Al/P‐ and Ga/P‐Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C−C and C−N Bond Formation

Abstract

Al/P‐ and Ga/P‐based frustrated Lewis pairs (FLPs) reacted with an azirine under mild conditions under cleavage of the heterocycle on two different positions. Opening of the C−C bond yielded an unusual nitrile–ylide adduct in which a C−N moiety coordinated to the FLP backbone. Cleavage of a C−N bond afforded the thermodynamically favored enamine adduct with the N atom bound to P and Al or Ga atoms. Ring closure was observed upon treatment of an Al/P FLP with electronically unsaturated substrates (4‐(1‐cyclohexenyl)‐1‐aza‐but‐1‐en‐3‐ynes) and yielded by C−N bond formation hexahydroquinoline derivatives, which coordinated to the FLP through P−C and Al−C bonds. Diphenylcyclopropenone showed a diverse reactivity, which depending on steric shielding and the polarizing effect of Al or Ga atoms afforded different products. An AltBu2/P FLP yielded an adduct with the C=O group coordinated to P and Al. The dineopentyl derivative gave an equilibrium mixture consisting of a similar product and a simple adduct with O bound to Al and a three‐coordinate P atom. Both compounds co‐crystallize. The Ga/P FLP only formed the simple adduct with the same substrate. Rearrangement resulted in all cases in C3‐ring cleavage and migration of a mesityl group from P to a former ring C atom by C−C bond formation. Diphenylthiocyclopropenone (evidence for the presence of P=C bonds) and an imine derivative afforded similar products.

Reactions of Al‐N Based Active Lewis Pairs with Ketones and 1,2‐Diketones: Insertion into Al‐N Bonds, C‐C and C‐N Bond Formation and a Tricyclic Saturated Tetraaza Compound

“…Simple adducts such as [Mes 2 PC(AlR 2 )=CHPh] · O=C(CPh) 2 (R = CH 2 t Bu 2 ) were formed via Al–O bond formation. The coordination of the C=O group to P and Al or Ga afforded spiro ‐compounds, and cleavage of the C 3 ring occurred (R = t Bu, CH 2 t Bu) . Ring cleavage is similar to the reaction of 1 , but a mesityl group migrated from P to a C atom of the opened ring, and the C=O group was coordinated to the FLP.…”

“…Ring cleavage is similar to the reaction of 1 , but a mesityl group migrated from P to a C atom of the opened ring, and the C=O group was coordinated to the FLP. [10a] With the related P–H functionalized FLP, R(H)PC(AlR 2 )=CHPh [R = CH(SiMe 3 ) 2 ], the H atom at P migrated in an analogous fashion to the opened cyclopropene ring. [10b]…”

“…[10a] With the related P–H functionalized FLP, R(H)PC(AlR 2 )=CHPh [R = CH(SiMe 3 ) 2 ], the H atom at P migrated in an analogous fashion to the opened cyclopropene ring. [10b]…”

Cooperative activation of azides by an Al/N-based active Lewis pair – unexpected insertion of nitrogen atoms into C–Si bonds and formation of AlCN3 heterocycles

“…Usually the O or N atoms of substrates (carbonyl compounds, imines) approach the Lewis acidic Al atoms of the FLPs or ALPs as the initiating step of the respective transformation. By the interaction with the metal atom the polarity of the C=O or C=N bonds and the electrophilicity of the C atoms increase and facilitate the nucleophilic attack of the Lewis basic ALP or FLP centers at this position by ring closure [7,8] or by initiating another secondary reaction such as an insertion into a M-C bond [12,13,16]. Azides with a homonuclear N 3 group react with Al/P-based FLPs by coordination of the terminal N atom to the Lewis acidic and basic atoms (see Introduction).…”

“…These highly promising materials are considered as attractive alternatives or complements for transition metal catalysts. The majority of FLPs is based on systems with B and P atoms as Lewis acids and Lewis bases [1][2][3][4], but it has recently been shown that Al/P based systems are also highly efficient in various transformations [5][6][7][8][9][10]. Al atoms possess an inherently high Lewis acidity, which makes an activation by electron-withdrawing substituents, as required in B based systems, unnecessary.…”