Formation of 1,2,3-Butatrienes

Several examples are known of the transition metal-catalyzed synthesis of 1,2,3-buta-trienes, which possess one more cumulated C=C double bond than allenes. Most of the reported examples of the butatriene synthesis involve dimerization of terminal alkynes and conjugated enynes are typical side products of the reactions. 

The first example of a butatriene preparation via dimerization of a 1-alkyne was reported in 1976 [129]. Treatment of a benzene solution of tBuC=CH (184a) at 

195a Ar = Ph 195b Ar= 1-naphthyl 195c Ar = P-CF3-C6H4 195d Ar = p-MeO-CeH4 

This chapter has discussed the transition metal-catalyzed synthesis of allenes. Because allenes have attracted considerable attention as useful synthons for synthetic organic chemistry, effective synthetic methods for their preparation are desirable. Some recent reports have demonstrated the potential usefulness of optically active axially chiral allenes as chiral synthons however, methods for supplying the enantiomerically enriched allenes are still limited. Apparently, transition metal-catalyzed reactions can provide solutions to these problems. From the economics point of view, the enantioselective synthesis of axially chiral allenes from achiral precursors using catalytic amounts of chiral transition metal catalysts is especially attractive. Considering these facts, further novel metal-catalyzed reactions for the preparation of allenes will certainly be developed in the future. 

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The observation by Fischer et al.18 that the 4,1-addition of dimethylamine to compound la is thermodynamically controlled at 20°C, whereas 2,1-addition/elimination is kinetically controlled at -115°C, turned out to be limited to few cases.20 It has been shown9a 9b 42 112 113 that for most cases, three competing reaction paths must be considered (i) 2,1-addition/elimina-tion with formation of (l-amino)alkynylcarbene complexes (= 2-amino-l-metalla-l-en-3-ynes) 98 (ii) 4,1-addition to give [(2-amino)alkenyl]carbene complexes (= 4-amino-l-metalla-l,3-butadienes) 96 and (iii) 4,1-addition/ elimination to (3-amino)allenylidene complexes (= 4-amino-l-metalla-1,2,3-butatrienes) 99 (Scheme 33, M = Cr, W). The product ratio 96 98 99 depends on the bulk of substituents R and R1, as well as on the reaction conditions. Addition of lithium amides instead of amines leads to predominant formation of allenylidene complexes 99.112 Furthermore, compounds 99 also can be generated by elimination of ethanol from complexes 96 with BF3 or AlEt3114 and A1C13,113 respectively. 

For R = H and Me, the derived values are [321.3 ( >1.9)] and [323.3 ( >1.4)] klmoF , respectively. A value of [326 ( > 4)] kJmoF for AHf(g, 1,2,3-butatriene) is thus credible. What is found for the cyclopropanation enthalpies of butatriene There are seemingly no relevant data for either of its monocyclopropanation products, dimethylenecyclopropane (25a) or vinylidenecyclopropane (25b). The two dicyclopropanation products have comparable enthalpies of formation dicyclopropylidene (26), 286.6 (1) and 324.3 (g), and meth-ylenespiropentane (27), 287.0 (1) and 320.9 (g), respectively". The 2-6 kJ moF decrease in enthalpies of formation for gaseous dicyclopropanated products is not particularly in accord with the 3 kJ moF increase per alkyl substituent of cyclopropanation of simple olefins. However, in that the allene —> methylenecyclopropane —> spiropentane (3 23 7) enthalpy of formation changes are still enigmatic, and error bars are absent for the dicyclopropanated products, we do not fret. But we eagerly await more thermochemical data. 

Shortly after discovery of a convenient dihalocyclopropane synthesis their conversion to allenes was effected This area has been reviewed very recently ", and therefore only some very new examples will be mentioned Employing a repetitive cyclopropanation/allene formation sequence, 1,2,3-cyclononatriene—presumably the smallest isolable cyclic butatriene—has become available (equation 120). ... 

Irradiation of a benzene solution containing l-diarylvinylidene-2,2,3,3-tetramethylcyclopropanes (7a-d) efficiently afforded l,l-diaryl-4,5,5-tetramethyl-l,2,3-hexatrienes (8a-d) in high yields. The quantum yields for the formation of 8a-d were not high (4> =0.01 0.02) as shown in Table 31.3. The photorearrangement of 2,2,3-trimethyl and 2,2-and 2,3-dimethyl derivatives 7e-f, 2a also took place, but the rates for the formation of the 1,2,3-butatrienes were quite slow and products were not isolated. The bicyclic vinyhdenecyclopropanes 7g-h also rearranged to the 1,2,3-butatriene derivatives 8e-. The structure of 8b is confirmed by x-ray crystallography. 

E)-1,3-pentadiene, and 2,3-pentadiene are accompanied by increases in enthalpies of formation of ca. 30 kJ mol [57] and so the enthalpy of formation of butatriene is 265 + (2-30) = 325 kJ mol . Combined with the enthalpy of formation of C4 from the JANAF tables we thus deduce the enthalpy of reaction 28 is 971-325 = 646 kJ mor. This value is relatively close to that found for reaction 27. Why should C2 be such a significant outlier [5S]... 

Buta-l,2,3-trienes. Newkome et al. were able to prepare the unstable (Z)-and (E)-l,4-diphenyl-l,4-di(2-pyridyl)butatrienes, (2) and (3), by didehydrox-ylation of (1) with P214 in anhydrous pyridine. Reduction of (1) with the more usual reagents (SnCl2 or PBra) resulted mainly in formation of (4). 

The combination of probably the oldest synthetic procedure for formation of a triple bond, i.e., the dehydrobromination of a vinyl bromide, with modern crown ether chemistry has resulted in one of the simplest yet very powerful methods for making highly strained cycloal-kynes. Thus, 1,5-cyclooctadiyne (56) can be made by treating l,5-dibromo-l,5-cyclooctadiene (55) with potassium rerf-butanolate in nonpolar solvents in the presence of 18-crown-6 [3 b, 24]. The nonpolar solvent protects the bent triple bond from nucleophilic attack by tert-butanol (Scheme 8-5). 1,5-Cyclooctadiyne had previously been made in very low yield by dimerization of butatriene (57), which is not a readily accessible compound [25]. Other important 1,2-elimination reactions generating cyclic alkynes are the oxidative degradation of... 

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