cations form Li(OPPh3)4 species. The phosphane are inert against water but they react readily with oxygen to give the phasphanoxides. This lithium-phosphane compound could be formed because the triphenyllithium-phosphane might be better stabilizing the complex with the coordinated lithium atom than the sulfur-nitrogen ligand, S(NtBu)42-. This compound is not completely characterized because the quality of the X-ray diffraction data was too poor for a detailed structure refinement. The synthesis of this Li(OPPh3)4 species was also not reproducible. The ¹H NMR spectrum of the

crystals dissolved in d8-THF showed multiplets of the phenyl rings at 7.36 and 7.67 ppm.

NMR studies of the reaction mixture were inconclusive. Also EI-MS and elemental analysis could not clarify which products had formed.

Table 3.2: Utilized coordinated metal halides and outcome of the reaction.

substance solution could not be increased by recrystallization from THF, toluene or pentane. The crystals from the reaction of [Fe2Cp2]PF6 and [Fe2Cp2]BF4 could be identified as FeCp2. Most attempts did not yield crystals. Some reaction mixtures resulted in powder being formed, which was, however, not the desired product.

3.2. Metal chlorides and lithium