Intramolecular C–F Activation in Schiff-Base Alkali Metal Complexes

Abstract

A series of alkali metal complexes (Li, Na, K) with different fluorinated phenoxo-imine ligands [M­{(O-2-(RNCH)-C6H4}] [R = C6F5; 2,4,6-F3C6H2F3; 2,6-C6H3F2; 2,3-C6H3F2; 2-C6H4F; 2-CF3­C6H4CH2; CF3CH2] have been synthesized and fully characterized. By using THF as solvent, suitable crystals for X-ray diffraction analysis were obtained, and the solid state structure of some of the prepared compounds has been determined. This analysis reveals the formation of a C–F bond activation product for the potassium compound with the five-fluorinated ligand. In stronger donor solvents, such as dimethylsulfoxide, this process occurs immediately, independently of the metal used. This C–F activation takes place in a smooth process at room temperature. The nature of this reaction is studied by 1H-DOSY NMR experiments with stoichiometric amounts of DMSO, monitoring the disaggregation process from the initial solution structure to the final activation product. The variation of the number of fluorine atoms and their location in the iminic ring is also studied, showing their influence in this intramolecular C–F bond activation process.

Main Group Metal‐Mediated Transformations of Imines

“…Substitution of the Schiff base with electron‐withdrawing fluorine substituents provides additional stabilising M⋅⋅⋅F interactions (Scheme 14), [52] allowing solid state isolation of the initial sodium aluminate complexes before redistribution in solution. Interestingly, no alkali metal salt (MF) elimination is observed in these reactions, highlighting the stability of the species compared to their pentafluoro‐analogues in which M–F elimination occurs [53] (see section 6).…”

“…Synthetically this has presented ac hallenge owing to the stronga nd inert nature of the bond, particularly in fluoroaromatics, (CH 3 ÀF = 115kcal mol À1 ; CH 3 ÀH = 105 kcal mol À1 ;P h ÀF = 127 kcal mol À1 ). [53] The selective preparation and functionalisation of fluorarenes has been a hot topic, both from the perspective of developing new compounds andt he safe degradation of existing fluorocarbons. To date there are some examples which exploit the imine moiety in functionalisingC ÀFb onds.…”

“…Previous synthetic strategies include thermolysis pathways. [127,128] However,i n 2019, ar eport emerged from Mosquera andc o-workerst hat developed an alkali metal approacht oa ccess thesem olecules via an intramolecular CÀFa ctivationo ft he Schiff base alkali metal complex [53] (Scheme 40). They adopted ao ne-pots ynthesis, using simple alkali metal bases such as LiHMDS, NaH and KHMDS to prepare the intermediate alkoxo-imino-alkalimetal complex.…”

“…Fluorinated compounds are of great interest in today's society with over 25 % of all pharmaceuticals and 30 % of agrochemicals containing at least one C−F bond, [121, 122] despite being the least abundant natural organohalide. Synthetically this has presented a challenge owing to the strong and inert nature of the bond, particularly in fluoroaromatics, (CH 3 −F=115 kcal mol −1 ; CH 3 −H=105 kcal mol −1 ; Ph−F=127 kcal mol −1 ) [53] . The selective preparation and functionalisation of fluorarenes has been a hot topic, both from the perspective of developing new compounds and the safe degradation of existing fluorocarbons.…”

Asymmetric Domino Heck Arylation and Alkylation of Nonconjugated Dienes: Double C–F···Sodium Attractive Noncovalent Interaction

“…Most examples have CF···M bond lengths of around 2.4 to 2.7 Å and the binding energies are estimated to be about half of those with ethers. The benefits of the noncovalent interactions of CF bonds of ligands with transition-metal centers have also been documented in metal-catalyzed polymerization of lactide, olefin metathesis and olefin polymerization …”