Pyrrolizine anion

Typical NMR data were compiled in CHEC-II(1996) <1996CHEC-II(8)2> for 3.77-pyrrolizine 1, its lithium salt, the pyrrolizin-3-one 2, and its regioisomer (pyrrolizin-2-one). More recently, Kissounko et al. <1998JOM(556)145> reported the H chemical shifts of the parent pyrrolizine anion and anion 21a as well as those of their silylated or stannylated derivatives 22-26. 

Influences of substituents on the positions of isomeric equilibria that have been calculated116 by the HMO route do not agree with experimental findings (see Section III,B,1). A correlation of the acidity of 3H-pyrrolizine(l)with the difference in HMO delocalization energies between the pyrrolizine anion and the neutral molecule has been attempted but showed limited success.115... 

The electronic structure of the pyrrolizine anion has been investigated by the HMO-,115 the ASMO-SCF-MI-,117 and a CNDO/2-SCF semiempirical method of Pople-Santry-Segal, taking into account all valence electrons... 

From HMO115 as well as SCF calculations117 an electrophilic attack should occur at the 3-position of the pyrrolizine anion, a conclusion that fits experimental findings (see Section III,B,4). 

The relatively high of 29 for deprotonation of 3H-pyrrolizine (cf. indene pK 18.5) indicates that formation of the 1 Oil-electron pyrrolizine anion adds only minor stabilisation relative to the simple pyrrole originally present. Its reactions are those of a highly reactive carbanion, for example benzophenone condenses to generate a fulvene-like product. ... 

Proton NMR spectra of 3H-pyrrolizines, IH-pyrrolizin-l-ones and 3H-pyrrolizin-3-ones have been compiled from the literature <84AHC(37)i). Chemical shifts and coupling constants of 3H-pyrrolizine (1) <68TLll8l), the pyrrolizine anion and the pyrrolizinones (2) <71CB2170) and (3) <82CB706) are grouped in Table 2. The H-NMR spectra of the pyrrolizine anion with various... 

Okamura and Katz have determined the pAl of 3H-pyrrolizine (1) measuring the rate of exchange of the protons with 5 M D2O in DMF containing 1 M triethylamine <67X2941 >. The value of pK = 29, which was attributed to the aromaticity of the pyrrolizine anion, however, seems surprisingly high compared with those of indene (pK = 18.2), cyclopentadiene (pK = 15) and fluorene (pAla = 22.8). 

Pyrrolo[l, 2 3,4]pyrimido[2,l,6-crf]pyrrolizines (182), as well as pyrrolizines (183) and (184), have been obtained from a one-pot reaction of the anion of pyrrole-2-aldehyde with l,2-bis(hetero) substituted ethylenes (181) in a route given in Scheme 24 (80CB614,81UP30801). 

Di- and polysubstituted pyrrolizines are uncommon. The only pyrrolizine isolated from the intramolecular aldol condensation of 2-acetyl-l-(butan-3-on-l-yl)pyrrole was the 6,7-disubstituted compound 223.122 A small amount of the 5,6-disubstituted pyrrolizine (145d) was obtained when the 2-formylpyr-ryl anion was condensed with l,2-di(phenylsulfonyl)ethene.90 By contrast with the production of the single isomer (223), the homologous diketone 224... 

Not surprisingly, the anion derived from 3//-pyrrolizine is a stronger nucleophile than compound 1 itself. Thus the anion reacts with deuterium oxide to give 31//,32//-pyrrolizine, but attempts at further enrichment give mixed deuteriopyrrolizines.83,115 The anion reacts with ketones to give azafulvenes such as 248 and 268.43,115 It seems possible that the reported formation of the oxime of 3//-pyrrolizin-3-one (269) from 3//-pyrrolizine and... 

The anion-radical of cycl[3.2.2]azine (pyrido[2,l,6-ct/]pyrrolizine) (181) has been known many years.598,599 The observed nitrogen splitting is very... 

Because tin-lifhium exchange and anionic cyclization are known to occur with retention of configuration at the metal-bearing carbon center [126], enantiomerically enriched organostannanes can serve as promising precursors for access to chiral heterocycles. On fhe basis of fhis idea Coldham prepared fhe optically active hexahydro-IH-pyrrolizine alkaloid (-i-)-pseudohehotridane with complete diastereoselectivity and enantiospecificity (Scheme 12.56) [127]. In sharp contrast with this success, six-membered ring formation by the same strategy resulted in a diastereomeric mixture of racemates (Scheme 12.57) [128]. 

Azapentalenyl anions,461 e.g., 237 and the 4-azapentalenyl anion 238,47 also show signals in the NMR spectrum consistent with extensive de-localization of the negative charge. The latter further exhibits a UY spectrum very similar to that of pentalenyl dianion. An attempt was made to relate the estimated pKa, 29, of 3/f-pyrrolizine (239) with the difference in HMO delocalization energies of the 7r-electrons in the anion and neutral compounds, but with only a limited degree of success.47... 

There is surprizingly little difference in the electronic absorption in 3H-pyrrolizine (1) and its anion <6712941 >. 

In a reaction of 3H-pyrrolizine (1) with a lithium dicarbenoid species described in Section 8.01.5.4, anion (63) was assumed to be an intermediate which formed the butenyne (64) by ring opening <79HCA540>. A similar ring-opening reaction has been described <82JCR(S)54> ... 

Deprotonation of the pyrrole N—H and addition of the anion to the alkenyl phosphonium salt gives an intermediate phosphonium ylide. This ylide undergoes intramolecular Wittig reaction to give the pyrrolizine 8. See E. E. Schweizer and K. K. Light, J. Am. Chem. Soc., 86 (1964), 2963. 

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