Cycloaddition reactions of alkenes

The normal electron-demand principle of activation of 1,3-dipolar cycloaddition reactions of nitrones has also been tested for the 1,3-dipolar cycloaddition reaction of alkenes with diazoalkanes [71]. The reaction of ethyl diazoacetate 33 with 19b in the presence of a TiCl2-TADDOLate catalyst 23a afforded the 1,3-dipolar cycloaddition product 34 in good yield and with 30-40% ee (Scheme 6.26). 

According to the Woodw ard-Hofmann rules the concerted thermal [2n + 2n] cycloaddition reaction of alkenes 1 in a suprafacial manner is symmetry-forbidden, and is observed in special cases only. In contrast the photochemical [2n + 2n cycloaddition is symmetry-allowed, and is a useful method for the synthesis of cyclobutane derivatives 2. 

Cycloaddition reactions of alkenes DIO with nitrones were also catalyzed by Yb(OTf)3, by Sc(OTf)3 (795), by chiral 2,6-bis(4R-trialkylsiloxymethyl-oxazolinyl)pyridine/Ni(II) (pybox) (Fig. 2.43) (796a), and by chiral bis(2-oxazolinyl)xanthene (xabox) (Fig. 2.44) (796b). 

Another class of intermolecular cyclizations are the Cycloaddition Reactions of Alkenes and Alkynes... 

Cycloadditions are useful for the preparation of cyclic ompounds. Several thermal and photoactivated cycloadditions, typically [4+2] (Diels-Alder reaction), are known. They proceed with functionalized electronically activated dienes and monenes. However, various cycloaddition reactions of alkenes and alkynes without their electronical activation, either mediated or catalysed by transition metal complexes under milder conditions, are known, offering a useful synthetic route to various cyclic compounds in one step. Transition metal complexes are regarded as templates and the reactions proceed with or without forming metallacycles [49]. 

The syntheses of diazetidines are known to proceed via a [2+2] cycloaddition reaction of alkene derivatives with azo compounds. A comprehensive literature overview has been provided in CHEC-II(1996) <1996CHEC-II(1B)911>. This chapter compiles some of the new synthetic methods and examples reported after 1994. 

Huisgen, R., R. Grashey, and /. Sauer Cycloaddition reactions of alkenes. In S. Patai The chemistry of alkenes, S. 739. London Interscience Publishers, 1964. 

Fig. 3. Some examples of postulated concerted [2 + 2] and [4 + 2] cycloaddition reactions of alkenes with silicon double bonds, as well as alternate descriptions in terms of a [1 + 2 + 1] reaction with silicon free radicals that would be expected to proceed in a non-concerted fashion through the intermediate [1 + 1] adduct shown.

The thermal and photochemical [2 + 2] cycloaddition reaction of alkenes with... 

Bach T, Bergmann H (2000) Enantioselective intermolecular [2+21-photo-cycloaddition reactions of alkenes and a 2-quinolone in solution. J Am ChemSoc 122 11525-11526... 

In this chapter we have attempted to summarize recent trends in nitrile oxide cycloaddition reactions of alkenes. We hope that this overview will stimulate and encourage continued work in the field. 

In the ensuing sections many more examples will be cited, such as the cycloaddition reactions of alkenes to benzene and related compounds (Cantrell, 1977 Gilbert and Heath, 1979 Bryce-Smith et al., 1980b) where the quenching of fluorescence can be related to the redox properties of the quencher and yet it has proved impossible to detect the formation of fluorescent complexes or radical ions. On the basis of this negative evidence it has to be assumed that reaction proceeds via a non-relaxed exciplex. 

Diazetidines and/or 5,6-dihydro-4//-l,3,4-oxadiazines are produced by cycloaddition reactions of alkenes and acyl or diacyldiazenes, depending on the structure of both the reaction partners, but the synthetic application of this cycloaddition is seriously limited by the occurrence of the ene reaction (Section 7.2.10.1.). 

The photochemical [2 + 2] cycloaddition reaction of alkenes, the light-induced cycloaddition of two carbon-carbon ir-bonds to produce a cyclobutane, is a potentially highly useful reaction in organic synthesis since two new carbon-carbon bonds are formed and a maximum of four new stereocenters are introduced in the process. Ciamician reported the first example in 1908 when he observed the formation of carvone camphor (2) on prolonged exposure of carvone (1) to Italian sunlight (equation 1). ... 

Finally, it should be mentioned that the photochemically allowed [2 + 2] cycloaddition reaction of alkenes can be considered to be a radical-mediated process. Photoexcitation of an alkene gives a 1,2-diradical. The radical at Cl adds to one terminus of the other alkene to give a 1,4-diradical, which then cy-clizes to give the observed product. Spectroscopic measurements at the femtosecond time scale have recently proven that the 1,4-diradical is a true intermediate along the reaction pathway of [2 + 2] cycloadditions. However, the lifetime of the 1,4-diradical is shorter than the rate of rotation about C-C a bonds, as [2 + 2] cycloadditions are stereospecific. 

Figure 11.15. Photochemically induced electron promotion in cycloaddition reactions of alkenes. [Reprinted...

Hastings DJ, Weedon AC (1991) Stereochemical studies of the photochemical cycloaddition reaction of alkenes with N-henzoylindole and N-carboethoxyindole evidence for biradical intermediacy. Can J Chem 69 1171-1181... 

Cycloaddition reactions of alkene radical cations have been the subject of a number of mechanistic studies and are potentially useful synthetic reactions. - - - Most of the initial work on radical cation mediated cycloadditions focused on the dimerization of arylalkenes. with one of the first examples being Ledwith s report of the chloranil-sensitized dimerization of M-vinylcarba-zole to generate a diarylcyclobutane. This work led to the development of the mechanism outlined in Scheme 2, in which addition of the radical cation to neutral alkene generates an acyclic 1,4-radical cation as the primary intermediate. This intermediate cyclizes to a cyclobutane radical cation that is then reduced by the neutral alkene and regenerates a second radical cation to carry the chain. 

Apart from the described radical reaction pathways, there are several important side and consecutive reactions that also proceed in the cracking furnace. The higher the product concentration in the stream (i.e., at high feedstock conversion), the higher is the probability of these side and consecutive reactions. Important side and consecutive reactions include isomerization, cyclization, aromatization, alkylation, and also condensation reactions. The aromatic compounds found in the steam cracker product stream are formed, for example, by cycloaddition reactions of alkenes and dienes followed by dehydrogenation reactions. Moreover, monoaromatic compounds transform into aromatic condensates and polyaromatics (see also Scheme 6.6.2) by the same reactions. Typically, more than 100 different products are found in the product mixture of a commercial steam cracker. 

Next, we will treat cycloaddition reactions of alkenes with the bare diamond surface, after high temperature vacuum annealing, which results in the formation of carbon carbon bonds [39 4l]. For example, if the diamond (100) surface is heated in vacuum to 1000°C, hydrogen desorbs, leaving surface C-C dimers. These have appreciable double-bond character and can react with alkenes under conditions appropriate for the Diels Alder cycloaddition reaction. Either the [2-h2] or the [2-1-4] product can be formed, with the latter being the energetically favored pathway. This type of modification can also be carried out photochemically, and this approach is the one that is used more commonly, as discussed in the next section. 

There are two principal types of photochemical modification techniques 1) cycloaddition reactions of alkenes with the bare diamond surface under UV illumination, resulting in carbon-carbon... 

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