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The competitive 1,5-electrocyclization versus intramolecular 1,5-proton shift in imidazolium allylides and imidazolium 2-phosphaallylides has been investigated theoretically at the DFT (B3LYP/6-311 + +G**//B3LYP/6-31G**) level. 1,5-Electrocyclization follows pericyclic mechanism and its activation barrier is lower than that for the pseudopericyclic mechanism by ∼5-6 kcal mol−1. The activation barriers for 1,5-electrocyclization of imidazolium 2-phosphaallylides are found to be smaller than those for their nonphosphorus analogues by ∼3-5 kcal mol−1. There appears to be a good correlation between the activation barrier for intramolecular 1,5-proton shift and the density of the negative charge at C8, except for the ylides having fluorine substituent at this position ( 7b and 8b). The presence of fluorine atom reduces the density of the negative charge at C8 (in 7b it becomes positively charged) and thus raises the activation barrier. The ylides 7f and 8f having CF3 group at C8, in preference to the 1,5-proton shift, follow an alternative route leading to different carbenes which is accompanied by the loss of HF. The carbenes Pr 7, 8b- e resulting from intramolecular 1,5-proton shift have a strong tendency to undergo intramolecular SN2 type reaction, the activation barrier being 7-28 kcal mol−1. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Effect of the number and positions of the methoxycarbonyl substituents in 2-phosphaindolizine on the feasibility of its Diels–Alder (DA) reaction with 1,3-butadiene has been investigated theoretically at the density functional theory (DFT) level. Among the series of four differently substituted 2-phosphaindolizines, 3-methoxycarbonyl-2-phosphaindolizine does not undergo the DA reaction due to the highest activation barrier (29.49 kcal mol-1) and endothermicity, whereas the activation barrier of the corresponding reaction of 1,3-bis(methoxycarbonyl)-2-phosphaindolizine is lowest (22.43 kcal mol-1) with exothermicity making it possible to occur. This reactivity trend is corroborated by FMO energy gaps as well as by global electrophilicity powers of the reactants. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Computational calculations have been used successfully to explain the reactivity of the >C=P- functionality in pyrido-annelated 1,3-azaphospholes. Theoretical investigation at the Density Functional Theory (DFT) level shows that the lone pair of the bridgehead nitrogen atoms is involved in extended conjugation, due to which electron density increases considerably in the five-membered azaphosphole ring. The electron density in the azaphosphole is further enhanced by the presence of an ester group at the 3-position making the >C=P- functionality electron-rich. Thus, 1,3-azaphospholo[5,1-a]pyridine, i.e., 2-phosphaindolizine having ester group at the 3-position only does not undergo Diels-Alder (DA) reaction with an electron rich diene, such as 2,3-dimethyl-1,3-butadiene (DMB). However, an ester group at 1-position acts as electron-sink, due to which transfer of the electron density to the >C=P- moiety is checked and DA reaction occurs across the >C=P- functionality. The coordination of the Lewis acid to the carbonyl group at the 3-position raises the activation barrier, while it is lowered remarkably when it is coordinated to the P atom. Furthermore, the attack of 1,3-butadiene on the Si face of the P-coordinated (o-menthoxy)aluminum dichloride complex is a lower activation energy path. Fukui functions could be used to explain relative reactivities of indolizine and 2-phosphaindolizine towards electrophilic substitution reactions. [ABSTRACT FROM AUTHOR]

Review: [36 refs. [ABSTRACT FROM AUTHOR]

Abstract: Computational calculations of the hetero-Diels–Alder (HDA) reactions of benzaldehyde with 2,3-dimethyl-1,3-butadiene (DMB) catalyzed by BF3, EtAlCl2 or TiCl4 at the density functional theory (DFT) (B3LYP/6-31G(d,p)) level reveal that the reactions involve activation energy barriers of 14.59, 14.34, and 17.44 kcal mol−1, respectively. In the first two cases, reaction occurs through a concerted mechanism, whereas in the presence of TiCl4, stepwise mechanism involving zwitterionic intermediate is followed. The reaction with isoprene proceeds with complete regioselectivity leading to the expected regioisomer having O/Me in 1:4 positions. The computational results have been verified experimentally when aromatic aldehydes react with 1,3-dienes in the presence of a catalyst to give 3,6-dihydro-2H-pyran derivatives in high yields. [Copyright &y& Elsevier]

Abstract: Theoretical calculations at the DFT (B3LYP/6-31+G**) level of the model Diels–Alder (DA) reactions of 1-methyl-3-(methoxycarbonyl)-2-phosphaindolizine with 1,3-butadiene in the presence of methylaluminium dichloride reveal that the co-ordination of organoaluminium reagent to the carbonyl oxygen increases the activation barrier compared to that for the uncomplexed 2-phosphaindolizine. On the other hand, co-ordination of the organoaluminium reagent to the σ2, λ3 P atom lowers the activation barrier by ∼6kcalmol−1. 1-Methyl-2-phosphaindolizines having an electron-withdrawing group at the 3-position only undergo DA reaction with 2,3-dimethylbutadiene in the presence of the ethylaluminium dichloride catalyst in methylene chloride at a low temperature to afford [2+4] cycloadducts. The formation of an intermediate having the ethylaluminium reagent co-ordinated to σ2, λ3 P atom has been detected by 31P NMR. The products have been characterized by 31P and 1H NMR studies. [Copyright &y& Elsevier]

Abstract: The three-components reaction of dimethyltin dibromide with imidazo[1,2-a]pyridine and allyl bromide in ethanol affords an ionic complex, bis(1-allylimidazo[1,2-a]pyridinium) dimethyltetrabromostannate(II). X-ray crystal structure analysis of the complex reveals the tin atom to be hexacoordinated, forming a dianion. Organotin(IV) halides on reacting with imidazo[1,2-a]pyridines yield 1:2 complexes. The X-ray crystal structure analysis of such a complex shows the tin atom to be hexacoordinated, with two imidazo[1,2-a]pyridine molecules coordinated through the N1 atom. Bis(imidazo[1,2-a]pyridine)dimethyltin dibromide does not show any reactivity towards allyl bromide or 1-(1-pyrrodinyl)cyclohexene, even on refluxing in ethanol. [Copyright &y& Elsevier]

The formation of some new pyridinium salts (II) (yields not given) and phosphaindolizines (III) by treatment of pyridinium bromides (I) with PCl3 is reported. [ABSTRACT FROM AUTHOR]