Fragments CpFe

I have reviewed, in Chapter No. 12, the activation of arenes by the strongly electron-withdrawing 12-electron fragment CpFe+, isolobal to Cr(CO)3 and Mn(CO)3+, and its application to the synthesis of dendritic cores, dendrons, dendrimers, and metallodendrimers, including molecular batteries. 

Scheme 22. Heterolytic C-O cleavage reaction in aryl ether complexes by tBuOK or KOH, induced by the activating 12-electron fragment CpFe+. This reaction is very useful and has been applied to the convenient one-pot synthesis of the pheno l-tri al lyl dendron (see Scheme 23).

Recently the first examples of complexes between the four-membered amidinato-Group 13 metal(l) heterocycles and transition metal fragments were reported. Complexes of the type CpFe(CO)2[M(X) But(NR)2 ] (M = Al, Ga, In X = Cl, Br R = Pri, Gy) were formed in salt-elimination reactions between Na[CpFe(CO)2] and [But(NR)2]MX2. A series of complexes between the four-membered amidinato-Group 13 metal(l) heterocycles and Group 10 metal(O) fragments have been prepared according to Scheme 35. ... 

Fig. 28. Orientation of orbital with ir-symmetry on CpFe(CO)2 fragment and the orientation of these groups in CpFe(CO)2 3Ga.

Flash photolysis has now been applied to a wide range of metal carbonyl species in solution, including Mn2(CO)10 (37), [CpFe(CO)2]2 (38), and [CpMo(CO)3]2 (39). In almost every case, interesting data have emerged, but, as with Cr(CO)5, the structural information is usually minimal. Thus, the radical Mn(CO)5 has been generated in solution by flash photolysis (37), the rate constant for its bimolecular recombination has been measured, but the experiments did not show whether it had Z>3h or Qv symmetry. Some experiments have been unsuccessful. Although the fragment Fe(CO)4 is well known in matrices (15), it has never been... 

The X-ray diffraction analysis of the major isomer revealed that the CO anti to the CpFe fragment in complex 31 had been replaced by the bulky PPhs ligand (Fig. 1.5.7). 

The nitrene transfer from PhI=NTos to alkenes catalyzed by the CpFe(CO)2+ fragment gave better results (85% for styrene) [25], but the characteristics of the chemistry of the cationic intermediates as postulated by the reaction mechanism are closely connected to the alternative formation of aziridines by a carbene transfer... 

The aziridination of imines catalyzed by the CpFe(CO)2+ fragment by carbene transfer is a most remarkable reaction [26], The imine is consumed first to form both the cis and trans products. The catalyst then coordinates predominantly to the trans product and leads to its decomposition, leaving the cis product untouched [27]. 

This diastereoselective decomposition can be rationalized by the steric hindrance in the cis products as the coordination of the CpFe(CO)2+ fragment is prohibited by the relative orientation of the substituents (Scheme 9.13). Consequently, the rate of decomposition is enhanced when the substituents stabilize cationic ring-opened intermediates, such as in the methoxy derivative 11 (0% yield). On the other hand, the rate of decomposition of the trans products is reduced, based on the destabilization of cationic intermediates when an electron-withdrawing substituent, such as in the nitro derivative 13, is used (78%, cis trans = 4 1). 

The formation of epoxides is a well-investigated synthetic problem and two approaches, either from a double bond system by transfer of oxygen starting from an alkene, or carbene transfer to a carbonyl group, have attracted much interest. The use of the CpFe(CO)2+ fragment was also investigated by Hossain and coworkers with a view to its use for the synthesis of epoxides (Scheme 9.15) [30, 31]. However, the CpFe(CO)2+ fragment not only catalyzes the carbene transfer, but also acts as... 

In a long-term research project, Hossain and coworkers investigated the usefulness of the CpFe(CO)2+ fragment [35-38] in the cydopropanation reaction of alkenes by a carbene transfer utilizing diazo esters as the carbene source (Scheme 9.17). The cydopropanation products of styrene derivatives could be obtained in good yields of up to 80% and excellent cis selectivity by using an excess of the alkene, whereas the cydopropanation of aliphatic alkenes was less effective, yielding the desired cyclopropane derivative in up to 51% yield. 

Scheme 9.16 Cydopropanation by a carbene transfer from a CpFe(CO)2 fragment.

The modification of the CpFe(L)2+ fragment in 39 with chiral phosphorus donor ligands based on chiral 1,2-dioles was investigated by Kxindig and coworkers and tested in the reaction of acrolein derivatives (Scheme 9.28) [72-74],... 

Cycloadditions can also be initiated by the CpFe(CO)2+ fragment, as demonstrated by Rosenblum s group in an early report where a CpFe (CO)2-CH2CH=CH2 complex reacts as the C3-component with the CpFe(CO)2+ adduct 49 of cyclohexenone to give the bicyclic product SO (Scheme 9.36) in very moderate yield of 10% [94],... 

Brookhart and co-workers found that the cationic a-silane complex [CpFe(CO)(PR3)(HSiEt3)]+ was observable by NMR at RT but only in the presence of excess silane (sacrificial removal of trace H20 as Et3SiOH) and could not be isolated as a solid (122). The [CpFe(CO)(PR3)] fragment catalyzed silane alcoholysis in the presence of the BArF counterion (123). Although rapid deactivation of the catalyst occurred with ethanol as substrate, phenol reacted continuously with turnover numbers up to 80 min-1. It was proposed (Scheme 9)... 

The use of nitro arenes as the nitrogen fragment donor in combination with CO under catalytic, and reductive, conditions has also been presented by Nicholas et al. who have found that [CpFe(CO)2]2 can be used as the catalyst [69]. A variety of different alkenes were tested, and a-methyl styrene gave the highest yield of the allyl amine 113 when nitrobenzene was used. The yield of the allyl amine depends markedly on the structure of both the alkene and the nitro compounds. 

Consistent with our analyses above, CpMn(CO)3 is a known 18-electron compound. The 17-electron CpMo(CO)3 fragment is found as a dimer with an M-M single bond (Figure 4.8). In order to be isolobal with CH3, the latter must use one t2g orbital plus one electron in bonding. Seventeen-electron CpFe(CO)2, with an unused, filled t2g set, is also isolobal with CH3 Further, 15-electron CpMo(CO)2 behaves as if it were isolobal with CH as it is found as a dimer with a short Mo-Mo bond attributed to a Mo=Mo triple bond. Hence, two of the t2g orbitals and three electrons must be used in the bonding. The Fe analog, 15-electron CpFe(CO) also forms a multiply bonded dimer which is isolated at low temperatures by photolysis of [CpFe(CO)2]2 in an inert matrix. It now must utilize one orbital from the t2g set to be viewed as isolobal with CH. 

The more appropiate axis choice for CpM(CO)2 is one in which the 2-axis is along the future Fe-thiophene bond. The mirror plane of this complex lies in theyz plane. The [CpFe(CO)2l fragment of 67, point group symmetry has at low energy three occupied metal-based orbitals (see left side of Figure 2) 17a (2, 3% yz, 18% —y, 52%), 18a ... 

Although SO2 insertion is a clean reaction for many metal-carbon complexes, some metal alkyls, e.g., PhCH2Cr(H20)s (4), decompose rapidly to unidentified materials upon treatment with sulfur dioxide. At the other end of the reactivity scale, a number of metal-carbon a-bonded compounds are inert to SO2. These include, in particular, perfluoro-alkyls and -aryls such as CF3Mn(CO)s (66) and CpFe(CO)2CeF5 (13), as well as other complexes with electron-withdrawing substituents in the o-bonded carbon fragment (71). 

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