Butatriene synthesis

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 cyclodimerization of 82 to 83 is an example of a Ni(0)-mediated synthesis of [4]radialenes from [n]cumulenes. Applications of this method to butatriene derivatives 9050,63, 9164 and 9365,66 are shown in Scheme 10. The usefulness of Ni(0) catalysis for this transformation was first demonstrated by West and coworkers63 and later explored in detail by Iyoda and coworkers42,54,67. In some cases, Ni(0) catalysis improves the... 

Scheme 2.63 Samarium-mediated synthesis oftrisubstituted furan 209 via butatriene 206. L= PPh3 X= OAc.

Reports on ruthenium catalytic activity focus more on mechanistic consideration of the prototypical phenylacetylene dimerization than in establishing its synthetic applicability. It is not unusual that changing the alkyne substituents results in reversed selectivity (i.e. R = Ph or SiMe3 gave ( )- or (Z)- isomers, respectively) [27]. Competitive alkyne cyclotrimerization (R = COOMe) [27] or butatriene formation (R= CH2Ph, Bu) [10, 21] have occasionally been reported as possible drawbacks in enyne synthesis. The operating mechanism restricts the reaction to terminal alkynes. 

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces trans-1,2,3-butatrienes (Eq. 102) 157). The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane31). 

Amino-l -metalla-1,2,3-butatrienes nuclear magnetic resonance, 195-197 synthesis, 194-196... 

After the synthesis of some 1-phosphaallenes (type I, n = l), 1-phospha-l,2,3-butatrienes (type II, n = 1) were synthesized recently for the first time (Scheme 16). 

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). ... 

After last year s preliminary report on the synthesis of pentatetraene by a retro-Diels-Alder reaction involving flash thermolysis of (131), Ripoll has now published the full details and has also shown that the similar decomposition of ll-vinylidene-9,10-ethano-9,10-dihydroanthracene (132) at 750°C-10mm-Hg pressure gives an 85% yield of butatriene (133). To complement these developments, a timely publication has appeared on the theoretical basis and the instrumentation necessary for flash thermolyses, suitable for such experiments on a preparative scale. 

Scheme 2.138. Synthesis of 1,2,3-butatrienes by fluoride ion induced 1,4-elimination.

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