Understanding conjugation and hyperconjugation from electronic delocalization measures

The concepts of conjugation and hyperconjugation play an important role to provide an explanation for several fundamental phenomena observed in organic chemistry. Because these effects cannot be directly measured experimentally, their assessment became a primary concern for chemists from the very beginning. In general, the stabilization produced by both phenomena has been studied by means of isodesmic reactions and energy based analysis such as the energy decomposition analysis. In recent years, electronic delocalization measures have been successfully applied to elucidate the nature of chemical bonding and the aromatic character of all kind of molecules. Because conjugation and hyperconjugation stabilizations are strongly linked to the concept of electron delocalization, this paper will give an account of both effects from the point of view of electronic delocalization measures calculated within the framework of the quantum theory of atoms in molecules. In particular, we focus our attention in the controversial case of the stabilization by conjugation in 1,3-butadiyne and 1,3-butadiene. Unexpectedly, theoretical calculations based on the scheme proposed by Kistiakowsky to quantify the extent of stabilization due to conjugation predicted that the conjugation of 1,3-butadiyne was zero. Subsequent energetic analyses contradicted this observation. These studies pointed out the presence of hyperconjugation stabilization in the hydrogenated product of 1,3-butadiyne and 1,3-butadiene that were used as reference systems in the Kistiakowsky's scheme. Consequently, the extra stabilization of 1-butyne due to hyperconjugation hides the stabilization by conjugation of 1,3-butadiyne. Our results based on electron delocalization measures confirm both the presence of conjugation in 1,3-butadiene and 1,3-butadiyne and hyperconjugation stabilization in their respective hydrogenated products, 1-butene and 1-butyne.     

Practical enantioselective synthesis of axially chiral biaryl diphosphonates and dicarboxylates by cationic rhodium(I)/Segphos-catalyzed double [2 + 2 + 2] cycloaddition

A cationic rhodium(I)/Segphos complex catalyzes a [2 + 2 + 2] cycloaddition of internal 1,6-diynes with a phosphonate- or ester-substituted 1,3-butadiyne leading to C(2)-symmetric axially chiral biaryl diphosphonates or dicarboxylates, respectively, in high yields with outstanding ee's. The use of a phosphonate- or ester-substituted 1,3-butadiyne as a cycloaddition partner and Segphos as a ligand is crucial for the success of this transformation.     

The conjugation stabilization of 1,3-butadiyne is zero

[reaction: see text] In contrast to 1,3-butadiene, the textbook example of "conjugation stabilization", G3(MP2) calculations yielding the enthalpy of hydrogenation Delta(hyd)H(298) of 1,3-butadiyne indicate that it is not stabilized by the conjugated configuration of its triple bonds. Differences between ethylenic and acetylenic pi bonds are examined in the light of CAS-MCSCF calculations on 1,3-butadiene and 1,3-butadiyne.     

Formation and decomposition of distonic o-, m-, and p-benzyne radical cations from photolysis of Mg(+)(o-, m-, p-C(6)H(4)F(2))

Distonic o-, m-, and p-benzyne radical cations (1-3) have been generated by a novel photolysis reaction of mass-selected Mg(+)-difluorobenzene complexes. The energy required for the formation of these radical cations is within 2.2 eV. The formation of o-benzyne cation is most facile. The benzyne radical cations dissociate further to yield ethyne and 1,3-butadiyne radical cation as major products given a sufficient amount of energy. The whole process involves only a single photon, and is very efficient. The calculated threshold for the formation of 1,3-butadiyne radical cation from Mg(+)(o-C(6)H(4)F(2)) is about 4.6 eV, quite comparable with the experimental estimate.     

新型紫外线吸收剂1,4-二芳基-1,3-丁二炔类化合物的合成及其性能研究

本文以含有端基炔的芳硫醚为原料，碘苯二乙酯为氧化剂，氯化亚铜为催化剂，室温下反应30 min，合成了1，4-二（2-芳硫基苯基）-1，3-丁二炔化合物。通过¹HNMR、¹³CNMR等对其进行了结构表征，并研究了其光学性质，以及作为紫外线吸收剂在紫外光固化涂料中的应用。结果表明：该类化合物能够有效吸收270~360 nm的紫外光，且光降解程度小，并具有与树脂、单体相容性好、无气味等特点。同时，合成的目标化合物在光固化过程中不参与自由基体系的固化，且增强了涂料及固化涂层的光稳定性，有益于紫外光固化涂料的存储，是一类具有发展前景的紫外光吸收剂。     

Synthesis of the end-capped 5-(N,N-dimethylamino)naphthyl-1-ethynyl derivatives and their 1,4-di[5-(N,N-dimethylamino)naphthyl]-1,3-butadiynes: anti rotamer structure

A new family of the end-capped 5-(N,N-dimethylamino)naphthylethynyl chains were synthesized, as terminal acetylenes or poly(yne) structures, by heterocoupling between 5-iodo-N,N-dimethylnaphthalen-1-amine and 2-methyl-3-butyn-2-ol or 4-(5-iodo-1-naphthyl)-2-methyl-3-butyn-2-ol, catalyzed by the palladium-copper system. Catalytic homocoupling of the terminal acetylenes, affords to 1,4-dinaphthyl-1,3-butadiyne nanostructures. X-ray diffraction analysis of 1,4-di(α-naphthyl)-1,3-butadiyne shows that the naphthalene rings are in the anti configuration along the acetylene axis. All the conjugated compounds show an important fluorescent emission radiation.     

Cyclic tetra- and hexaynes containing 1,4-donor-substituted butadiyne units: synthesis and supramolecular organization

Cyclic bis(1,3-butadiynes) with sulfur centers placed in the alpha-position to the 1,3-butadiyne units (2(n)) were synthesized by Glaser coupling of the corresponding open chain dithia-alpha,omega-diynes 1(n). In a second protocol we applied a four-component cyclization by reacting alpha,omega-dithiocyanatoalkanes 6(n) or alpha,omega-diselenocyanatoalkanes 7(n) with dilithium-1,3-butadiynide. This concept afforded either the cyclic dimers (S, 2(n); Se, 9(n)) or the cyclic trimers (S, 8(n); Se, 10(n)). Most of the molecular structures of 2(n) and 9(n) adopt chairlike conformations in the solid state. Tubular structures in the solid state with short distances between the chalcogen centers of neighboring stacks were encountered for 2(5), 9(5), 8(4), 10(4), and 10(5). Recrystallization of 10(5) from various polar and nonpolar solvents yielded inclusion of the solvent guest molecules. The solvent-accessible volume was calculated to vary from 19% (n-hexane) to 25% (mesitylene). The elastic properties of our cycles are due to the flexible methylene chains and the easily variable torsional angles between the rigid 1,3-butadiyne rods.     

Perthio- and perseleno-1,3-butadienes, -but-1-ene-3-ynes,and -[3]-cumulenes: one-step syntheses from 1,4-dilithio-1,3-butadiyne

[reaction: see text] Treatment of 1,4-dilithio-1,3-butadiyne (1) with dichalcogenides RSSR or RSeSeR affords dithio- and diseleno-1,3-butadiynes (2, 3), perthio- and perseleno-[3]-cumulenes (4, 5), perthio- and perseleno-1,3-butadienes (6, 7), and/or perthio- and perseleno-but-1-ene-3-ynes (8, 9). The products can be controlled by stoichiometry and temperature, by the presence or absence of oxygen, and by choice of the "R" group. By X-ray crystallography, hexa(methylthio)-1,3-butadiene is highly twisted, with a torsion angle [Phi(CCCC)] of 84.7 degrees and an elongated C(2)-C(3) distance of 1.484(3) A.     

Regio- and stereoselective hydrohalogenation of ynamide components in 1,3-butadiynes with in situ generated HX

A simple protocol for regio-, and stereoselective hydrohalogenation of ynamide moieties in 1,3-butadiyne structures is described. The facile approach enables exact syn-addition of HX to the diyne components, giving just single isomer of the corresponding dienes and enynes.     

How large is the conjugative stabilization of diynes?

The conjugation stabilization energies of dienes and diynes are considerably larger than estimates based on heat of hydrogenation differences between 1,3-butadiyne and 1-butyne as well as between 1,3-butadiene and 1-butene. Such comparisons do not take into account the counterbalancing hyperconjugative stabilization of the partially hydrogenated products by their ethyl groups. When alkyl hyperconjugation is considered, the conjugation stabilization of diynes ( approximately 9.3 kcal/mol) is found by two methods (involving isomerization of nonconjugated into conjugated isomers and heats of hydrogenation) to be larger than that of dienes ( approximately 8.2 kcal/mol).     

A general procedure for the synthesis of alkyl- and arylethynyl-1,2,3-triazole-fused dihydroisoquinolines

A general procedure for the synthesis of the title compounds has been devised starting from the available 2-halophenylethyl azides, by means of click reactions with trimethylsilylacetylene or 1-trimethylsilyl-1,3-butadiyne followed by a transition metal-catalyzed functionalization of C-H bond. A further extension of this procedure led us to devise the synthesis of more complex 4,4'-bitriazole-fused dihydroisoquinolines.     

Synthesis of polyacetylenic montiporic acids by means of organosilicon compounds

A straightforward synthesis of polyacetylenic montiporic acids A and B has been developed, based upon the selective and sequential substitution of the two trimethylsilyl groups of the readily available 1,4-bis(trimethylsilyl)-1,3-butadiyne.     

Stereoselective total synthesis of (S)-Virol C and (S)-1-dehydroxyvirol A

A stereoselective total synthesis of (S)-Virol C and (S)-1-dehydroxyvirol A has been developed, based upon the selective and sequential substitution of the two trimethylsilyl groups of readily available 1,4-bis(trimethylsilyl)-1,3-butadiyne.     

Functional benzoxazines containing ammonium salt of carboxylic acid: Synthesis and highly activated thermally induced ring‐opening polymerization

1,3‐Benzoxazine monomers having ammonium salt of carboxylic acid have been developed. These 1,3‐benzoxazines 1a and 1b were easily synthesized from the corresponding tetrabutylammonium salts of glycine and β‐alanine, respectively. The glycine‐derived benzoxazine 1a exhibited remarkably high reactivity, which allowed its thermally induced ring‐opening polymerization in bulk at 100 °C, at which N‐methyl‐1,3‐benzoxazine 1d did not undergo the polymerization at all. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Hyperbranched carbosilane oxazoline-macromonomers: polymerization and coupling to a trimesic acid core

Attachment of oxazoline-based hyperbranched macromonomers to a trimesic acid core affording trimers of the hyperbranched fragments as well as polymerization of the oxazoline groups is reported. Polymerization and copolymerization with 2-phenyl-1,3-oxazoline were investigated for two macromonomers (M _w = 1700 and 8 000, respectively), resulting in hyperbranched analogs of combburst polymers. Homopolymerization of the macromonomers yielded a degree of polymerization DP ≈ 15. Hyperbranched trimers and the polyethyleneimines with apolar hyperbranched carbosilane side-chains form superstructures in solution and in bulk.     

Structural and co-conformational effects of alkyne-derived subunits in charged donor-acceptor [2]catenanes

Four donor-acceptor [2]catenanes with cyclobis(paraquat-p-phenylene) (CBPQT4+) as the pi-electron-accepting cyclophane and 1,5-dioxynaphthalene (DNP)-containing macrocyclic polyethers as pi-electron donor rings have been synthesized under mild conditions, employing Cu+-catalyzed Huisgen 1,3-dipolar cycloaddition and Cu2+-mediated Eglinton coupling in the final steps of their syntheses. Oligoether chains carrying terminal alkynes or azides were used as the key structural features in template-directed cyclizations of [2]pseudorotaxanes to give the [2]catenanes. Both reactions proceed well with precursors of appropriate oligoether chain lengths but fail when there are only three oxygen atoms in the oligoether chains between the DNP units and the reactive functional groups. The solid-state structures of the donor-acceptor [2]catenanes confirm their mechanically interlocked nature, stabilized by [pi...pi], [C-H...pi], and [C-H...Omicron] interactions, and point to secondary noncovalent contacts between 1,3-butadiyne and 1,2,3-triazole subunits and one of the bipyridinum units of the CBPQT4+ ring. These contacts are characterized by the roughly parallel orientation of the inner bipyridinium ring system and the 1,2,3-triazole and 1,3-butadiyne units, as well as by the short [pi...pi] distances of 3.50 and 3.60 A, respectively. Variable-temperature 1H NMR spectroscopy has been used to identify and quantify the barriers to the conformationally and co-conformationally dynamic processes. The former include the rotations of the phenylene and the bipyridinium ring systems around their substituent axes, whereas the latter are confined to the circumrotation of the CBPQT4+ ring around the DNP binding site. The barriers for the three processes were found to be successively 14.4, 14.5-17.5, and 13.1-15.8 kcal mol-1. Within the limitations of the small dataset investigated, emergent trends in the barrier heights can be recognized: the values decrease with the increasing size of the pi-electron-donating macrocycle and tend to be lower in the sterically less encumbered series of [2]catenanes containing the 1,3-butadiyne moiety.     

Cycloaddition of Alkenes and Alkynes to the P-centered Singlet Biradical [P(μ-NTer)]2

The reaction of biradical [P(μ-NTer)]₂ ( 1 , Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl) towards different alkenes (R = 2,3-dimethyl–butadiene, 2,5-dimethyl-2,4-hexadiene, 1,7-octadiene, 1,4-cyclohexadiene) and alkynes (R = 1,4-diphenyl-1,3-butadiyne) was studied experimentally. Although these olefins can react in different ways, only [2+2] cycloaddition products ( 1R ) were observed. The reaction with 2,3-dimethylbutadiene also led to the [2+2] product ( 1dmb ). Thermal treatment of 1dmb above 140 °C resulted in the recovery of biradical 1 upon homolytic bond cleavage of the two P–C bonds and the release of 2,3-dimethylbutadiene. In contrast to this reaction, all other [2+2] additions products ( 1R , R = 1,7-octadiene, 1,4-cyclohexadiene, 1,4-diphenyl-1,3-butadiyne) began to decompose at temperatures between 200 °C and 300 °C. Only unidentified products were obtained but no temperature-controlled equilibrium reactions were observed. Computations were carried out to shed light into the formal [2+2] as well as the possible [4+2] addition reaction.     

New zirconium-mediated approach toward regio- and stereocontrolled synthesis of trans-enediynes

[reaction: see text] The coupling reactions of alpha-alkynylated zirconacyclopentene based on 1,4-bis(trimethylsilyl)-1,3-butadiyne with unsaturated compounds are described, which provide an efficient, regio- and stereocontrollable synthesis of trans-enediynes in a one-pot procedure. An interesting alkynyl group shift from alpha- to beta-position of the zirconium center during the reaction was observed, which was accountable for the novel transformations to trans-enediynes.     

Tetranuclear rare earth metal polyhydrido complexes composed of "(C5Me4SiMe3)LnH2" Units. Unique reactivities toward unsaturated C-C, C-N, and C-O bonds

The tetranuclear Lu and Y polyhydrido complexes [(C5Me4SiMe3)Ln(mu-H)2]4(THF) (Ln = Lu, Y) undergo novel multiple hydrogenation reactions with unsaturated organic compounds such as benzonitrile, gamma-butyrolactone, styrene, and 1,4-bis(trimethylsilyl)-1,3-butadiyne to afford a series of structurally characterizable polynuclear complexes that possess novel structures and are otherwise difficult to access. Most of these reactions are unprecedented and can be attributed to the unique cooperative effects of multiple active sites in the polyhydrido rare earth metal complexes.     

A straightforward method for the synthesis of unsymmetrically substituted 1,3-diynes

An efficient synthetic approach to a variety of unsymmetrically substituted conjugated diynes has been developed, starting from the readily available 1,4-bis(trimethylsilyl)-1,3-butadiyne, based upon the selective and sequential substitution of the trimethylsilyl groups with alkyl, aryl and vinyl groups.