Pyrrole, complex

Azacyclopentadienyl complexes may be obtained by the reaction of the anion of pyrrole (sodium or potassium salt) with transition metal compounds [equations (9.101) and (9.102)]. 

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NH2)50s(2,3-T -L)], where L = furan, pyrrole, and thiophene. Although neither the furan nor thiophene complexes react with maleic anhydride over a period of 10 days, the pyrrole complex (15) reacts rapidly at room temperature and 101.3 kPa to form a mixture of endo (17) and exo (16) complexes. An a2omethine ylide intermediate was postulated as the key intermediate through which maleic anhydride added to the 2- and 5-positions of the coordinated pyrrole ring. 

Me, R = R = H) [64JOM(l)471 68JOM(14)405 70JINC441 84JA1646 86JCS(F2)1543 90JOM(388)175]. During the formation of 20, it is possible to anticipate the existence of an intermediate a-pyrrolate complex 21, which then... 

Di-rerr-butylsodium pyrrolate serves as a source of the complexes of lanthanides [93CB2657 95JOM(495)C12]. Thus, with cyclooctadienyl chlorides of samarium, thulium, and lutetium, it affords species 89 [96JOM(507)287]. The N-coordinated samarium(II) calix-pyrrole complex is known [99AG(E)1432]. 

Figure 34 The structure of the pyrrolate complex CpMg(Et20)(rj2-Me2NCH2(C4H2N)CH2NMe2) 63.

Pentaamineosmium(ll) 4,5 -coordinated pyrrole complexes have been reacted with methylacetonitrilium triflate in the presence of an excess of proton sponge to afford the corresponding iminium salt of 2-[ 1 -(iV-methylirnino)ethyl]-pyrrole-Os complexes 181 (Scheme 80).371... 

Pyrrole complexes with group VlIB metals, 12 242... 

The synthesis of r -bound pyrrole complexes has recently been reported and studies have shown remarkable differences between the reactivity of... 

Reaction of the pyrrole complex 341 with acrylate gives 343 via 342, and indole ring 344 is formed by oxidation with DDQ [84], Similarly, the aniline complex 345 reacts with acrylate to afford 347 via 346 [85]. 

Figure 5. Synthesis of rj -pyrrole complexes and ORTEP drawing of [Os(NH3)5(4/5-ri2-2,5-dimethylpyrrole)] (OTf)2 (22).

Table 1. Yields and Substitution Half-Lives for ri2-Pyrrole Complexes...

For all of the pyrrole complexes studied, the metal is bound predominately across C-4-C-5.16 Isomerization of the metal to the 2,3-position occurs rapidly at room temperature for symmetric pyrrole complexes, and involves the formation of an intermediate azomethine ylide isomer in which the metal is bound across C-3-C-4. The rate of this isomerization depends strongly on the substituents on the pyrrole ring. For example, for the 1-methylpyrrole complex 21, the specific rate of isomerization is approximately 100 s-1 at 20 °C (see Table 5).14 In the case of the 2,5-dimethylpyrrole complex 22, the steric interaction between the methyl substituents and the metal destabilizes the 4,5- and 2,3-isomers, and the isomerization rate is increased (k > 300 s-1). 

A. Protonation and Isomerization Unsubstituted and a-Substituted Pyrrole Complexes... 

Table 2. pKa Values for Selected tj2-Pyrrole Complexes and Pyrrolium Ions11... 

Figure 8. Protonation and isomerization of P-substituted ri -pyrrole complexes.

The 1-methylpyrrole complex 21 undergoes clean acylation at the P-position in the presence of Ac20/DMAP to give the 3-acetyl-1-methylpyrrole complex, 38 (Figure 10).12b Propionic anhydride reacts with 21 in a similar manner to form 39. Under these reaction conditions, the parent pyrrole complex (20) undergoes exclusive A-acylation to give 40, and the 2,5-dimethylpyrrole complex (22) gives a 7 3 ratio of N- to P-acylation. 

Treatment of either the pyrrole (20) or 1-methylpyrrole (21) complex with methylacetonitrilium triflate gives the iminium-substituted 1//-pyrrole complexes 41 and 42, respectively (Figure 10).llb,12b These complexes are surprisingly resistant to deprotonation with Proton Sponge ... 

Figure 11. Lewis Acid promoted aldol reactions of rf-pyrrole complexes.

Electrophilic additions with activated alkynes also occur readily with ri2-pyrrole complexes.12bl3b In methanol solution, the pyrrole (20) and 1-methylpyrrole (21) complexes undergo conjugate addition cleanly at C-3 with 3-butyn-2-one to give the p-enone-substituted pyrrole complexes 56 and 58 in high yield (Figure 13). In contrast to what is observed... 

Figure 12. Alkylation and Michael addition of ri2-pyrrole complexes.

Electrophilic addition of r -pyrrole complexes with activated... 

In contrast to MVK or 3-butyn-2-one, the reaction of methyl acrylate with all of the pyrrole complexes studied requires activation of the... 

The preceding sections describe regioselective electrophilic addition of pyrrole complexes at the 3-position with various electrophiles to give ( -substituted lZ/-pyrrole or 3//-pyrrolium isomers. The latter compounds, in contrast to their noncomplexed counterparts, are only moderately acidic (pKa 6) and therefore resist rearomatization and multiple alkylations. For example, when uncomplexed 2,5-dimethylpyrrole is treated with 1 equiv of methyl acrylate and TBSOTf, a statistical 1 2 1 ratio of starting material, monoalkylated 105 (vide infra), and 3,4-bis-alkylated product is found.12 Treatment of 1-methylpyrrole under the same conditions results in at least four alkylated products along with starting material. In contrast, coordination by osmium results in smooth... 

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