Relative Stabilities of Spirocyclopropanated Cyclopropyl Cations

Abstract

AbstractDispiro[2.0.2.1]hept‐1‐yl triflate (3), 7‐bromo‐7‐phenyldispiro[2.0.2.1]heptane (4) and 7‐chloro‐7‐phenylsulfanyldispiro[2.0.2.1]heptane (6) were prepared from bicyclopropylidene (5) in 50, 77 and 90% overall yield, respectively. 7‐Bromo‐7‐cyclopropyldispiro[2.0.2.1]heptane (8) and 7‐bromo‐7‐methyldispiro[2.0.2.1]heptane (11) were obtained by hydrobromination of 7‐cyclopropylidene‐ (7) and 7‐methylenedispiro[2.0.2.1]heptane (10) (78 and 95% yield, respectively). Methanolyses of triangulane derivatives 4, 6, and 8 as well as acetolysis of 6 all proceed with retention of the dispiro[2.0.2.1]heptane skeleton yielding the corresponding 7‐substituted 7‐methoxydispiro[2.0.2.1]heptanes 15, 21, and 23 as well as 7‐acetoxy‐7‐phenylsulfanyldispiro[2.0.2.1]heptane 22 in 90, 100, 100 and 88% yield, respectively. Methanolysis of 1‐bromo‐1‐cyclopropylcyclopropane (24) also gave mainly the ring‐retained product 25 (66%) along with the ring‐opened product 26 (33%). Apparently, an increasing number of spiro‐annelated three‐membered rings stabilizes a cyclopropyl cation against ring opening under solvolysis conditions. The rate of solvolysis, however, is only slightly affected by this spiroannelation, as the rate coefficient for the triflate 3 in sodium acetate‐buffered methanol was determined to be k = 3.5 × 10−4 s−1 at 50 °C and 1.6 × 10−4 s−1 at 40 °C which is virtually the same as that for cyclopropyl triflate itself (4.02 × 10−4 s−1 at 70 °C in acetone/H2O 3:2). No reaction was observed for the bromotriangulane 11 in pure methanol, and solvolysis of 11 in aqueous methanol only led to products 27−30 which were formed by ring‐enlarging and ring‐opening rearrangements of the initially formed 7‐dispiro[2.0.2.1]heptyl cation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)