Organic cycloadditions

Further work from this group centered on mechanistic investigation of the cycloreversion reaction. The fact that strained alkenes could undergo oxidation provided an experimental variable in evaluation of kinetic behavior. Shea and Kim had recently demonstrated that concerted organic cycloadditions showed characteristic variation of rate with strain in the double bond.81 This is due to the partial rehybridization of carbon from sp2 to sp3 at the transition state. For example, in the cycloaddition of organic azides to strained alkenes, approximately 60% of the strain energy is released at the transition state. The corollary is that concerted extrusions will show a complementary effect, in which the barrier to extrusion increases as the strain energy in the product alkene is increased. 

According to O Fig. 3.2, with respect to B, complete electron transfer from B to A will be favorable and A will act as an oxidizing agent. With respect to C, the A(LUMO) - C(HOMO) perturbation will be favorable and A will act as an acid. With respect to D, the A(HOMO) -D(LUMO) perturbation will be favorable and A will act as a base. Lastly, with respect to E, complete electron transfer from A to E will be favorable and A will act as a reducing agent. For F species, the frontiers orbitals energies are identical with those of A. Here neither species is clearly the donor or acceptor and species may display both behaviors simultaneously (a case of multisite interactions encoimtered in concerted organic cycloaddition reactions). 

ACIEE 1984, 23,876 ACJEE 1977, 16, 10 Organic Reactions 1984, 32, 1 W. Carruthers Cycloadditions Reactions in Organic Synthesis (Pergamon Press, Oxford) 1990... 

The trans isomer is more reactive than the cis isomer ia 1,2-addition reactions (5). The cis and trans isomers also undergo ben2yne, C H, cycloaddition (6). The isomers dimerize to tetrachlorobutene ia the presence of organic peroxides. Photolysis of each isomer produces a different excited state (7,8). Oxidation of 1,2-dichloroethylene ia the presence of a free-radical iaitiator or concentrated sulfuric acid produces the corresponding epoxide [60336-63-2] which then rearranges to form chloroacetyl chloride [79-04-9] (9). 

In the [2+4] pencyclic cycloaddition reaction known as the Diels-Alder reaction, fluonne-containing compounds have been widely used as dienes, dieno-philes, or both Much of the fundamental work, including many comprehensive and systematic studies, was done before 1972, and Hudlicky provides an exeellent summary of this work [9] Additional sources for early work in this area are reviews in Organic Reactions [61] and Fluorine Chemistry Reviews [62]... 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

Covalent fluondes of group 3 and group 5 elements (boron, tin, phosphorus, antimony, etc ) are widely used m organic synthesis as strong Lewis acids Boron trifluoride etherate is one of the most common reagents used to catalyze many organic reactions. A representative example is its recent application as a catalyst in the cycloadditions of 2-aza-l,3-dienes with different dienophiles [14] Boron trifluoride etherate and other fluonnated Lewis acids are effective activators of the... 

Another triflate ester that recently has found growing application in organic synthesis is commercially available trimethylsilylmethyl trifluoromethanesul fonate. This powerful alkylating reagent can be used for the synthesis of various methylides by an alkylation-desilylation sequence A representative example is the generation and subsequent trapping by 1,3-dipolar cycloaddition of indolium methanides from the corresponding indole derivatives and trimethylsilylmethyl trifluoromethanesulfonate [108] (equation 54)... 

Similarly, the regiospecific 1,3-dipolar cycloaddition reaction of 1-methyl-1,2-dihydropyridines 41 with cyanogen azide (50a) and selected organic azides 50c and 50g afforded 2-methyl-2,7-diazabicyclo[4.1.0]hept-4-enes 57, which can be elaborated to 1-methyl-l,2,5,6-tetrahydropyridylidene-2-cyanamide (58) and 1-methyl-2-piperidylidenes 59a-d (85CJC2362). 

Application in organic synthesis of optically active isoxazolidones obtained by asymmetric cycloaddition of nitrones with allenes 97T403. 

Cycloaddition Reactions in Organic Synthesis. 1 Edited by S. Kobayaslii and K. A. Jorgensen Copyright 2001 Wiley-VCH Verlag GmbH ISBNs 3-527-30159-3 (Hardcover) 3-527-60025-6 (Electronic)... 

Other advances include the construction of seven- and nine-membered rings via the analogous [4-1-3] and [6-1-3] cycloadditions with dienes and trienes respectively. Heterocycles, such as tetrahydrofurans and pyrrolidines, are accessible using carbonyl compounds and imines as substrates. The following discussion is organized around these recent discoveries. It serves to illustrate the versatility and the high degree of selectivity which are some of the distinctive features of the Pd-TMM chemistry. 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

See e.g. (a) W. Cahhuthehs, Cycloaddition Reactions in Organic Synthesis, Tetrahedron Organic Chemistry Series Vol. 8 Pergamon Press Elmsford, NY 1990 (b) I. OjiMA, Catalytic Asymmetric Synthesis, VCH Publishers. Inc. New York. 1993 ... 

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