1,2,3-Butatriene

Figure 1, Coordinates used for describing the dynamics of a) H -I- H2 (6) NOCl, (c) butatriene, (a), (b) Are Jacobi coordinates, where and are the dissociative and vibrational coordinates, respectively, (c) Shows the two most important normal mode coordinates, Qs and Q a, which are the torsional and central C—C bond stretch, respectively.

Figure 4. Wavepackec dynamics of photoexcitadon, shown as snapshots of the density (wavepacket amplitude squared) at various times. The model is a 2D model based on a single, uncoupled, state of the butatriene redical cation. The initial structure represents the neutral ground-state vibronic wave function vertically excited onto the A state of the radical cation.

The Hamiltonian provides a suitable analytic form that can be fitted to the adiabatic surfaces obtained from quantum chemical calculations. As a simple example we take the butatriene molecule. In its neutral ground state it is a planar molecule with D2/1 symmetry. The lowest two states of the radical cation, responsible for the first two bands in the photoelectron spectrum, are and... 

As indicated in the general scheme below, butatrienes are the first products from base-induced 1,4-elinination of hydrogen and a suitable leaving group. The butatriene in general very readily undergoes isomerization into enynes, if sufficiently "acidic" protons are available (see Chapter 11 in Ref. 3a). In aprotic media cumulenic ethers are fixed as their lithio derivatives if an excess of alkyllithium is applied... 

The extremely unstable butatriene, HjC=C=C=CH2, can be obtained in more than 90%... 

The reaction of l,4-bis(trimethylsilyl)-l,3-butadiyne (174) with disilanes, followed by treatment with methylmagnesium bromide, produces i,l,4,4-tetra(-trimethylsilyl)-l,2,3-butatriene (175) as a major product[96]. The reaction of octaethyltetrasilylane (176) with DMAD proceeds by ring insertion to give the six-membered ring compounds 177 and 178[97], The l-sila-4-stannacyclohexa-2,5-diene 181 was obtained by a two-step reaction of two alkynes with the disilanylstannane 179 via the l-sila-2-stannacyclobutane 180[98],... 

In a deceivingly simple process apparently involving a butatriene intermediate, a one-pot preparation of ethyl 5-methylpyrrole-2-carboxylate (6) from diethyl acetamidomalonate (4) and l,4-dichloro-2-butyne (5) has been described <96JOC9068>. 

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). 

C(sp2)=C(sp2) bond imparts properties which more closely resemble those of the central double bond in butatriene than of the one in a simple tetrasub-stituted ethylene [12]. For this reason, bicyclopropylidene (3) undergoes cycloadditions with 1,3-dienes, and these showed an interesting dependence on the structure of the diene. Whereas cyclopentadiene (6) gave the [4 + 2] cycloadduct 28 exclusively, 1,3-cyclohexadiene (26) and 1,3-butadiene (12) led to mixtures of the [4 + 2] and [2 + 2] cycloadducts, with the proportion of the [2 + 2] adduct increasing respectively [13] (Table 3). 

Figure 7. The PES of the X2B2S and A -Bi, states of the butatriene radical cation, (a) Diabatic surfaces, (b) Adiabatic surfaces. The surfaces are obtained as eigenfucations of the vibronic coupling model Hamiltonain that fitted to reproduce quantum chemical calculations. The coordinates are shown in Figure lc. See Section III. D for further details.

The urethane-substituted polydiacetylenes exhibit thermo-chromic transition with low and high temperature crystal phases favoring acetylenic and butatriene backbone, respectively (4-6). Our interest in the application of epitaxial polymerization to diacetylenes has been the possibility of substrate control over orientation, structure, and the single crystal nature of thin films. 

Poly(TCDU) in thin film exists in the acetylenic phase 2, as opposed to the butatriene phase 1 found in bulk crystals polymerized from macroscopic monomer crystals (7). Polymerization to completion in thin film, thereby removing the constraint of the monomer lattice, could account for the acetylenic phase. 

FIGURE 13. Franck-Condon envelopes of the low-energy part of the PE spectra of (a) butatriene 158 and (b) tetradeuteriobutatriene 159. The central feature labeled 1 is the mystery band mentioned in the text... 

Aryl group substituted butatrienes and hexapentaenes can be selectively reduced with Zn-ZnCl2-H20 to result in aryl-substituted 1,3-butadienes and hexa-l,5-dien-3-ynes, respectively (equations 191 and 192)315. 

An allenyl cation 65 is involved as an intermediate in the room-temperature isomerization of l-chloro-l,4,4-tris(4-methylphenylthio)butatriene (66) to the tris(4-methylphenylthio)butenyne (67)43 (equation 23). 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

It appears that neither the lithium carbenoid pathway nor the cyclopropanation of buta-trienes are general routes to [3]radialenes. More successful is the cyclotrimerization of 1,1-dihaloalkenes via copper or nickel carbenoids, provided the substituents at the other end of the C=C double bond are not too small. Thus, tris(fluoren-9-ylidene)cyclopropane 27 was formed besides butatriene 28 from the (l-bromo-l-alkenyl)cuprate 26 generated in situ from (9-dibromomethylene)fluorene (Scheme 3)10. The cuprate complexes formed... 

The most recent strategy to prepare [3]radialenes is the treatment of 1,1-dihaloalkenes with activated nickel. Thus, the aryl-substituted [3]radialenes (Z,E,E)-30 and (E,E,E) 30, 27 and 32 were obtained together with the corresponding butatrienes (29, 28, 31) from the 1,1-dibromo- or 1,1-dichloroalkenes with the help of nickel activated by ultrasound (Scheme 4)11. It is worth mentioning that the mixed-substituted radialene 33 was produced, when the nickel carbenoid derived from 9-(dichloromethylene)xanthene was generated in the presence of butatriene 2811. 

Radialenes represent the biggest and best known subset of the radialene family this is not surprising in view of the fact that more methods to prepare them exist than for any other class of radialenes. The major strategies are the transformation of appropriate cyclobutane derivatives, the thermal or Ni(0)-catalyzed cyclodimerization of butatrienes or higher cumulenes and the cyclotetramerization of (l-bromo-l-alkenyl)cuprates. 

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... 

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