Bond lengths in conjugated polyenes

Some conjugated polyenes have been examined using the valence-bond approximation including σ-bond compressional energy.The appropriate exchange and coulomb integrals were obtained as functions of bond length, using spectroscopic and thermal data for benzene and ethylene.These semi-empirical functions were then used in investigating linear cyclic polyenes.It was found that the bond lengths predicted for butadiene were in excellent agreement with experiment and that there should be appreciable bond alternation in longer chains. Similarly for n > 3, it was found that bonds should alternate in the cyclic polyenes C_2nH_2n and that the degree of bond alternation tends to increase with n.This agrees qualitatively with the recent application of simple molecular-orbital theory (including σ-bond compression) to long chain and large cyclic polyenes.     

Novel Kumada coupling reaction to access cyclic (2-azaallyl)stannanes. Cycloadditions of cyclic nonstabilized 2-azaallyllithium species derived from cyclic (2-azaallyl)stannanes

A Kumada cross-coupling reaction involving organomagnesium reagents and (3-methylthio-2-azaallyl)stannanes with a Ni(0) catalyst provided cyclic nonstabilized (2-azaallyl)stannanes in moderate to good yields. Primary alkyl, aryl, and allylic organomagnesium reagents can be used as the cross-coupling partner. In general, NiCl(2)dppp in toluene at room temperature provided the shortest reaction times and most consistent yields. The azomethine ylides and 2-azaallyllithium species derived from these stannanes were shown to undergo efficient [3 + 2] cycloaddition reactions to provide azabicyclo[n.2.1]alkanes as the endo cycloadducts. These cycloadducts were found to be useful as starting materials for further elaboration into aza-bridged bicyclic natural and unnatural products of biological interest. Although cyclic 2-azaallyllithium species have been generated previously, this work reports the first generation and cycloaddition of entirely nonstabilized 2-azaallyllithium species. In addition a novel extension of the Kumada coupling was developed to allow for the preparation of the cyclic (2-azaallyl)stannanes, which are precursors to the nonstabilized 2-azaallyllithium species.     

Synthesis and photonics of polyfunctional conjugated polyenes

Methods for the synthesis of various types of polyfunctional conjugated polyenes based on aminals of unsaturated -amino aldehydes are surveyed. The influence of the interaction of chromophores on the photophysical properties of polyene bis(amino)-substituted ketones and polymethine salts based on them as well as on the properties of absorption and fluorescence spectra of the cation-anionic polymethine dyes are considered.     

Synthesis of conjugated polyenes via sequential condensation of sulfonylphosphonates and aldehydes

Selective metalation of sulfonylphosphonates results in sufficiently stable carbanions that undergo chemoselective Julia-Kocienski condensation with various aldehydes to provide (E)-allylic phosphonates in good yields and selectivities. The subsequent Horner-Wadsworth-Emmons condensation with aldehydes is used to synthesize various unsymmetrical trans-dienes, trienes, and tetraenes. This methodology is utilized for the concise synthesis of a naturally occurring fluorescent probe for membrane properties, β-parinaric acid.     

Isomer combinatorics for acyclic conjugated polyenes: enumeration and beyond

A combinatorial approach to fully conjugated acyclic polyenes (C _N H _{N +2}) is considered with a view to the extension of standard enumeration techniques to treat a widened range of chemically interesting features. As a first step, enumerations are made respecting: placement of single and double bonds, the occurrence of cis/trans isomers, and the degree (and type) of “radicality” of such conjugated networks. As a further extension, several structural (graph-theoretic) invariants averaged over various types of isomer classes and sub-classes are made, and then these invariants are utilized to estimate several physicochemical properties averaged over these same classes or sub-classes. The properties currently so considered are heats of formation, indices of refraction, and magnetic susceptibilities. Finally, the asymptotic behaviors of isomer counts and isomer properties in the many-atom limit is elaborated. Received: 10 August 1998 / Accepted: 17 November 1998 / Published online: 16 March 1999     

MCSCF study of polaron- and bipolaron-like defects in smallall-trans conjugated polyenes

Summary A Multiconfigurational Self-Consistent Field study of neutral, singly and doubly ionized C₄H₆, C₆H₈, C₈H₁₀, and C₁₀H₁₂ as model systems for polaron and bipolaron defects in polyacetylene has been carried out. In all computations a double zeta quality basis set was used. The post-Hartree-Fock computations were carried out in the complete active subspace and the fully optimized structures are reported. Comparison with SCF results reveals that the inclusion of electron correlation attenuates the double bond/single bond alternancy in conjugated polyenes as well as the charge waves associated with the cations.Dedicated to Prof. Klaus Ruedenberg     

Practical synthesis of 1,5-dimethyl substituted conjugated polyenes from geranyl acetate

A protocol to synthesize 1,5-dimethyl substituted conjugated polyenes via dehydrogenation of geranyl acetate was established. C₅ unit elongation to multi-1,5-dimethyl substituted conjugated polyenes was also achieved via Horner-Wadsworth-Emmons olefination in good yields and good selectivities.     

Cycloadditions of nonstabilized 2-azaallyllithiums with cycloheptatriene

[reaction: see text] Transmetalation of tin-bearing cyclic imidates gave 2-azaallyllithiums that underwent [pi6s + pi4s] cycloadditions with cycloheptatriene to produce tricyclic adducts, which may be useful as analogs of cocaine. The peri- and stereoselectivity of this process are discussed.     

Chemical relevance of a pure combinatorial problem: Isomers of conjugated polyenes

A correspondence is pointed out between the triangulation of convex polygons on the one hand and the enumeration of isomers of conjugated polyenes on the other.     

Approximate symmetry-breaking in the independent particle model of monocyclic completely conjugated polyenes

We examine the singlet stability of symmetry adapted, restricted Hartree-Fock (RHF) solutions and the implied symmetry breaking for various planar, π-electron systems with conjugated double bonds as described by the semiempirical Pariser-Parr-Pople Hamiltonian. In particular, we explore the energy and charge- density waves (CDWs) in various real and hypothetical structures that result by a systematic deformation of the nuclear framework: we start with a highly symmetric cyclic polyene C _N H _N having a nondegenerate ground state (N = 2n = 4ν + 2, ν = 1, 2,...), whose sites form a regular N-gon (D _Nh point group), and proceed to structures with lower symmetry (D _6h , D _3h , D _2h point groups), or with only the planar symmetry of the conjugated π-electron system (C _1h ). The objective of this study is to explore the phenomenon that could be referred to as a breaking of an approximate symmetry or an implied symmetry breaking.     

Collective electron excitations in long molecules with conjugated bonds (cyclic and linear polyenes)

Conclusions The technique used above for calculating electronic excitations is equivalent to the random phase approximation, but permits a clearer understanding of the approximations made. The linearization with respect to Ø in the derivation of the equations for the excited states means that the approximation made is valid only for small changes in the spin (or electron) density in the atoms in the excited states from that in the ground state. This is always the case for fairly small excitation energies. The proposed calculation technique may be used to calculate excitations both in long conjugated molecules and in ordinary molecules just as well as the Pariser-Parr-Pople and random phase approximations [14, 17, 18].We note that another approach was used in [6] to find the energy of the first triplet level in polyenes. In that paper the wave function of the generalized Hartree-Fock approximation was projected onto a singlet (ground) and a triplet state. The latter was treated as a very low triplet excited state. However, as shown in [1, 2], the energies of these (the singlet and triplet) states differ by a quantity that decreases like N^–2 or even faster as N. On the other hand, as shown in [7], the energy of the first triplet excitation should decrease like ₁ 1/N as N. This implies that the interaction between electrons above the generalized Hartree-Fock approximation must be taken into account in order to obtain the first triplet state.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 10, No. 6, pp. 723–731, November–December, 1974.     

Non-linear polamzabilities of conjugated chains: regular polyenes,solitons, and polarons

The hyperpolarizabilities of linear polyenes are shown to be extremely sensitive to confonnational changes and to the presence of charges along the conjugated chain. Geometries corresponding to those of regular polyenes and soliton and polaron defects are examined.     

Application of many-body perturbation theory in the localized representation for the all-trans conjugated polyenes

Ab initio self-consistent field (SCF ) calculations are performed with the standard 6-31G* basis set for all-trans conjugated oligomers C_2n+2H_2n+4. The canonical occupied and virtual molecular orbitals (MO s) are separately localized by unitary transformations. Due to the localization, the perturbation operator is partitioned into two-particle and into single-particle terms; the MBPT is, therefore, a double-perturbation expansion in this case. By using the localized representation of the MBPT , the correlation energy contributions can be partitioned into local and nonlocal effects. It can be shown that the local effects are very important and well transferable, which makes possible the calculation of the correlation energy of larger molecules if the localized molecular orbitals (occupied and virtual) of smaller related molecules are known. © 1994 John Wiley & Sons, Inc.     

Comments on the optical properties of fully conjugated polymers: Analogy between polyenes and polydiacetylenes

The similarity of the π-electron backbones of the polyenes and the polydiacetylenes suggests an analogy between these two fully conjugated, linear polymers. The extent of the analogy is discussed through the examination of a series of model compounds: the polyenes and the polyenynes. The optical band gaps and the polarizabilities of polyenynes are compared to those of the polyenes and to the predictions of a modified free-electron model. It is concluded that the chain-dependent electronic properties of the polyenes may be extrapolated with some confidence to the polydiacetylenes.     

Cycloadditions with Polar Intermediates

A discussion of the criteria that have been used to distinguish experimentally between one-step and two-step cycloadditions shows that in most cases useful information can be obtained only by isolation or interception of intermediates; the isolation of intermediates is also of preparative importance.