The thermal ring opening of 3,3-disubstituted cyclobutenes

Abstract

The rates of the thermal unimolecular isomerisation of 3-t-butyl-3-methylcyclobutene and 3-methyl-3-phenylcyclobutene have been measured in the gas phase over the temperature ranges 158-190' and 120-180", respectively. They are correlated by the Arrhenius expressions k = 1 014-08 * o*80exp( -1 50.8 f 2.5/RT) sand k = 1 012.60 * O*lzexp( -1 26.1 5 f 0.95/RT) s-l, respectively. The ratio of Z-to €-isomer of the butadienes formed has been measured both for these two and for the 3-methyl-3-R-cyclobutenes, where R = isopropyl, n-propyl, cyclopropyl, pentadeuterioethyl, 4 -methoxyphenyl, 3-methoxyphenyl, and 4-cyanophenyl. In each case the proportion of Z-isomer formed is much larger than would be predicted on steric grounds. For the aryl compounds an attractive interaction during the formation of the Z-diene is proposed to account for its excess formation. For the alkyl substituents, a repulsive electronic interaction between the group and the developing remote double bond should be larger for methyl than for the other alkyl groups, accounting for the observed product ratios. The rates of isomerization of the 3-aryl-3-methylcyclobutenes at 161' correlate with the LUMO of the aromatic ring. The conclusions are shown to apply to results which have been reported by other authors.

Stereoselective Olefination of Unfunctionalized Ketones via Ynolates

“…The olefination of ketones via ynolates corresponds to the ring-opening of 3,3-dialkyl(or aryl)cyclobutenes, which was extensively studied by Stevens 20 and Houk. 25 Stevens reported that 3-tert-butyl-3-methylcyclobutenes preferentially gave the E-isomer as the major isomer, which is not in accord with our results (Figure 3).…”

A vector‐based representation of the chemical bond for predicting competitive and noncompetitive torquoselectivity of thermal ring‐opening reactions

“…In this investigation, therefore, we select five ring‐opening reactions where the Frontier molecular orbital (FMO) theory has not been satisfactory (see R1 and R2‐R5 ) in explaining the reaction mechanism; 3‐methyl‐cyclobut‐1‐ene ( R1) , 3‐(ethyl, methyl)‐cyclobut‐1‐ene ( R2 ), 3‐(isopropyl, methyl)‐cyclobut‐1‐ene ( R3 ), 3‐(terbutyl,methyl)‐cyclobut‐1‐ene ( R4 ), and 3‐(phenyl, methyl)‐cyclobut‐1‐ene ( R5 ), see Scheme and Table . We choose a series of reactions R1 ‐ R5 with contrasting nuclear skeletons and activation energies, conventionally considered to be either competitive or noncompetitive on the basis of Δ TS < 1.0 kcal/mol, see Scheme , Table , and Figure .…”

“…The cut‐off for stereospecificity is defined to be for experiment yields of approximately 70% for R1 and for R2‐R5, the dominant product is displayed for clarity in a bold font.…”

Electronic Control of Stereoselectivities of Electrocyclic Reactions of Cyclobutenes: A Triumph of Theory in the Prediction of Organic Reactions

“…Curry and Stevens published a systematic study of the stereochemistry of ring-opening of 3,3-disubstituted cyclobutenes in 1980, and in interpreting their surprising results, they made the first attempt to rationalize the stereochemistry of the ring-opening process in terms other than steric effects. However, the full demise of the steric rationale did not occur until the papers of Dolbier, Burton, Koroniak, et al and Houk, Rondan, and Kirmse appeared in 1984, wherein it became crystal clear that electronic effects , not steric effects, were dominant in determining both the kinetic and the stereochemical behavior of 3-substituted cyclobutenes.…”