Bisallenes

What about measurements of enthalpies of combustion of condensed phase species 49 and 50 and accompanying enthalpies of vaporization Enthalpies of formation of the gaseous hydrocarbons can be directly obtained from these studies as well. There are two recent studies that provide us with useful information. The first42 results in the values of 104.6 0.6 and 104.8 0.6 kJmol-1 respectively. The second accompanies the earlier cited cyclic bisallene (and polycyclic monoolefin) study, in which the authors20... 

We can narrow the difference from 10 kJmol-1 even further once it is remembered that in the comparison of meso-bisallene, 27, and (Z, Z)-diene, 29, there are two extra alkylallene and alkylolefin interactions for which a stabilization of ca 3 kJ mol-1 for the latter was already suggested. Admittedly, comparison with the corresponding 1,5-cyclooctadiyne suggests strain-derived anomalies. From the enthalpy of hydrogenation, and thus derived enthalpy of formation, of this diyne from W. R. Roth, H. Hopf and C. Horn, Chem. Ber., 127, 1781 (1994), we find 1/2S (bis-allene, bis-acetylene) equals ca — 80 kJ mol-1. We deduce that the discrepancy of this last 5 quantity from the others is due to strain in the cyclic diyne. 

Furthermore, this protocol can be employed for the highly efficient introduction of two (159) and even three allene entities (161) into an aromatic workbench (Scheme 2.51). Thus, by starting with two different halides, e.g. 162 (or with identical halides in different positions of a heteroaromatic substrate), two diverse allenic groups can be introduced by sequential coupling reactions. Furthermore, a structurally different bisallene 166 was also assembled via a twofold coupling of the bispro-pargyl bromide 165 with the functionalized aryl iodide 164 [85],... 

The classical examples of these two routes are the conversion of 2,5-dimethyl-2,4-hexadiene (113) via the bisdibromocarbene adduct 114 into the terminally fully methylated bisallene 115 (Scheme 5.15) [43] and the reductive coupling of propargyl bromide (116). 

To prepare the parent bisallene 118, 116 is first converted into its Grignard reagent (known from spectroscopic studies to possess the allenic structure), from which, presumably, by the addition of cuprous chloride the organocopper intermediate 117 is generated. Addition of further 116 subsequently provides a mixture of 118 and propargylallene (l,2-hexadien-5-yne) (29) (see below) in a 2 3 isomer ratio [44],... 

The conformationally locked [46] bisallene 127 was first prepared by subjecting the allene dimer 126 to the DMS allene synthesis (Scheme 5.17) [47]. 

Hydrocarbon 136 subsequently cyclizes to a bismethylencyclobutene again, 137, in which the triple bonds of the substituent are so close that they can engage in a [2+ 2] cycloaddition, leading to the doubly annelated hydrocarbon 138. For less highly unsaturated bisallenes, a similar behavior is observed and they allowed the determination of the stereochemistry of the first ring-closure step [51]. 

Although conjugated bisallenic hydrocarbons display a rich chemistry (see Section 5.5), more and more functionalized derivatives are being described in the chemical literature [52, 53],... 

The cross-conjugated bisallene 226 is obtained when the diacetylene 224 is treated with potassium tert-butoxide in THF at -78 °C for 30 min (Scheme 5.34) [76]. 

In semicydic allenic hydrocarbons, one of the terminal allene carbon atoms is part of an alicyclic ring system, as illustrated by the general structure 37 in Scheme 5.3. Numerous hydrocarbons of this type are known, some of them carrying more than one allene group, such as in the case of the conjugated bisallenes 127 and 129 (see Scheme 5.17), and many of them are described in the review literature [2] and will not be repeated here. However, since Chapter 6 on cycloallenes does not treat these derivatives, some new developments in this area will be briefly presented, limited to the two cases in which cydopropane rings form the end groups of the allene moiety, i.e. 246 and 249. 

Although not involving a pure hydrocarbon, the thermal isomerization of the methylenecyclobutene 268 to the benzocyclobutene 271 shows some typical allene hydrocarbon behavior (see below). In the first step of this sequential reaction, the substrate opens to give a vinylallene , 269, which, at the same time is also a derivative of the (Z)-bisallene 230 already referred to. As such, it readily electrocydizes to the o-xyly-lene 270, which, in a last and also characteristic step, ring closes to 271 [113]. 

Five-membered ring compounds are obtained from conjugated bisallenes under a variety of conditions. Whereas oxidation with m-chloroperbenzoic acid (MCPBA) provides ketones of type 323 [137], carbonylation by iron pentacarbonyl in THF at 50 °C leads to the symmetrical dialkylidene cyclopentenones 326 [45]. With phosphi-nidene complexes such as 325, biallenyls react in a stepwise process that eventually leads to adducts such as 327 [138], mimicking the behavior of singlet carbenes [139]. 

With bisallene 11 having a three-carbon chain between the allenic parts, the intramolecular [2 + 2]-cycloaddition occurred at the inner carbon-carbon double bonds to afford 6,7-dimethylenebicyclo[3.2.0]heptane 12 [11]. 

On the other hand, bisallene 13 having a two-carbon tether underwent dimerization at the terminal carbon-carbon double bonds at lower temperature. Aromatic compound 14 was ultimately formed through elimination and a hydrogen shift [12]. 

Vinylallenes and bisallenes participate in the Diels-Alder-type cycloaddition as the diene component, providing a powerful tool for the construction of complex ring systems. They also undergo thermal electrocydic ring closure to form methylenecy-clobutene derivatives. 

Bisallene (1,2,4,5-hexatetraene) reacts with reactive dienophiles to afford the corresponding [4 + 2]-adducts [182],... 

A [2.2]paracydophane was prepared by dimerization of p-quinodimethane 232, which was obtained by a [4+ 2]-cycloaddition reaction of bisallene with DMAD [184], This sequence represents one of the most general approaches to functionalized para-cyclophanes. 

A convenient route for the preparation of yne-allenes was recently described by Saalfrank et al. [19]. Products 29a/b were formed by Stille cross-coupling of allenyl bromides 27a/b with alkynylstannanes such as 28 (Scheme 14.9). Allenyl phospho-nates such as 30 were also suitable substrates in palladium-catalyzed couplings with propargylstannane 31 (Eq. 14.1). Bisstannylated acetylene 33 as alkyne component furnished the expected yne-bisallene 34 in reasonable yield, but without any diaster-eoselectivity (meso-34 (R,R)-/(S,S)-34 =50 50) (Eq. 14.2). 

On the other hand, the reaction of bisallenes with Bu3SnSnBu3 may generate cis-bisallene Pd(SnBu3)2 (C) and/or a chelated cj-allylpalladium complex D reversibly. The fast carbocydization of these intermediates would give rise to a vinylpalladium complex E, which then yields a ds-compound by reductive elimination and/or a cis-bicyclodiene through (T-bond metathesis as a kinetically controlled product (Scheme 16.66). 

The isomerizations have also proven to be very useful in the synthesis of a series of 1,3-diarylallenes [49-55], even tolerating other functional groups such as aryl chlorides, aryl bromides [56-58] and vinyl bromides [59]. Mixed systems with an alkene on one side and an arene on the other could also be prepared [41, 60], as well as products with two olefinic substituents [61] or bisallenes [62-64],... 

Tomita and Endo have shown that three-component coupling of bisallene 163, aryl dihalide, and a malonate nucleophile takes place with a palladium catalyst. Arylpalladium species derived from the halide attacks the central carbon of allene to form a Tt-allylpalladium intermediate, which is then attacked by the malonate anion to form C-C... 

Various arylene dihalides and heteroarylene dihalides are applicable to the reaction of bisallene 163, and polymers 164 of various structures are available in good to excellent yields (M of 15 000-22000 with PDI of ca. 2.0). Photoluminescent and electroluminescent properties of the polymer 164 are also studied. 

---