Synthesis, Structure and Reactivity of Trimethylsilyl‐Substituted Phosphametallocenes

Abstract

Keywords: Metallocenes / P ligands / Silicon / Phosphaferrocenes / Conformation analysis New sterically crowded phosphaferrocene and -ruthenocene derivatives were synthesized via the 2,5-bis(trimethylsilyl)-phospholide anion 5. Their reactivity with regard to the exchange of the Me 3 Si groups for other functional groups was examined. X-ray structures were determined for the mono-(6) and the 1,1Ј-diphosphaferrocene (8). The activation barrier for ring-ring rotation in 8 was determined by NMR spec-

Double Friedel–Crafts Acylation Reactions on the Same Ring of a Metallocene: Synthesis of a 2,5‐Diacetylphospharuthenocene

“…Recently, both Ganter et al [48] and Mathey et al [79] have drawn attention to the particular importance of 2,5-diacyl-or 2,5-diester-substituted phosphametallocenes as potential precursors for phosphametallocene analogues of porphyrins and/or related macrocycles. Matano, Imahori et als exquisite studies [81][82][83][84][85][86][87][88][89][90][91][92][93][94][95] of compounds such as 34 to 36 (Scheme 14), which are founded upon the chemistry of 2,5-difunctionalised phospholes, give an indication of their possible use-and to the extreme scarcity of derivatives of this kind.…”

“…For example, acyl groups are readily installed in the 2-position of phosphaferrocenes and -ruthenocenes, [40,48] but electrophiles such as boron tribromide, [110,111] benzyl bromide, [19] and methyl iodide [19] react at phosphorus, often with decomposition of the complex (Scheme 15). For the acylation, the unsaturated P-bound carbon of the P-acyl intermediate should migrate easily to the 2-position because of a combination of low PÀ CA C H T U N G T R E N N U N G (acyl) bond energy [112] and a low energetic barrier to tetracoordination of the carbon atom that undergoes the shift; [113] conversely, the P-alkylated derivatives have a strong P À C sp 3 bond, and the alkyl group migration requires an energetically prohibitive pentacoordinate carbon atom in Scheme 14.…”

“…Doubly functionalised phosphametallocene complexes have been recognised as interesting synthetic targets for some time. Recently, both Ganter et al 48. and Mathey et al 79.…”

“…In this light, it is interesting to note that an intimate involvement of the phosphorus atom in the reaction with an incoming electrophile is consistent with the striking contrast between the ease of phosphametallocene acylation and formylation reactions and the absence from the literature of the analogous Friedel–Crafts alkylations. For example, acyl groups are readily installed in the 2‐position of phosphaferrocenes and ‐ruthenocenes,40, 48 but electrophiles such as boron tribromide,110, 111 benzyl bromide,19 and methyl iodide19 react at phosphorus, often with decomposition of the complex (Scheme ). For the acylation, the unsaturated P‐bound carbon of the P‐acyl intermediate should migrate easily to the 2‐position because of a combination of low PC(acyl) bond energy112 and a low energetic barrier to tetracoordination of the carbon atom that undergoes the shift;113 conversely, the P‐alkylated derivatives have a strong PC sp 3 bond, and the alkyl group migration requires an energetically prohibitive pentacoordinate carbon atom in any transition state that leads to the 2‐alkylated derivative.…”

“…Considered globally, the data above might be taken to imply a parallel and quite classical relationship between cyclopentadienyl and phospholyl complexes. However, relatively recent demonstrations that a range38 of late‐transition‐metal phosphametallocenes can be built around metals other than iron (M=Ru ( 16 );39–48 Co + , Rh + , Ir + ( 17 );49–51 Co ( 18 );52, 53 Ni + ( 19 );54 Ni ( 20 );54 Scheme ) have allowed reactivity studies to be pushed further and two unusual results, obtained with phospharuthenocenes, imply a more complex relationship. In an early paper,40 we showed that the phospharuthenocene complex 22 shows an activity towards trifluoroacetyl electrophiles that is comparable with the homologous phosphaferrocene 21 (Scheme ) 27.…”