On the Intermediate Occurrence of 1,2‐Cyclohexadiene

Abstract

1,3-Diphenylbenzo[c]furan ( I ) has proved of particularvalue for the detection of lower-membered cycloalkynes [I].In order to ascertain that it is cyclohexyne which is trapped, in accordance with equation (a), and not perhaps 1,2-cyclohexadiene, we have generated the cyclic allene by an independent route and allowed it to react with ( I ) . Sublimed potassium t-butoxide was treated with l-bromocyclohexene in dimethyl sulfoxide in the presence of ( I ) for several hours at 40OC. Chromatography on A1203 then gave, among other products, two compounds, ( 3 4 (m.p. 137-138 "C) and (3b) (m.p. 195-196 "C), in a total yield of 37 %; these are isomers of compound (2) (m.p. 170-171 "C) and are clearly differentiated from it by mixed melting point determinations and by IR and N M R spectroscopy. Whereas only aliphatic and aromatic protons can be recognized in the spectrum of ( 2 ) , the spectra of (3u) and (3b) both show a multiplet at o = 4.3, due to an olefinic proton. This indicates that the exo-and endo-isomers of 1,2-~yclohexadiene adducts have been formed. Control experiments confirmed that the uNene was trapped: compound (2) does not isomerize to (3u) or (3b) under the conditions used./ Treatment of 1-bromocyclohexene with potassium t-butoxide on dimethyl sulfoxide in the absence of ( I ) affords in 7 % yield a hydrocarbon (m.p. 53-54 "C) whose structure (4) was proved by its mass spectrum, elemental analysis, NMR, infrared, and ultraviolet spectra ( A , , , = 237 mp, log E = 4.1761, and a diene reaction with tetracyanoethylene to give (5) [21.In contrast, reaction of 1,2-dibromocyclohexene with lithium amalgam or magnesium gives a trimer, (C&8)3, and a tetramer, (C6H8)4, whose structures and probable modes of formation have been determined [31. Analogous experiments have been carried out on compounds with 7-and 8-membered rings [41. Received: July 28th, 1966 [Z 298 I€] German version: Angew. Chem. 78, 905 (1966) [I] G . Witgig acd A . Krebs, Chem. Ber. 94,3260 (1961); G . Witfig and R. Pohlke, ibid. 94, 3276 (1961).In order to obtain a further protected 2-nitroacrylic acid derivative[ll, we treated potassium mi-nitroacetonitrile, m.p. 150-155 "C (decomp.), obtained from nitroacetonitrile in anhydrous ether and an equivalent amount of a solution of potassium methoxide in anhydrous methanol, in 4 0 % aqueous solution with about the two-fold molar amount of formaldehyde solution at room temperature; on vacuum evaporation of thz mixture we isolated potassium 3-hydroxy-2-nci-nitropropionitrile ( I ) , yield 75 %, m.p. 188-120 "C (decomp.) (from methanol/ether). When kept for 24 hr in the seven-fold volume of 2 N sulfxic acid, this salt evolves nitrogen oxides and gives 2,4,6-tricyanopyridine 1-oxide (2), yield ca. 50 '4, m.p. 222°C (from acetone/water). The structure of the oxide (2) was ascertained by elemental analysis, molecular-weight determination, N M R spectrum (only a sharp singlet at T -= 8.82 ppm), mass spectrum (peak at M-16; characteristic for N-oxides [2, *I), and IR spectrum.Reduction of the oxide (2) ...

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes

“…In the modern era, benzyne ( 1 ) and other cyclic alkynes are readily used to make natural products, medicinal agents, agrochemicals, materials, tools for chemical biology, and ligands for catalysis . The related intermediate 1,2‐cyclohexadiene ( 2 ) has generally received less attention despite being validated not long after benzyne ( 1 ) in 1966 . Historically, theoretical studies of 2 and its derivatives have been popular .…”

A molecular electron density theory study of the [3 + 2] cycloaddition reaction of nitrones with strained allenes

“…6 Unlike arynes and cyclic alkynes, whose structure and reactivity have been widely studied, [1][2][3]7 highly strained allene species have been studied to a much lesser extent. Since 1966, when Wittig reported, for the rst time, the existence of 1,2-cyclohexadiene (CHDE) 10, 8 the chemistry of this highly strained species has received little attention especially compared to its aryne and alkyne counterparts. Only some theoretical studies devoted to [2 + 2] 9-13 and [4 + 2] 14,15 cycloadditions of CHDE 10 yielding cycloadducts 12 and 14, respectively, have been reported (see Scheme 2).…”

Reactions of Cyclohexenyliodonium Salts

“…OBu-t ucts formed under these conditions varied over a wide range, from 98 : 2 to 56 : 44, probably due to change in the fraction of intermediate 1,2-cyclohexadiene [16].…”

“…A usual precursor of cyclohexyne is 1-halocyclohexene which is subjected to strongly basic conditions, e.g., to the action of potassium tert-butoxide or amide [14,15]. Such strong bases can induce isomerization of cyclohexyne into 1,2-diene, and formation of an appreciable amount of the latter isomer always accompanies the main product [16]. Thus a mild and general method for selective generation of cyclohexyne is awaited for the study of its reactivity.…”