Unexpected Formation of the First Doubly Benzylic 1,1-Di-Grignard Reagent

Abstract

Reaction of l-bromo-2-(l-chloro-l-trimethylsilylmethyl)benzene (3) with magnesium in THF expectedly gave the corresponding 1,3-di-Grignard reagent lb in 44 % yield, but in addition, the isomeric 1,1-di-Grignard reagent 4 was obtained in equal amounts. Compound 4 is remarkable for two reasons: it is one of the few examples of a 1,1-di-Grignard reagent, and its formation involves rearrangement by 1,3-migration of a trimethylsilyl group which provides further evidence for the occurrence of carbanionic intermediates such as 11 during the formation reaction of Grignard reagents. INTRODUCTIONDi-Grignard reagents XMg-Y-MgX are of interest for several reasons [1,2]. In the first place, they may be useful synthons because reaction with electrophiles leads to other difunctional derivatives; this is particularly attractive for the synthesis of a great variety of organometallic heterocycles. Secondly, they sometimes show unusual structures, in particular when the bridge -Y-connecting the two organomagnesium functions consists of three carbon atoms or less, the so-called "small" di-Grignard reagents [3,4], Thirdly, these reagents often pose the challenge of not being easily accessible by the ordinary approaches, for instance the well-known Grignard reagent formation reaction of the corresponding dihalide with magnesium metal. For this reason, the number of known 1,1-di-Grignard reagents, which have two magnesium atoms attached to the same carbon, is rather limited [1,2], Fortuitously, we came across such a species in the context of our investigations on the benzylic/aromatic 1,3-diGrignard reagent la; it is connected by the well-known Schlenk equilibrium with the corresponding diorganylmagnesium compound 2a. The latter has been applied for the synthesis of a number of organometallic heterocycles [1,2,[5][6][7][8][9][10][11].

Carbanions as Intermediates in the Formation of Grignard Reagents

“…Therefore, one may conclude the reaction of 1b with magnesium leads not only to the unrearranged Grignard reagent 2b but also to the rearranged 3b (Scheme ); this process occurs only during the formation reaction and not afterward, and this again is ascribed to the highly reactive carbanionic intermediate 9b analogous to 9a (Scheme ). We recently reported that in the related case of the formation of di-Grignard reagents from 54 , only intramolecular migration of the trimethylsilyl group occurred, leading to a mixture of 55 and 56 14 …”

OLIGOMERIC 1,2-DIHYDRO-1-MAGNESACYCLOBUTA[a]NAPHTHALENE

“…Presumably, it has an oligomeric or polymeric structure, as it is rather poorly soluble; even in THF, which normally is a powerful solvent for organomagnesium compounds, the solubility is only about 0.0016 M. On one hand, this property is convenient if one wishes to obtain pure la, because on preparation from the corresponding dihalide 2a and magnesium, the primarily formed di-Grignard reagent 3a undergoes the Schlenck equilibrium with la from which the latter precipitates in pure form. While the formation of 3b' was of considerable mechanistic interest as it proceeds via a carbanionic intermediate [10], 3b could not be obtained pure, and hence the corresponding diorganylmagnesium lb was not accessible by this route. A conceivable strategy to enhance the solubility is the introduction of bulkier groups which might act not only by increasing the lipophilicity of the reagent, but also by counteracting association.…”