Pentadienyl cation analogs

Dienones, which because of the electron deficiency induced at the carbonyl carbon may be regarded as analogs of pentadienyl cations, are known to close photochemically.103 These closures occur readily in cyclic systems where the geometry requires the disrotatory mode, but the proposed intermediate product, formally a 1,3 diradical, is ordinarily stabilized through rearrangement.104 Woodward and co-workers have demonstrated the predicted photochemical... 

A interesting variation on this theme employing the isomeric enynol acetates (Scheme 24) has been developed by Rautenstrauch. The cyclizations are induced by a Pd" catalyst in warm acetonitrile. The proposed mechanism is intriguing. Reaction is initiated by an anchimerically assisted palladation to (35) followed by opening the dioxolenium ion to a pentadienylic cation (36). The closure of (36) is analogous to the silicon-directed Nazarov cyclization in the ejection of the Pd" electrofuge from (37). Both secondary and tertiary acetates can be employed as well as both acyclic and monocyclic systems. 

The cis-pentadienyl cation [C6H7Fe(CO)3] (387) and its 1-methyl-substituted analog (386) upon reaction with water undergo geometrical inversion leading to formation of the corresponding trans alcohol (84) (R = H and CHg, respectively). A mechanism involving a hydrated tt-allylic complex as intermediate has been postulated (386). Solvolysis... 

Few electrocyclic reactions leading to boron heterocycles have been reported as such some examples can be identified, however. Since it is unlikely that the formation of boroles from stannoles occurs by two simultaneous boron-tin exchanges (36) [Eq. (11)], the presence of an open-chain boron-tin intermediate can be inferred (19). A conrotatory ring-closure of the 1,3-alkadienylborane (analogous to the pentadienyl cation), followed by elimination of MeaSnClz, would yield the borole. 

For the protic acid catalyzed reaction, the cyclization commences with protonation of the divinyl ketone 9 and formation of a pentadienyl cation 10. An analogous process is operational in the case of Lewis acid-catalyzed reactions. The pentadienyl cation 10 then undergoes a 47t electrocyclic closure to give a cyclopentenylic cation 11. This cyclization is a pericyclic reaction and is governed by the rules for conservation of orbital symmetry. Namely, this means the cyclization occurs stereospecifically in a conrotatory fashion, with predictable relative configurations of the substituents (i.e., the R groups in 11 are anti). Elimination of a proton, followed by tautomerization gives product 13. ... 

The analogous cycloheptadienyl complex (5 equation 1) was similarly prepared by Dauben and Ber-telli,4 but the acyclic pentadienyl systems were a little more difficult to obtain. The triphenylmethyl cation does not remove hydride from tricarbonyl(frans-pentadiene)iron (6 equation 2). The corresponding c/s-pentadiene complex (7 equation 3) cannot be prepared directly from the diene and an iron carbonyl,... 

Whereas NMR spectroscopy is generally applied to the diamagnetic allyl, pentadienyl, or cyclopentadienyl cations or anions, this technique is generally not applicable to the analogous radical species, although CIDNP (chemically induced dynamic nuclear polarization) has been observed for allyl140 and pentadienyl radical species139. More useful for the radicals, naturally, is EPR spectroscopy. 

The alkenyliron cations 1-4 represent an optimum series of electrophiles for the study of nucleophilic interactions as ion pairs since two distinctive processes are clearly delineated, despite minimal changes in reactant structures. The pentadienyl and hexadienyl derivatives 1 and 2 cleanly yield the nucleophilic adducts (5), whereas the cyclic analogs 3 and 4 yield only the homodimers (6) according to the stoichiometries in Eqs. (48) and (49), respectively. Indeed, these processes occur concurrently in the case of (hexadienyl )Fe(CO)3+ which in combination with CpMo(CO)3 produces both the nucleophilic adduct 5b in 75% yield together with the homodimers in 25% yields. It is important to emphasize that 5b is not an intermediate in the formation of 6d and 7 since its rate of decomposition [Eq. (52)] is much too slow to account for the amounts of homodimers observed. 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

The first phenolic derivative of ferrocene to be reported, di(l-hy-droxy-3-methyl-cyclopentadienyl) iron, was prepared by the reaction between 3-methyl-2-cyclopentanone, sodamide, and ferrous chloride in liquid ammonia. This compound is remarkably unstable, being extremely sensitive to air. It is a yellow substance which sublimes at 130-140° at 0.1 mm pressure, and is readily soluble in methylene dichloride but insoluble in carbon tetrachloride and in n-heptane. In water saturated with nitrogen a yellow solution is produced which rapidly bec( nes blue-green on exposure to air. The compound forms a benzoate (mp 121-123.5°) which is stable in air. An analogous derivative of the dicyclo-pentadienyl cobalt (III) cation has also been obtained (6). 

As discussed earlier (Section 6.3.4), acyclic (pentadienyl) iron cations are intermediates in reactions involving protonation or Lewis acid activation of iron dienol or dienol acetate complexes. With appropriate functionalization, these cations may also be trapped intramolecularly (Scheme 81). In the case of the formation of tetrahydrofurans (278) and (279), equilibration of the transoid and cisoid cations is observed. However, this equilibration is not observed in the stereoselective cyclization to provide (racemic) oxocene (280). Analogous intramolecular cyclizations of sulfur and nitrogen... 

The cation t V(CO) Arene] is electronically analogous to the cyclo-pentadienyl complex C/(CO) C iy (3). The inftared data suggest that they are also structurally similar. In addition to that, the infrared measurements... 

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