P–H Functionalized Al/P-Based Frustrated Lewis Pairs in Dipolar Activation and Hydrophosphination: Reactions with CO₂ and SO₂

Abstract

The P−H functionalized FLPs R(H)PC(AlBis 2 )C(H)-CMe 3 (1a, R = Bis; 1b, R = Mes; Bis = CH(SiMe 3 ) 2 ) combine the typical FLP properties based on Lewis acidic Al and basic P atoms with the reactivity of a P−H bond. They allow the coordination of substrates followed by hydrophosphination with the activated P−H group. Reactions of 1a with R′−NCS (R′ = Ph, CMe 3 ) or of both FLPs with Et−N CCPh 2 afforded five-membered AlCPCS/N heterocycles (4 and 5) via coordination of CS or CN bonds to the FLP backbone. Isomerization by a 1,3-H shift from P to N or C (6 and 7) was only achieved in the presence of an auxiliary base (DABCO; 1−5 mol %). (Z)-1a coordinated CO 2 to yield the adduct 8, which features a five-membered AlCPCO heterocycle with an exocyclic CO bond. Addition of bases such as DABCO and DBU afforded by deprotonation of the P atom the compounds [Bis-PC(AlBis 2 )C(H)−CMe 3 (CO 2 )] − [HB] + (9a, B = DABCO; 9b, B = DBU) that displayed hydrogen bonding between the ammonium ions and the exocyclic O atom of the FLP adducts in solution and the solid state. The stronger base n-BuLi afforded the dimeric Li compound 10, in which the Li cation was coordinated in a chelating manner to the oxygen atoms of one FLP adduct and additionally to the exocyclic O atom of the second adduct, resulting in a fourmembered Li 2 O 2 heterocycle. The related reaction of (Z)-1a with DABCO(SO 2 ) 2 led in contrast to the elimination of BisH and formation of a SO 2 -bridged dimer that features a central (AlOSO) 2 heterocycle with Al−O and S−O single bonds. The resulting unusual structural motifs may be derived from those of dialkylcarbamic or dialkylamidosulfinic acids with the N atoms replaced by P atoms.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

“…Attempts to convert these stoichiometric reactions to a catalytic protocol failed. After the first demonstration of the high reactivity of Al‐based FLP's, [21] the field has been enormously expanded [22–26] . By far most of these FLP applications concern stoichiometric molecule activation and only a limited number of catalytic procedures partially focused on FLP polymerization have been reported [27–30] .…”

“…After the first demonstration of the high reactivity of Al-based FLP's, [21] the field has been enormously expanded. [22][23][24][25][26] By far most of these FLP applications concern stoichiometric molecule activation and only a limited number of catalytic procedures partially focused on FLP polymerization have been reported. [27][28][29][30] This is inherently due to the much higher reactivity of aluminium complexes when compared to boron reagents, often leading to decomposition of the Al-based Lewis acid.…”

Tri-insertion with dearomatization of terminal arylalkynes using a carborane based frustrated Lewis pair template

“…Frustrated Lewis Pairs (FLPs), in which Lewis acid/base adducts are not formed due to steric hindrance, can activate a variety of small molecules. 1 After the first example of H 2 activation by FLPs, 2 this concept has been widely used and FLPs showed reactivity toward a series of unsaturated compounds such as alkenes and alkynes, 3,4 carbonyl compounds, 4,5 carbon oxides, 6,7 nitrogen oxides, 8 SO 2 , 7,9 etc. Among these, reactions of FLPs with alkynes were widely studied, which could be broadly classified into four types of reaction: C–H bond activation (type I), 3 c C C bond addition (type II), 3 b – d 1,1-carboboration (type III) 10 and 1,2-hydroboration (type IV) 4 b ,11 ( Scheme 1a ).…”

Solid‐State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane‐Phosphane Frustrated Lewis Pairs and Secondary Adducts

“…Besides the widely studied borane‐phosphane FLPs, the alane‐phosphane homologs present a viable alternative in chemical reactivity and homogeneous FLP catalysis. [ 17 , 18 , 19 , 20 , 21 , 22 , 23 ] With few exceptions[ 20 , 21 , 22 , 23 ] the field of intramolecular Al/P systems has been focusing on geminal compounds,[ 19 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ] where the relative orientation of the orbitals limits the degree of possible overlap, rendering these generally more reactive in comparison to borane‐phosphane FLPs. To the best of our knowledge, solid‐state NMR techniques have never been used for the structural characterization of these systems.…”