Synthesis and some properties of mixed alkyl (-) menthyltin dihydrides

Organotin hydrides have found many applications in organic synthesis not only as reducing reagents [1] but also as intermediates in the generation of carbon-carbon bonds [2], and the synthesis of macrocycles via cyclohydrostannation [3]. Since one of the seminal papers on the reduction of carbonyl compounds with organotin hydrides (mono-, di-, and triorganotin hydrides) was published in 1961 [4], mainly the triorganotin hydrides (R3SnH) have been used routinely for the free radical reduction of alkyl halides and many other functional groups [1,2]. It should be mentioned that at present triphenyltinand tri-n-butyltin hydrides are commercially available. Triorganotin hydrides containing mixed alkyl and (-)-menthyl ligands have been reported [5]. On the other hand, there are less reports on the uses of diorganotin dihydrides (R2SnH2), mostly restricted to the use of Ph2SnH2 and n-Bu2SnH2 as reagents for the reduction of carbonyl compounds. We have not found reports on the synthesis of diorganotin dihydrides containing mixed alkyl and (-)menthyl ligands. In this communication, we report the synthesis of several diorganotin dihydrides containing a bulky chiral (-)menthyl substituent and alkyl groups of increasing steric volume. The synthesis was carried out taking into account the reported sequence [6] for the easy of brominolysis of carbon-tin bonds of tetraalkyltins shown below.


Introduction
Organotin hydrides have found many applications in organic synthesis not only as reducing reagents [1] but also as intermediates in the generation of carbon-carbon bonds [2], and the synthesis of macrocycles via cyclohydrostannation [3].Since one of the seminal papers on the reduction of carbonyl compounds with organotin hydrides (mono-, di-, and triorganotin hydrides) was published in 1961 [4], mainly the triorganotin hydrides (R3SnH) have been used routinely for the free radical reduction of alkyl halides and many other functional groups [1,2].It should be mentioned that at present triphenyltin-and tri-n-butyltin hydrides are commercially available.Triorganotin hydrides containing mixed alkyl and (-)-menthyl ligands have been reported [5].
On the other hand, there are less reports on the uses of diorganotin dihydrides (R2SnH2), mostly restricted to the use of Ph2SnH2 and n-Bu2SnH2 as reagents for the reduction of carbonyl compounds.We have not found reports on the synthesis of diorganotin dihydrides containing mixed alkyl and (-)menthyl ligands.
In this communication, we report the synthesis of several diorganotin dihydrides containing a bulky chiral (-)menthyl substituent and alkyl groups of increasing steric volume.The synthesis was carried out taking into account the reported sequence [6] for the easy of brominolysis of carbon-tin bonds of tetraalkyltins shown below.
Dihalogenation of (-)-menthyltrimethyltin (1) with 2 equiv of bromine in methanol gives a mixture of (-)-menthylmethyltin dibromide (3) and monobromide 2. The 119 Sn showed that in the mixture the ratio 3/2 was 3. As the chromatographic separation of mono and dibromides 2 and 3 was inefficient and vacuum distillation led to high loses, the mixture dissolved in ether was treated with aqueous NaOH.The (-)-menthylmethyltinoxide present in the mixture formed (A) precipitated as a compound with a wax like consistence, insoluble in both the aqueous and the ether layers.The oxide was washed with water, decanted, and then treated with conc.HCl.The new (-)-menthylmethyltin dichloride (5) thus obtained, was extracted with pentane, dried, and solvent elimination gave 5 (65% yield from 1 as a dense oil.The reduction of dichloride 5 with LiAlH4 in diethylether led to (-)-menthylmethyltin dihydride (12) in 91% yield.
The (-)-menthyldimethyltin bromide (2) was alkylated i-propylmagnesium bromide, n-butyl magnesium bromide, and t-butylmagnesium bromide afforded the corresponding (-)menthylalkyldimethyltin derivatives 6-8 in average yields of 82%.The reactions of tetraalkyltins 6-8 with 2 equiv of bromine in methanol led, again, to the mixtures (B) of the corresponding (-)menthylalkyltin dibromides and monobromide.The mixtures B were dissolved in diethyl ether and [a] Universidad Nacional del Sur, Argentina. [b] CIC-PBA, Argentina. [c] CONICET, Capital Federal, Argentina treated with an aqueous solution of NaOH.The resulting mixtures of oxides (C) were handled as before, and the treatment with conc.HCl led to the corresponding (-)-menthylalkyltin dichlorides 9-11.The reduction of dichlorides 9-11 with LiAlH4 in diethylether under Ar atmosphere led to the (-)menthylalkyltin dihydrides 13-15 in very good to excellent yields.
The 13 C NMR spectra of these compounds showed that all of them were obtained without epimerization at the menthyl carbon directly attached to tin [C(1)] atom.In all cases the 10 menthyl a In CDCl3 (compounds 5, 6, 9, 10,11) and C6D6 (12-15); chemical shifts in ppm; 1 J(Sn,C) and 1 J(Sn,H) coupling constants (in brackets) in Hz.
Dihydrides 12-15 are dense and, most of them, colorless oils.They remain active for few days in the fridge under an argon atmosphere.At the open air and r.t.they decompose in few hours.These new dihydrides will be used in studies connected with the stereoselective reduction and hydrostannation of prochiral unsaturated systems.

Table 1 .
distinguishable, and the DEPT experiments together with the magnitude of the n J( 13 C, 119 Sn) coupling constants | 1 J| > | 3 J| > | 2 J| > |4J| enabled the easy assignment of the signals, and, therefore, to establish the structure of the new compounds.Selected 1 H and 13 C NMR values