The C---F bond as a tool in the conformational control of amides

The conformation of 2-fluoro-N-(2-fluoroethyl)-propionamide 4 in the solid state indicates the influence of both a P-fluorine-amide gauche effect and an alpha-fluoroamide effect. The structure reveals the influence of two recently observed stereoelectronic effects associated with the C-F bond, which has resulted in the successful prediction of the solid state conformation of amide 4. A gauche relationship (-69.9degrees) was observed for atoms N(1)-C(4)-C(5)-F and a syn planar (2.0degrees) relationship was observed for N(1)-C(3)-C(2)-F. The paper demonstrates the predictive power of using the C-F bond as a tool in influencing the conformation of amides and peptides. (C) 2002 Elsevier Science B.V. All rights reserved.</p

Activation and functionalization of vinylic C-F bonds by transition metal compounds: The factors determining reactions between nucleophiles and a (perfluorovinyl)diiron(I) complex; Syntheses of diiron derivatives containing new C-N, C-S, C-H and C-O bonds

International audienceThe reactions between alcohols ROH (R = Me, Et) and RR(OH)2 (RR = CH2-CH2) and the [perfluoro(sulfanyl)vinyl]- diiron complex [_Fe(CO)3₂_μ-C(SMe) (CF3)CβCαF2_] (1) in THF at room temperature involve substitution at the Cα atom to give new (alkoxymethylene)thiaferracyclobutadiene compounds [_Fe(CO)3₂_μ-S(Me)C (CF3)CβCα(OR)2_] [R = Me (2a), Et (2b); RR = CH2CH2 (2c)]. Treatment of 1 with aniline and (methoxycarbonyl)hydrazine, bases of intermediate strength according to Pearson, produces α,β-substituted thiaferracyclopentadiene [_Fe(CO)3₂_μ-S(Me)C (CF3)Cβ(NHR)Cα-(NHR)_] [R = Ph (5a), NHC(O)OMe (5b)] complexes. It is suggested that these compounds form through initial nucleophilic attack at Cα to give the zwitterionic intermediate [_Fe(CO)3₂_μ-S(Me)C (CF3)CC(NRH)2_] (f). Thermally induced C-F bond activation is a feature of these reactions. When 1 reacts with thioamides containing three "hard" competitive primary and secondary amine functions and one "soft" thione function, the nucleophile first attacks at Cα through a secondary amine and, in a second step, at Cβ through the thiol function to give α,β adducts [_Fe(CO)3₂_μ-S(Me)C (CF3)CβSC(=NR2)N (R1)Cα(Cβ-Cα)] [R1 = NH2, R2 = CH3 (6a); R1 = CH3, R2 = H (6b); R1 = R2 = CH3 (6c)l possessing novel iminothiazolidine systems fused to the five-membered metallacyclic ring. Treatment of 1 with acetylhydrazine, which has three "hard" nucleophilic functions, results in the formation of the compound [_Fe(CO)3₂_μ-S(Me)C (CF3)C(H)C(10)NNC(Me)O(C(10)-O)_] (7a), in which an oxadiazole function is fused to the thiaazaferracyclohexene ring. The position of the fluorine substituent on the CαCβCγ(CF3)S(Me) ring of ferracyclopentadiene complexes has been found to be the essential factor determining the specific activation of the C-F bond. A single fluoro substituent attached to the β-carbon atom of the chain can be activated by nucleophiles to give α,β adducts [_Fe(CO)3₂_μ-S(Me)C (CF3)Cβ(X)Cα(NRR′)_] [X = OR″, R = R′ = R″ = Me (3a); X = OR″, R = R″ = Me, R″ = C(O)NMe2 (3b); X = SR″ (4)]. In contrast, complexes in which a single fluorine atom is linked to the α-carbon atom do not undergo C-F bond cleavage reactions. X-ray structures of compounds 2a, 3b, 5a, 5b, 6a and 7a are reported