A Simple Synthesis of Bicyclo [1.1.0]butane and its Relation to the Internal Conversion of Electronic Energy in 1,3-Butadiene

Perturbing π-clouds with Substituents to Study the Effects on Reaction Dynamics of gauche-1,3-Butadiene to Bicyclobutane Electrocyclization

The conical intersection (CI) governs the ultra-fast relaxation of excited states in a radiationless manner and are observed mainly in photochemical processes. In the current work, we investigated the effects of substituents on the reaction dynamics for the conversion of gauche-1,3-butadiene to bicyclobutane via photochemical electrocyclization. We incorporated both electron withdrawing (-F) and donating (-CH₃ ) groups in the conjugated system. In our study, we optimized the minimum energy conical intersection (MECI) geometries using the multi-configurational state-averaged CASSCF approach, whereas, to study the ground state reaction pathways for the substituted derivatives, dispersion corrected, B3LYP-D3 functional was used. The non-adiabatic surface hopping molecular dynamics simulations were performed to observe the behaviour of electronic states involved throughout the photoconversion process. The results obtained from the multi-reference second-order perturbation correction of energy at the XMS-CASPT2 level of theory, topography analysis, and non-adiabatic dynamics suggest that the -CH₃ substituted derivatives can undergo faster thermal conversion to the product in the ground state with a smaller activation energy barrier compared to -F substituted derivative. Our study also reveals that the GBUT to BIBUT conversion follows both conrotatory and disrotatory pathways, whereas, on substitution with -F or -CH₃ , the conversion proceeds via the conrotatory pathway.

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

The unique chemistry of small, strained carbocyclic systems has long captivated organic chemists from a theoretical and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as laboratory curiosities. However, new approaches for preparing, functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.

Photochemical Valence Isomerization to High Energy Products-Bicyclobutanes and Oxabicyclobutanes†

DFT calculations were used to determine the structures and energies of bicyclobutane and oxabicyclobutane as valence isomers derived from electronic excitation of their corresponding precursors, 1,3-butadiene and acrolein, respectively. Proton affinities of these strained compounds were determined and compared with their simple ring components, cyclopropane and ethylene oxide. The basicity as determined from proton affinities showed that bicyclobutane is the most basic saturated hydrocarbon, even more basic than oxabicyclobutane. Strain energies of these valence tautomers were computed which showed oxabicyclobutane to be significantly more strained than bicyclobutane. Qualitative reasons are provided to account for the difference in strain energies.

Regioselectivity and mechanism of dihalocarbene addition to benzocyclopropene

Dihalocarbenes add regioselectively to aryl-substituted benzocyclopropenes to produce dihalobenzocyclobutenes. The regioselectivity of addition is not due to steric effects but depends on the electronic donor or acceptor ability of the substituent. B3LYP/6-31G* calculations show preferential :CCl2 addition to substituted benzocyclopropene through electrophilic attack on the benzocyclopropene pi-system (Ea = 1.1-2.4 kcal/mol) rather than C-C sigma-bond insertion into the cyclopropenyl moiety (Ea = 5-24 kcal/mol). pi-Addition proceeds regioselectively through a single transition state to xylylene intermediates or directly to benzocyclobutene products.