Linear conjugation

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Reduction of linearly conjugated 4,6-dien-3-ones with lithium-ammonia yields either 5-en-3-ones or 4-en-3-ones depending upon the work-up procedure. Protonation of the dienyl carbanion intermediate (58) occurs at C-7 to give ultimately the enolate ion (59) kinetic protonation of (59) occurs largely at C-4 to give the 5-en-3-one (60). ... 

Other linearly conjugated dienones also are reduced to unconjugated... 

Application of the Corey reaction to linear conjugated dienones gives the y,(5-methylene-a,/9-unsaturated ketones. ... 

However, on deprotonation even by weak bases the linearly conjugated system is transformed by tautomerization into the cyclically conjugated aromatic [22]pentaphyrin(1.1.1.0.0). 

The orbital phase is continuons in the linear conjugate triene and discontinuous in the cross-conjugate trine. The electron delocalization between the terminal bonds is favored in the linear triene and disfavored in the cross-conjugate triene. The linear triene is more stable. The continuity-discontinuity of orbital phase underlies the thermodynamic stabilities of non-cyclic conjugated molecules. 

Similar arguments lead to the prediction that the cross conjugate TMM dication should be more stable than the linear conjugate BD dication. The cyclic orbital interaction is favored by the continuity of orbital phase in the TMM dication, but the orbital phase is discontinuous in the BD dication. 

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

The cross a conjugation of a C-C bond with two antiperiplanar C-H bonds is more stable than the linear conjugation (Scheme 26). 

This article summarizes efforts undertaken towards the synthesis of the cyclo[ ]carbons, the first molecular carbon allotropes for which a rational preparative access has been worked out. Subsequently, a diversity of perethynylated molecules will be reviewed together, they compose a large molecular construction kit for acetylenic molecular scaffolding in one, two and three dimensions. Finally, progress in the construction and properties of oligomers and polymers with a poly(triacetylene) backbone, the third linearly conjugated, non-aromatic all-carbon backbone, will be reviewed. 

Oxidative polymerization of trans-bis-deprotected 79 under Hay coupling conditions [54] yielded, after end-capping with phenylacetylene, the high-melting and readily soluble oligomers 80a-e with the poly (triacetylene) backbone [87,106] (Scheme 8). Poly(triacetylene)s [PTAs,-(C=C-CR=CR-C=C) -] are the third class of linearly conjugated polymers with a non-aromatic allcarbon backbone in the progression which starts with polyacetylene [PA,... 

Fig. 7. Schematic representation of possible conjugation pathways in perarylated tetra-ethynylethenes. Paths a and b depict trans- and czs-linear conjugation, respectively, and path c depicts geminal cross-conjugation. (D donor, A acceptor)...

Third, substituents raising the effective electronegativity of boron will be expected to enhance B—C 7r-overlap (cf. 25a and 26). Finally, there can be competition between linear conjugation (36) and cross-conjugation (24). 

The photolysis of dienone (52) to the linearly conjugated dienone (56) involves a four-photon sequence and nicely illustrates the involvement of spiro intermediates<39,43> (Table 7.3). 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

Linearly conjugated cyclohexadienones usually photorearrange with ring fusion to a czs-diene-ketene. The reaction is reversible, so that in the absence of a nucleophile little change is observed. A good example of this type of transformation is the formation of photosantonic acid ... 

Shang, Q., X. Dou, and B.S. Hudson. 1991. Off-axis orientation of the electronic transition moment for a linear conjugated polyene. Nature 352 703-705. 

FIGURE 6.1 (a) The molecular structure of 3-carotene, which consists of a linear, conjugated, carbon back-... 

Many of the linear conjugated tricyclic systems have interesting fluorescence or other electrophysical properties. Bis-pyrazolepyridines such as compound 30 have been incorporated into polymers as fluorescent chromophores <1999JMC339>, and used in doped polymer matrices <1997JMC2323>. They are electroluminescent at 425 nm and photoluminescent at 427 and 430 nm in a poly(vinylcarbazole) matrix with a quantum efficiency of 0.8. 

Sterically strained linear conjugated dienes and polyenes. . 35... 

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