Ring opening of cyclopropylidenes to allenes: reactions with bifurcating transition regions, free internal motions, steric hindrances, and long-range dipolar interactions

Journal Article · Wed May 28 00:00:00 EDT 1986 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00271a078· OSTI ID:5583329

Valtazanos, P; Elbert, S T; Ruedenberg, K

The mechanism of the ring opening of cyclopropylidene to allene, the prototype bond fission of a cyclic carbene due to ring strain, has implications for many organic reactions. We have mapped out the energy surface E(phi,delta/sub 1/,delta/sub 2/) in its entirety where, for each triple of (phi,delta/sub 1/,delta/sub 2/) values, the molecular geometry was optimized with respect to 12 internal coordinates. An examination of the energy surface reveals the following features of the cyclopropylidene ring opening: (i) singlet cyclopropylidene is stable at phi = 59.5/sup 0/ and about 9 kcal/mol lower than the triplet (stable at 65.5/sup 0/). (ii) The CH/sub 2/ planes are perpendicular to the CCC plane (C/sub 2nu/) and approximately remain so during the initial stages of the ring opening. (iii) Shortly before phi reaches 70/sup 0/ the CH/sub 2/ groups begin a disrotatory motion, preserving C/sub s/ symmetry. (iv) Around phi approx. 81/sup 0/, with a disrotatory inclination of about 35/sup 0/, a conrotatory component admixes to the reaction path advance and C/sub s/ symmetry is lost. (v) No inherent preference exists between the two branches. (vi) The bifurcation occurs before the transition states (which are located around (phi,delta/sub 1/,delta/sub 2/) = (81/sup 0/, 50/sup 0/, 120/sup 0/) and (81/sup 0/, 60/sup 0/, 130/sup 0/)) and extremely close to it. (vii) The downhill path from the transition state to the product with phi opening to 180/sup 0/, while conrotatory in a general way, leads across slopes of isoenergetic valleys. (viii) The allene stereoisomerization requires a concerted twisting and bending, the transition state occurring at phi = 133/sup 0/.

• Research Organization: Iowa State Univ., Ames
• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 5583329
• Journal Information: J. Am. Chem. Soc.; (United States), Vol. 108:11
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ALLENE
CHEMICAL REACTIONS
CYCLOALKENES
DIPOLE MOMENTS
ELECTRONIC STRUCTURE
MOLECULAR ORBITAL METHOD
POTENTIALS
QUANTUM MECHANICS
ROTATIONAL STATES
STEREOCHEMISTRY
STRUCTURAL CHEMICAL ANALYSIS
THEORETICAL DATA
WAVE FUNCTIONS
ALKENES
DATA
DIENES
ENERGY LEVELS
EXCITED STATES
FUNCTIONS
HYDROCARBONS
INFORMATION
MECHANICS
NUMERICAL DATA
ORGANIC COMPOUNDS
POLYENES
400301* - Organic Chemistry- Chemical & Physicochemical Properties- (-1987)