NH bond activation of ammonia and amines by ditetrelenes: key insights into the stereochemistry of nucleophilic addition

Abstract

The NH activation of ammonia by tetramesityldisilene takes place in three steps: formation of the anti-ammonia-disilene adduct, inversion at the β-silicon, and intramolecular syn-transfer of the proton to give the syn-product.

On the Stereochemistry of the NH Bond Activation of Ammonia or Amines by Disilenes: Predictability

“…It has long been recognized that the disilenes are able to activate the O-H bond in water and alcohols; in fact, the reaction with water and alcohols is one of the most extensively investigated reactions of disilenes. [6][7][8] Disilene activation of σ-bonds in other important small molecules, such as NH 3 [9][10][11][12][13] 1 and H 2 , [14] has recently been realized. With the ability to selectively give the NH activated product in high yield rather than the coordinated donor adduct, the promise of disilenes in NH activation chemistry is particularly attractive and, to realize the potential, a thorough understanding of the mechanism and stereochemistry of this fundamental reaction is needed.…”

“…[11] Upon examination of the data provided, [11] the stereochemistry of the intermediate observed upon nucleophilic addition is the antidonor adduct. [13] Baines and Özpinar also investigated the addition of ammonia and amines to disilenes using tetramesityldisilene (3), [16] a prototypical, tetraaryldisilene with a fold angle of 18°a nd a twist angle of 7°. [16c] Experimentally, the addition of NH 3 and amines proceeds to the expected σ-addition products (4) in low to excellent yields depending on the bulk of the amine (Scheme 2).…”

“…[16c] Experimentally, the addition of NH 3 and amines proceeds to the expected σ-addition products (4) in low to excellent yields depending on the bulk of the amine (Scheme 2). [13] The mechanism of the addition was investigated computationally where the individual mesityl substituents can be tracked, and hence, the stereochemistry of the addition could be elucidated. The most energetically favored reaction pathway takes place in three distinct steps: formation of the anti-ammonia-disilene donor adduct, Int B, inversion at the silicon bearing the lone pair to give Int C, followed by intramolecular syn-transfer of the proton to give the syn-isomer of 4a (Scheme 3).…”

“…The most energetically favored reaction pathway takes place in three distinct steps: formation of the anti-ammonia-disilene donor adduct, Int B, inversion at the silicon bearing the lone pair to give Int C, followed by intramolecular syn-transfer of the proton to give the syn-isomer of 4a (Scheme 3). [13] Given the energetic preference for the formation of the anti-donor adduct, Int B, the anti-isomer of the product may only be formed in one of two ways. Protonation of the antidonor adduct in Int B by a second molecule of ammonia would give the anti-isomer of 4a; however, intramolecular syn-transfer of a proton is energetically more favoured (ΔG � = 13.7 kcal/ mol) compared to intermolecular transfer (ΔG � = 47.8 kcal/mol), and thus, the intermolecular pathway was ruled out as a reasonable alternative.…”

“…Protonation of the antidonor adduct in Int B by a second molecule of ammonia would give the anti-isomer of 4a; however, intramolecular syn-transfer of a proton is energetically more favoured (ΔG � = 13.7 kcal/ mol) compared to intermolecular transfer (ΔG � = 47.8 kcal/mol), and thus, the intermolecular pathway was ruled out as a reasonable alternative. [13] Secondly, rotation about the SiÀ Si bond in Int B to give an eclipsed conformation (Int C) followed by intramolecular syn-transfer of the proton would also lead to the anti-isomer of 4a. In the system under study, the pathway involving rotation about the SiÀ Si bond was shown to have a high activation energy barrier of 20 kcal/mol making it unfavourable in comparison to inversion and resulting in the Scheme 1.…”