Carbanions as Intermediates in the Formation of Grignard Reagents

Abstract

The reaction between certain aryl bromides RBr having substituents with group 14 elements and magnesium in THF to form the corresponding Grignard reagents RMgBr were found to be accompanied by unusual migrations of the group 14 functionality; it partially exchanges position with that of the original aryl bromide function. This migration is purely intramolecular for organosilicon derivatives but both intra-and intermolecular for the corresponding tin compounds. For example, 1-bromo-2-((trimethylsilyl)methyl)benzene (1b) gives rise to 1-(bromomagnesio)-2-((trimethylsilyl)methyl)benzene (2b) and to its intramolecular rearrangement product 1-((bromomagnesio)methyl)-2-(trimethylsilyl)benzene (3b), whereas the trimethylstannyl analogue 1a forms, in addition to the two corresponding Grignard reagents 2a and 3a formed by the intramolecular pathway, the intermolecular exchange products 1-(trimethylstannyl)-2-((trimethylstannyl)methyl)benzene (4a) and 1-(bromomagnesio)-2-((bromomagnesio)methyl)benzene (5′). These results, together with those from analogous reactions of compounds such as 1-bromo-2-(trimethylstannyl)benzene (18), its 5-methyl derivative 23, 2-bromo-2′-(trimethylstannyl)biphenyl (40), and its silicon analogue 57, can be convincingly explained by invoking the intermediate formation of the highly reactive carbanion R: -, which may in some cases be slightly stabilized by formation of an (intra-or intermolecular) stannate (or silicate) complex. Another serious candidate for such migrations would have been the radical R • , the occurrence of which is well-documented in Grignard reactions. However, this alternative route could be excluded by producing this radical in an unambiguous fashion from 1a and samarium(II) diiodide; under these conditions, hydrogen abstraction by R • strongly predominated, and minor quantities of rearranged products were shown to be due to further reduction of R • to R: -.

Palladium‐Catalyzed Intermolecular Coupling of 2‐Silylaryl Bromides with Alkynes: Synthesis of Benzosiloles and Heteroarene‐Fused Siloles by Catalytic Cleavage of the C(sp3)Si Bond

“…However, in contrast to many reports on the cleavage of the C(sp 3 ) À Si bonds through stoichiometric reactions [8,9] or through transition-metal-catalyzed reactions of activated C(sp 3 ) À Si bonds, [3c, 4b-d, 10] little is known about the transitionmetal-catalyzed cleavage of unactivated C(sp 3 )ÀSi bonds. [11][12][13][14][15] Rauf and Brown reported the cleavage of a C(sp 3 )ÀSi bond in an SiMe 3 group by using the SiMe 3 group as a methyl source in a Pd-catalyzed oxidative methylation of olefins.…”

9‐Stannafluorenes: 1,4‐Dimetal Equivalents for Aromatic Annulation by Double Cross‐Coupling

“…[7] We anticipated that the cyclic form would have a beneficial effect on the reactivity toward the first coupling of the annulation owing to the strain. [8] In addition, the use of 9-stannafluorenes is favorable for lowering metal waste as compared with the corresponding 2,2'-distannylbiphenyls. Initially, we chose 9,9-dimethyl-9-stannafluorene (1 a) and 1,2-dibromobenzene (2 a) as the coupling partners.…”

Clear-cut difference in the rearrangement of 1-bromo-2-(2-phenyl-3-butenyl)benzene under anionic or radical conditions

“…No cyclized product 6 was formed (<1%). In a series of works, Bickelhaupt's group showed [45][46][47][48] that the reaction of aryl halides with magnesium involves reactive aryl carbanion intermediates whose presence is evidenced by well identified by-products formed in yields of 4-85%. The detailed interpretation of these results involves an aryl bromide radical anion whose cleavage provides an aryl radical which is reduced extremely rapidly into a carbanion.…”