[2+2]‐Cycloadditions of Allyl Cations

Abstract

Allyl cations are versatile building blocks for the construction of carbocycles. Hoffmann has reported the syntheses of six-and seven-membered ring systems via [2+4]-and [3 + 4]-cycloadditions of allyl cations to 1,3-dienes"I.We previously found that [3 + 2]-cycloadditions of allyl cations to alkenes provide an easy access to cyclopentenesIZal, and now report [2 + 21-cycloadditions of allyl cations to afford cyclobutane derivatives.When a mixture of 4-chloro-2-methyl-2-pentene 1 and 2,3-dimethyl-2-butene 2 is treated with zinc chloride-ether at -78 "C, the cyclobutane 3l3"I is obtained in 72% yield1']. Similarly, 4-chloro-2,4-dimethyl-2-pentene 4 and 1-ethoxy-2-methyl-1-propene 5 give 80% 613b1. In both cases only one (probably the sterically favored trans) isomer is detected by N M R spectroscopy. No linear intermediates were detected. Lewis acid catalyzed rearrangements of 3 and 6 to give thermodynamically more stable five-membered rings, which would have to proceed via secondary carbenium ions, were not observed. Treatment of 3 with zinc chloride-ether at 20°C yields a complex mixture of products, which contains 2-chloro-2,3-dimethylbutane; its formation can be rationalized by [2 + 2]-cycloreversion of the cyclobutylcarbinyl cation and addition of HCl to 2. However, not all 1,l-dialkylated, 2-unsubstituted allylic systems react to form four-membered rings: under identical conditions reaction of l-chloro-3-methyl-2-butene with isobutene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene yields exclusively linear addition products12b1. According to molecular mechanics calc~lations[~1, these acyclic adducts are more stable than the isomeric cyclobutane derivatives. On the other hand, the heat of formation of 3 is calc~l a t e d~~l to be 3.8 kcal/mol more negative than that of the isomeric 6-chloro-2,4,5,5,6-pentamethyl-2-heptene; gauche-interactions destabilize the linear isomer to a greater extent than the cyclic system (gem-dialkyl effecd5'). Since steric effects should be similar in the corresponding carbenium ions, the different behavior of 1 and l-chloro-3-methyl-2-butene in addition reactions with tetramethylethylene 2 can be rationalized.[ 59, 18.93, 21.23, 23.60, 25.84, 32.67, 32.91 (7q) NMR Studies of Yeasts Grown on Substrates at "Zero" Level RadioactivityBy Ricardo Basosi, Claudio Rossi, Enzo Tiezzi*, and Gianni Valensin The cultivation of different protein-rich microorganisms which utilize hydrocarbons (petrochemicals) as sources of carbon and energy has been frequently suggested'31. The nutritional and biological value as well as the level of toxic substances have still to be studied; the effect induced by a prolonged alimentation on substrates which have less than the natural level of I4C radioactivity has also still to be investigated.Since many correlations have been found between the N M R parameters of water and the physiological or pathological states of cells, as well as the conformations of cell constituents". l0-''1 . we have used water relaxation studies