Dimerization of alkenes

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of alkenes to cyclobutanes (15-61) gives very poor results in most cases, except when... 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

The first solid-state linear dimerization was observed with N-vinylpyrrolidi-none. It was first quantitatively converted to its Markovnikov HBr addition product (by application of HBr gas at -40 °C), which upon warming to room temperature lost HBr and formed ( )-l,r-(3-methyl-l-propene-l,3-diyl)bis-(2-pyrrolidinone), but the yield was less than 100% [58]. Interestingly, such head-to-tail dimerizations of alkenes lead to shrinking and that may create reactivity even if the crystal lattice does not allow for molecular migrations due... 

This addition of one molecule of alkyne to another is formally analogous to the dimerization of alkenes under the influence of sulfuric acid (see Section 10-9), but the mechanisms are quite different. 

Dimerizations of alkenes bearing heteroaromatic substituents such as pyridine have been widely reported. 2-(/ -Styryl)quinoline (239), for example, yields on irradiation in the solid state a dimer which can be represented either as the head-to-head dimer (240) or the head-to-tail dimer (241).225 The stereochemistry of ring fusion in this and many other related dimers is not known. The structures of the dimers obtained by solid state photolysis of /3-2-furyl-, jg-2-thienyl-, and /9-3-pyridylacrylic acid have been interpreted in terms of crystal packing.226... 

The cycloadditions that do riot occur thermally, for example the dimerization of alkenes and of dienes, have 4nn electrons in their anti-aromatic transition states... 

The French Petroleum Institute has developed an ionic liquid based process for the dimerization of alkenes (Dimersol process) and it has been patented as the Difasol process. Interestingly, it can be retrofitted and operated in existing Dimersol plants. However, its biphasic nature offers several advantages over the traditional, homogeneous Dimersol process (Table 10.6). 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of alkenes to cyclobutanes (15-63) gives very poor results in most cases, except when photochemically induced. Woodward and Hoffmann, and Fukui have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, for example, mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three... 

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