Butatrienes

The silylated acetylene alcohol 1589, however, is converted by ethyllifhium in 87% yield into the 1,2,3-butatriene system 1590 and Me3SiOLi 98 [8] (Scheme 10.2). 

A completely different approach to butatrienes was reported in 2000 [139]. Treatment of l-aryl-2-bromo-l-buten-3-ynes 195 with a variety of carbon soft nucleophiles 196 in the presence of 5 mol% of a Pd-dpbp catalyst gave the corresponding 1,2,3-butatrienes 197 in moderate yields (Scheme 3.98). The reactions showed essentially no /Z-selectivity all the butatrienes were obtained as mixtures of the two isomers. 

Bohlmann and co-workers furthermore described the sole natural products with a 1,2,3-butatriene sub-structure (Scheme 18.33) [99]. These compounds 103-106 (which were isolated between 1965 and 1971 from the roots of camomile and other plants) are highly unstable, which made their structural assignment difficult. 

Schwarz and coworkers115 used 1,2,3-butatriene, along with 1,3-butadiyne, as a precursor for the generation of neutral 1,2,3-butatrienylidene in a neutralization/reionization mass spectrometric sequence (C4H4 - C4H2- - C4H2 - C4H2+ ). 

The dimerization of alkynes is a useful method for forming compounds such as enynes from simple alkynes [13]. The iridium-catalyzed dimerizahon of 1-alkyries was first reported by Crabtree, and afforded (Zj-head-to-head enynes using [Ir(biph)(PMe3)Cl] (biph = biphenyl-2,2 -diyl) as a catalyst [14]. Thereafter, an iridium complex generated in situ from [Ir(cod)Cl]2 and a phosphine ligand catalyzed the dimerizahon of 1-alkynes 1 to give (Tj-head-to-head enyne 2, fZj-head-to-head enyne 3, or 1,2,3-butatriene derivatives 4 in the presence of hiethylamine... 

The reaction of 1,2,3-butatriene 211 with excess lithium metal affords dilithiated compound 212 with the central carbon-carbon bond bridged by the two lithium centres (Scheme 73). In the sohd state, the C4 unit is bent, rather than linear, due to the... 

Three metal stabilized seven-membered rings containing 1,2,3-butatriene units have been prepared [Eq. (62)). In the metallacycle 367, independently synthesized by Hsu et al.153 and Burlakov et a/.114 by essentially the same method, zirconium serves as both a member of the seven-membered ring and as a cumulene-stabilizing metal. An X-ray crystal structure excluded the alternative structure 368, which was proposed as a possible intermediate in the reaction. The X-ray of 367 is consistent with the cumulene formulation in that bond lengths for C2-C3 [1.337(6) A], C3-C4 [1.298(6) A], and C4-C5 [1.337(6) A] indicate bonds of roughly similar bond order. The bond angles for C2-C3-C4 [148.8(5)°] and C3-C4-C5 [160.1(5)°] are also consistent with a severely distorted (from linearity) cumulene. 

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). 

A. 4-Amino-1-chroma-1,3-dienes, 2-Amino-1-metalla-1-en-3-ynes, and 4-Amino-1-metalla-1,2,3-butatrienes by Aminolysis... 

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. 

A. 2,4-Dloxy-1-metalla-1,3-butadienes and 3-Oxy-1-metalla-1,2,3-butatrienes by Alcoholysis... 

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

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. 

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