The thermal behavior of meso‐ and d,1‐3,4‐dimethyl‐1,5‐hexadiyne‐3,4‐diol (meso‐ and d,1–4) has been studied at 400°C in the gas phase. In a tandem reaction the diols isomerize via the bisallenes 5 first to cis‐ and trans‐3,4‐diacetylcyclobutene (cis‐ and trans‐8) which have been detected as intermediates by 13C‐NMR spectroscopy as well as catalytic hydrogenation to 1,2‐diacetylcyclobutane (9). Subsequently, 8 stabilizes itself by ring opening to (E,Z)‐ and (E,E)‐3,5‐octa‐dien‐2,7‐dione (E,Z) and [(E,Z)‐6]. As a side reaction retroene cleavage of 4 to 2‐butynone (10) and methyl vinyl ketone (12) is observed. By pyrolysis of the pure diastereomers 4 it is shown that the retro‐ene process is more pronounced in the d,l than in the meso series.

The thermal behavior of meso-and d,l-3,4-dimethyl-l,5-hexadiyne-3,4-diol (meso-and dJ4) has been studied at 400°C in the gas phase. In a tandem reaction the diols isomerize via the bisallenes 5 first to cis-and trans-3,4-diacetylcyclobutene (cis- In 1963 Huntsman and Wristers[*] first described that 1,5-hexadiyne (1, bipropargyl) on heating above 250°C smoothly and quantitatively cyclizes to 3,4-bis(methylene)-cyclobutene (3); the Arrhenius parameters, determined in the 210-300°C temperature range (E, = 34.4 kcal/mol, AS* = -9 cal K-' mol-'), were indicative of a concerted process [3]. Subsequent work showed that the cycloisomerization actually begins with a [3.3]-sigmatropic process which converts 1 into 1,2,4,5-hexatetraene (2, biallenyl) which subsequently electrocyclically closes to 3[41.Scheme 1. Propargyl-Cope rearrangement of 1,5-hexadiyne (1) 1 2 3Although the basic reaction mechanism of this "propargyl-Cope-type" isomeri~ation[~I hence appears to be established; the reaction is, in fact, more complicated. Even at the temperatures employed by the original workers 3 is not the only product formed, both fulvene and benzene being produced in small amounts. At higher temperatures (400-600°C) these latter C6H6 isomers become the main products as was shown by Jones et a1.L6]. Depending on the pyrolysis temperature, either 3 or fulvene can thus be made the main product, making the process a viable and simple method for the preparation of either hydrocarbon (isolated yields of fulvene 30%). Further insights into the details of this part of the C6H6 energy surface were obtained by Bergman and Henry who followed the isomerization process by means of a deuterium tracer171. Still, as we show in a forthcoming the high-temperature behavior of 1-3 is far from being totally understood, a fact underlined by the continuing interest of theoreticians in these But it is not only the mechanistic aspect of the propargylCope rearrangement which deserves a second look. Preparatively as well the 1 -+ 3 cyclization has only been exploited to a very limited extent, the work of Criegee on the use of bismethylenecyclobutenes as starting materials for cyclobutadiene-metal complexes being a notable exception ["]. In principle, though, propargyl-Cope processes could be synthetically as valuable as the Cope rearrangement (of lS-hexadienes) itself. To test this proposal one would have to study simple, functionalized 1,5-hexadiynes, not hydrocarbons, and then proceed to more complex structures and applications. One of the few thermal isomerizations of functionalized bipropargyls has been described by Chuche and Manisse" 'I, and it involves the title compound 3,4-dimethyl-l,5-hexadiyne-3,4-diol (4)[121.When 4 is heated to 350°C the only products formed are 3,5-octadien-2,7-dione (6), 2-butynone (10) and methyl viChem.

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The title 1,2‐diol derivative, C10H12O2, crystallizes with two independent but closely similar mol­ecules in the asymmetric unit. Only two of the four OH groups are involved in classical hydrogen bonding; the mol­ecules thereby associate to form chains parallel to the short c axis. The other two OH groups are involved in O—H⋯(C[triple‐bond]C) systems. Additionally, three of the four C[triple‐bond]C—H groups act as donors in C—H⋯O inter­actions. The 1,4‐diol derivative crystallizes with two independent half‐mol­ecules of the diol (each associated with an inversion centre) and one water mol­ecule in the asymmetric unit, C12H16O2·H2O. Both OH groups and one water H atom act as classical hydrogen‐bond donors, leading to layers parallel to the ac plane. The second water H atom is involved in a three‐centre contact to two C[triple‐bond]C bonds. One acetyl­enic H atom makes a very short `weak' hydrogen bond to a hydr­oxy O atom, and the other is part of a three‐centre system in which the acceptors are a hydroxy O atom and a C[triple‐bond]C bond.