Computational Study of Cyclobutane-1,3-diylidene Dicarbenes: Ground-State Spin Multiplicity and New Strategy toward the Synthesis of Bicyclo[1.1.0]but-1(3)-enes

Abstract

Coupled-cluster calculations were performed for cyclobutane-1,3-diylidene dicarbenes 2 at the CCSD(T)//CCSD/ cc-pVDZ level of theory, in which the ground-state spin multiplicity and the structures of unique molecules were investigated in detail. The closed-shell singlet state 2(S σπ ) with a bicyclo-[1.1.0]but-1(3)-ene (BBE) structure found to be the groundstate was much lower in energy than the corresponding singlet dicarbene structure 2(S ** ), the quintet state 2(Q), and the triplet state 2(T), suggesting that the hitherto experimentally unknown BBE structure can be synthesized by the intramolecular dimerization of two carbene units. The energy gap between the BBE structures 2(S σπ ) and corresponding quintet states 2(Q) with electron-withdrawing substituents (X = F) at the C2 and C4 positions was found to be larger than that with electrondonating substituents (X = SiH 3 ), i.e., ca. 100 kcal mol ¹1 for 2b (X = F) > ca. 85 kcal mol ¹1 for 2a (X = H) > ca. 70 kcal mol ¹1 for 2c (X = SiH 3 ). Two unique structures, 2(T σ ) with a C1σC3 bond and 2(T π ) with a C1πC3 bond, were found to be the equilibrium structures for the triplet state of cyclobutane-1,3-diylidene dicarbenes 2.

Computational study on 1,3‐disilacyclobutane‐1,3‐diylidene disilylenes: A synthetic strategy for cis‐bent disilenes

“…All the C 2v structures in compound 2 , which have cis ‐bent geometry, were found to be more stable than the planar D 2h structures by approximately 10 kcal mol −1 (entries 5‐7). The structural preference for the cis ‐bent geometry and a shorter E(1)–E(3) distance than that in the D 2h geometry were also found in dicarbene compound 1 …”

“…The energetic stabilization of the symmetric Si–Si π bond orbital (π F ) by the C–F σ* orbital is depicted in Figure B. The low‐lying σ* orbital can interact with the Si–Si π orbital to increase the π bond energy between the 2 silicon atoms.…”

“…Recently, we found that the formation of experimentally hitherto unknown bicyclo[1.1.0]but‐1(3)‐ene ( C – S σπ ; E═C) with the cis ‐bent alkene structure is possible during intramolecular dimerization of dicarbenes 1 (E═C) (Figure ) . In the present study, the effects of substituent X on the ground‐state spin multiplicity and structure of 1,3‐disilacyclobutane‐1,3‐diylidene disilylene derivatives 2 (E═Si) were examined computationally at the B3LYP/cc‐pVDZ and CCSD(T)//CCSD/cc‐pVDZ levels of theory to examine the possibility of forming rare cis ‐bent disilene structures .…”

A Tetrasilicon Analogue of Bicyclo[1.1.0]but‐1(3)‐ene Containing a Si=Si Double Bond with an Inverted Geometry

“…[5] Such nonclassical bonds with an inverted geometry should also be possible for multiple bonds.Bicyclo[1.1.0]but-1(3)-ene (B;F igure 1a)r epresents one extreme example of compounds with an inverted double bond (II' ',F igure 1b), wherein the direction of the double bond is again formally oriented in the diametrically opposite direction to that expected for an ormal C = Cd ouble bond geometry (II, Figure 1b). [6,7] Although B and its derivatives have been investigated theoretically as constitutional isomers of C 4 H 4 (including fascinating molecules such as tetrahedrane and cyclobutadiene) [8] and postulated based on experimental evidence as potential intermediates, [9] they have not yet been isolated and their structures have not yet been investigated experimentally.I nt his context, the N-heterocyclic gallylene-bridged Ge 2 species C (Figure 1c), in which the [Ge 2 Ga 2 ]f ramework is topologically similar to that of B,h as recently been isolated by Fisher,Frenking,and co-workers. [10] At heoretical investigation of C suggested that the central Ge À Ge bond consists of a p bond and aw eak s bond with contributions from equivalent mesomeric structures (Ge + Ge À $Ge À Ge + ), or from as tructure with two weakly coupled unpaired electrons (Ge›Gefl)a tt he germanium atoms.V ery recently,Abe and co-workers reported atheoret- ical study on the structures of 1,3-disilicon analogues of bicyclo[1.1.0]but-1(3)-ene (D;F igure 1c), which were calculated at the CCSD(T)/cc-pVDZ//B3LYP/cc-pVDZ level of theory.The results revealed that the structures depend on the substituents Ronthe carbon atoms of the saturated ring.…”

Is π-Single Bonding (C–π–C) Possible? A Challenge in Organic Chemistry

“…Very recently, the triplet state of cyclobutane-1,3-diylidene dicarbenes was predicted to possess the π single bonding between the carbene centers ( Figure 9). 32 Therefore, the π single bonding would be not the specific case for the singlet 1,3-diradicaloids.…”