Norbornanes. Part 21. Bridging strain in norbornyl and oxanorbornyl cations

Abstract

(1 1. V. 88)Further evidence is presented that the 2-norbomyl cation is stabilized primarily by C(2)-C{6) bridging, and that C(2)-C(7) bridging leads to prohibitive strain. Thus, a comparison of the heats of hydrogenation of nortricyclene 17 and bicycl0[3.2.0.0~.~]heptane 18 indicates that the strain energy of the latter is ca. 21.5 kcal/mol higher than that of 17. Furthermore, 6-exo-2-oxabicyclo[2.2. ljheptyl sulfonates 8 ionize with strong O(2) participation to the bridged oxonium ion 12. In contrast, 2-endo-7-oxabicyclo[2.2.1]heptyl sulfonates 11 ionize without O(7) participation to form the unbridged carbenium ion 15.As pointed out in [la], displacement (&2) reactions of bi-and tricyclic halides and sulfonates with nucleophilic solvents are sterically hindered, if the nucleofuge is adjacent to a bridgehead atom. In these cases, solvolysis reactions occur via carbenium ions (&1) and tend to be slow unless assisted by bridging of the cationic center by neighboring atoms.In carbocyclic compounds, bridging involves weak bonding between the electrophilic cationic center and neighboring C-atoms, thereby generating some of the strain associated with the subdivision of cyclic structures into smaller rings [2]. Differential bridging of neighboring C-atoms, therefore, accounts for the frequently different rates and products of epimeric bi-and tricyclic sulfonates [lb], such as the exo-and endo-2-norbornyl p -bromobenzenesulfonates (brosylates) and p -toluenesulfonates (tosylates) 1 (X = BsO) and 2 (X = TsO), respectively, which differ by factors of more than 3002).In their original communications [3], Winstein and Trifan attributed the higher rate of 1-OBs to anchimeric assistance of exo-ionization by the antiperiplanar C( 1)-C(6) bonding electrons, i.e. by so-called CT participation3), which is absent in the endo-epimer 2-OBs. According to a modified version of this rationale [6], the transition state for exo-ionization is stabilized by C,C hyperconjugation with the strained and antiperiplanar C( 1)-C(6) CT bond. Thus, both interpretations stress the role of the C( 1)-C(6) bonding electrons in controlling the relative rates of I-OBs and 2-OBs. On the other hand, the role of the equally strained C( 1)-C(7) bond, which is somewhat deflected from the plane ') 2, 3,The IUPAC name for norbornane: 8,9,1U-trinorbornane. The rate ratio at 25"is 350 in AcOH [3], 580 in 80% EtOH [lb], and cn. 2000 in H2O [4].For a definition of u participation, see [5].

Heats of Formation of Medium-Sized Organic Compounds from Contemporary Electronic Structure Methods

“…Second, only small organic and inorganic molecules for which accurate formation enthalpies are known have been used to equilibrate the reactions (i.e., were used as A, B, and D in eq ), in particular acetylene, ethylene, ethane, CO, methane, H 2 , H 2 O, H 2 S, HCN, N 2 , and NH 3 . We expect under these conditions that the possible cancellations of errors are minimal, and better insights into the ability of the electronic structure theory methods to provide accurate reaction enthalpies can be gained. …”

Accurate prediction of norbornadiene cycle enthalpies by DLPNO‐CCSD(T1)/CBS method

“…Although the results of three experimental studies of nortricyclane are relatively close to each other, varying from 78.3 to 84.5 kJ/mol, [1,6,19] a lower value of 70.2 kJ/mol determined from combustion calorimetry by Kozina et al [15] is regarded as the most reliable in Table 1. This value is in excellent agreement with the calculated value.…”