Substituent effects cycloaddition

There are almost no studies of substituent effects on additions to carbon-carbon triple bonds extant in the literature. Bowden and Price (208) have reported a correlation of rates of addition of hydrogen bromide to 3-substituted propiolic acids with the Hammett equation using the Op constants. Unfortunately, there are only three substituents in the set. Sufficient data are available for a single set of 1,3-dipolar cycloaddition. The set studied is shown in Table XXXIII, and the results of the correlation are in Table XXXIV. The correlation was significant the delocalized effect is predominant in this set. 

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

Certain specific steric effects are operative on intramolecular nitrile oxide— olefin cycloadditions. These effects are governed by both ring size and character of substituents. Thus, cycloadditions to the exomethylene group are successful with substituted methylenecyclohexanones 334 (m = 1, 2 n = 2) and gave tricyclic 335 (m = 1, 2), but do not occur with methylenecyclopentanones 334 (m = 1, 2, 3 n = 1). Activation energies calculated by molecular mechanics are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives cA-decalone 336 (403). 

Substituent effects on intramolecular dipolar cycloadditions can be illustrated by the gem-dicarboalkoxy effect (404). This effect (rel. rate >20) has been... 

Herndon, W. C. Substituent Effects in Photochemical Cycloaddition Reactions. 46, 141-179... 

The reactions of 534 with substituted quinones produced mixtures of regioisomers. The substituent effect on the regioselectivity of the [8 + 2] cycloaddition reactions was said to be dependent on steric as well as electronic effects. Equation 156 shows the reaction between 534 and 2-methylbenzoquinone (539). The reaction afforded a mixture of two regioisomeric adducts 540 and 541, which were transformed to azulenes 542-545 under the reaction conditions applied318. 

Density functional and semiempirical AMI molecular orbital calculations have been used to investigate substituent effects on site selectivity in heterocumulene-hetero-diene4 + 2-cycloadditions between ketene imines and acroleins.The new and novel heterocumulenes a, /3-unsaturated thioaldehyde S -oxides (97) behave as both diene... 

Some other data showing substituent effects on the rate constants for cycloadditions with electron-deficient alkenes is given in Table 7.2 (5). 

With so many variables to contend with—large numbers of dipoles and dipolarophiles, seemingly unpredictable and not always consistent patterns of reactivity and regiochemistry, and substituent effects that can support or override the core patterns—it is not surprising that almost no one can master 1,3-dipolar cycloaddition chemistry. For the present it is enough for you to recognize a 1,3-dipolar cycloaddition when you see it, and to appreciate the nature of some of the problems that exist. 

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