Relative stabilities of spirocyclopropanated cyclopropyl cations

Abstract

Dispiro[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 x 10(-4) s(-1) at 50 degreesC and 1.6 x 10(-4) s(-1) at 40 degreesC which is virtually the same as that for cyclopropyl triflate itself (4.02 x 10(-4) s(-1) at 70 degreesC 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).