Preparation of trialkylgermyl and -stannyl derivatives of methyl isothiocyanate by in situ trapping of anionic intermediates with chloro(trialkyl)germane and –stannanes

Addition of an ether solution of one or two equivalents of lithium diisopropylamide to strongly cooled 1:1-or 1:3-molar mixtures of methyl isothiocyanate and chloro(trimethyl)germane in THF afforded (isothiocyanatomethyl)(trimethyl)germane and [isothiocyanato (trimethylgermyl)methyl](trimethyl)germane, respectively, in fair to good yields. Using this in situ trapping procedure (isothiocyanatomethyl)(trimethyl)stannane and tributyl (isothiocyanatomethyl)stannane were obtained in high yields from the corresponding chloro (trialkyl)stannanes and methyl isothiocyanate .


Introduction
As part of our investigations with heterocumulenes and polar organometallics [1][2][3] we recently developed a method for the convenient preparation of silylated methyl isothiocyanates 4 (Scheme 1).Our procedure is similar to the one used to trap short-lived anions generated from allylic and benzylic halides 5 and consists of the in situ trapping with chloro(trimethyl)silane of anion 2, generated from methyl isothiocyanate 1 and lithium diisopropylamide (LDA).Depending on the molar ratios of the reactants, we succeeded in obtaining mono-, bis-and tris(trimethylsilyl)substituted methyl isothiocyanates 3, 4 and 5 in good to excellent yields.
Applying similar procedures we have successfully introduced one and two silyl groups into benzyl isothiocyanate. 6We wish to point out in this connection that generation of isothiocyanate anions and their functionalization with electrophiles in two separate and consecutive operations appears to be unfeasible.Attempts to realize this two step reaction sequence have resulted in the formation of thiazole and imidazole derivatives. 7

Results and Discussion
To our knowledge halo(trialkyl)germanes and -stannanes have not been used so far as reagents in in situ trapping experiments.Especially, isothiocyanatoalkylstannanes may have synthetic importance in view of the possibility to substitute a trialkylstannyl moiety by electrophilic groups such as an acyl group.In analogy with the trapping experiments with chloro(trimethyl)silane, mixtures of chloro(trimethyl)germane hloro(trimethyl)stannane or tributyl(chloro)stannane and methyl isothiocyanate (1) were treated with solutions of LDA (Scheme 2).
In an attempt to prepare the tris-germylated methyl isothiocyanate we added three equivalents of LDA to a 1:3 molar mixture of 1 and chloro(trimethyl)germane.However, in the crude product the expected compound was present only in 14%, distillation of the crude product afforded 7a in 73% yield.It seems that the removal of the methine proton from 7a proceeds less effectively, possibly due to steric hindrance.
The preparation of (isothiocyanatomethyl)(trimethyl)stannane (6b) was achieved with excellent results using 1 and LDA in excess with respect to chloro(trimethyl)stannane. A similar procedure with equal molar amounts of 1, LDA and tributyl(chloro)stannane afforded tributyl(isothiocyanatomethyl)stannane (6c) in high yield.No attempts were made to introduce two or three trialkylstannyl groups.

Experimental Section
General Procedures.Solvents and diisopropylamine were dried over machine-powdered potassium hydroxide and distilled after filtration.Methyl isothiocyanate and tributyl(chloro)stannane are commercially available (from Aldrich).Solutions of LDA were prepared by adding diisopropylamine at approx.−10 °C to a solution of n-butyllithium in hexane (1.6 M; Chemetall) and diethyl ether or THF.Chloro(trimethyl)germane and tetramethylstannane, from which chloro(trimethyl)stannane was prepared, were gifts (from Dr. K. D. Bos).Liquid nitrogen was used as coolant of the reactions.The reaction products were isolated by dry work-up: After removal of the solvents on the rotary evaporator under reduced pressure, about 70 mL of pentane was added to the syrupy residue; lithium chloride was filtered off and rinsed with pentane; after concentration of the pentane solutions under reduced pressure, the crude product was subjected to vacuum distillation. 1H NMR spectra were recorded on a Varian EM-390 (90 MHz) spectrometer.Chemical shifts are given in ppm relative to TMS, carbon tetrachloride was used as solvent.IR spectra were recorded on a Perkin-Elmer spectrophotometer (model 283).GLC analyses were carried out on a Varian 3400 gas chromatograph (15 m capillary column coated with a 1.5 µ DB-5, internal diameter 0.53 mm).