Double bonds conjugated systems

Cobalt is another metal which has been successfully used in asymmetric cyclopropanation. A chirally modified catalytic system for selective cyclopropanation of phenyl-, vinyl- or alkoxy-carbonyl-conjugated terminal double bonds with diazoacetates is formed from cobalt(ll) chloride and (+)-a-camphorquinonc dioxime27,69 71 and similar systems 09. Best optical yields are achieved with styrene and the bulky 2,2-dimethylpropvl diazoacetate which gives 2,2-dimethylpropyl /ra .v-2-phenyl-l-cyclopropanecarboxylate in 88% ee and the as-isomer in 81%ee7n. No cyclopropanation occurs with alkyl-substituted or cyclic alkenes, cyclic or sterically hindered acyclic 1.3-dienes, vinyl ethers and phenylethyne. 

Such delocalization occurs for systems that are referred to as conjugated. Conjugation means a lack of an intervening atom between tt bonds or between TT bonds and lone-pair electrons. Whenever you encounter a structure that has conjugated double bonds or lone-pair electrons conjugated with double bonds, you should consider that resonance is likefy. 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

FIGURE 10 5 (a) Isolated double bonds are separated from one another by one or more sp hybridized carbons and cannot overlap to give an extended it orbital (b) In a conjugated di ene overlap of two it orbitals gives an extended it system encompassing four carbon atoms... 

The major product corresponds to addition of a proton at C 1 and bromide at C 2 This mode of addition is called 1,2 addition, or direct addition The minor product has its proton and bromide at C 1 and C 4 respectively of the original diene system This mode of addition is called 1,4 addition, or conjugate addition The double bond that was... 

Elimination is typically regioselective and gives a conjugated diene rather than an isolated or cumulated diene system of double bonds... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

---