Polyyne

Carbynes are a form of carbon with chains of carbon atoms formed from conjugated C(sp )=C(sp ) bonds (polyynes) ... 

From X-ray diffraction studies of short chain (C4-Cg) polyynes [16] C=C bond lengths ranged from 119-121 pm while C-C bond lengths ranged from 132-138 pm, depending upon the local molecular enviromnent, cf. Table 2. 

Fig. 3. A, A kinked polyyne chain model for linear carbynes (after [23]) B, eyclo C-18 earbync [25].

X-ray diffraction peaks were rather broad with coherence lengths as low as 20 nm and this was attributed to rapid quenching. It was proposed that the carbon atoms are arranged in polyyne chains (n = 4) along the c-axis. The density of Carbolite (1.46 g-cm ) is lower than values for other carbynes and for diamond and graphite - hence the name - and this was attributed to a rapid quenching process. 

In addition to questions over interpretation of diffraction data, diere are reservations about the stability of carbynes. Lagow et al [32] note that the condensation of the compound Li-C=C-Br to form carbon chains is potentially explosive. There is also the possibility of cross-linking between carbyne chains and the nature of the termination of the carbyne chains is unclear. Eastmond et al [33] showed that polyyne compounds of the type ... 

In reaetions with nueleophilie reagents diaeetylene behaves as aeetylene aeti-vated with aeeeptor group that is eommon to eonjugated polyynes. Therefore, the nueleophilie addition of amines, aleohols, and thiols oeeurs to its terminal position and leads to the formation of the eorresponding l-heteroalk-l-en-3-ynes readily involved in diverse eyelization reaetions. 

Much current research is centering on polyynes—linear carbon chains of sp-hybridized carbon atoms. Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

Delaney, Acetylenic Polymers For Solid Propellants , Report RMI-157-L2, Reaction Motors Inc,Denville, Contract No NORD-17851 (1957) 5) Anon, Polyynes for Solid Propellants , C EN, 41-42 (Sept 28, 1959) 5a) W.W. 

Fig. 1. Cyclo[n]carbons in their polyyne and their cumulenic forms...

The in situ deprotection/alkynylation protocol has proven to be exceedingly useful. In addition to graphdiyne models, an array of PAM/PDM hybrids have been prepared (76 (R=H), 81, 97-99) [66]. Extension of the simple, one-pot procedure to various iodoarenes has allowed preparation of a series of bis(tri-isopropylsilyl)-protected a,m-polyynes in very good yields. Subsequent depro-... 

With sp bond angles calculated to be around 162°, macrocycle 131 would be highly strained and was therefore expected to be quite reactive [79]. The octa-cobalt complex 132, on the other hand, should be readily isolable. Indeed, 132 was prepared easily from 133 in five steps, and was isolated as stable, deep maroon crystals (Scheme 30). All spectroscopic data supported formation of the strain-free dimeric structure. Unfortunately, all attempts to liberate 132 from the cobalt units led only to insoluble materials. Diederich et al. observed similar problems when trying to prepare the cyclocarbons [5c]. Whether the failure to prepare these two classes of macrocycles is due to the extreme reactivity of the distorted polyyne moiety or to the lack of a viable synthetic route is not certain. Thus, isolation and characterization of smaller bent hexatriyne- and octatetrayne-containing systems is an important goal that should help answer these questions. 

The penultimate example of macrocycles based on phenyl and acetylenic units has been the very recent report by Tobe [801 and Rubin [81] of cyclophane 134. Both groups generated 134 in the mass spectrometer by laser desorption of hexa-protected polyynes 135 (robust) and 136 (unstable), respectively (Scheme 31). 

Until 1992, the only ethynylated cyclobutadiene complexes pertinent in the literature were 22 - 24, prepared by Fritch and Vollhardt using [2 -i- 2]-cycloaddition of suitable polyynes over CpCo(CO)2 [24]. No alkynylated derivatives of 25, however, had been prepared. 

Balza, E. and Towers, G. H. N. 1990. Dithiacyclohexadiene chlorohydrins and related sulphur-containing polyynes from Ambrosia chamissonis. Phytochemistry 29 2901-2904. 

Trans-polyenes H-(-HC=CH-),, -H, trans-polyenynes H-(HC=CH-C=C) -H, cumulenes H2C=(C=C) =CH2 and polyynes H-(C=C) -H have been studied (M=N-1). For eentrosymmetrie molecules, the first order hyperpolarizability p is equal to zero so that non linear effects are of second order nature. Furthermore, (the x axis goes through the middle of the C-C bonds of the polyenes, or is the intemuclear axis in the case of linear molecules) is the most important component of the second order y hyperpolarizability tensor, the other components being negligible. Both y and the mean hyperpolarizability... 

These monodimensional compounds, rich in it electrons, have been the object of several experimental[35] and theoretical work[20,35,36]. Perry et al.[35], using a powder SHG technique, have studied diaryl polyynes and have shown that some of them exhibit second order hyperpolarizabilities of very high magnitude. On another hand, Jameson and Fowler[36] carried out ab initio calculations in order to study basis sets effects on the electrical properties of acetylene and diacetylene. Furthermore, Chopra et al [20], then Maroulis and Thakkar [37] have been interested in the influence of the lengthening of the... 

In Table 6, are given the MNDO y values of the four series of polymers,polyenes (A), polyenynes (B), cumulenes (C) and polyynes (D). 

It should be emphasized that the electrochemical carbonization proceeds, in contrast to all other common carbonization reactions (pyrolysis), already at the room temperature. This fact elucidates various surprising physicochemical properties of electrochemical carbon, such as extreme chemical reactivity and adsorption capacity, time-dependent electronic conductivity and optical spectra, as well as its very peculiar structure which actually matches the structure of the starting fluorocarbon chain. The electrochemical carbon is, therefore, obtained primarily in the form of linear polymeric carbon chains (polycumulene, polyyne), generally termed carbyne. This can be schematically depicted by the reaction ... 

F. Sato developed titanium (Il)-based c/s-reduction of alkynes as shown in Scheme 5 [14], and the method was applied to the synthesis of pheromones by Kitching (Scheme 6) [15]. This titanium (Il)-based reaction gives pure (Z)-alkenes. Kitching summarized the contemporary methods for the synthesis of skipped polyynes and their reduction to skipped polyenes [15]. 

Compounds of this type, listed in Table 9, are available by interaction of triallylboranes with certain polyyne derivatives of silicon and tin. Their formation involves 1,1- and 1,2-allylboration of triple and double carbon-carbon bonds. 

Having initiated the chemistry within the atmospheric model, the elongation of the carbon-chain polyyne species occurs by successive addition of the C2H radical ... 

It is clear that a complex hydrocarbon polymer chemistry must exist in the atmosphere of Titan involving polyyne species, polynitrile species and mixtures of the two, and additional routes to polyaromatic hydrocarbon formation. This presents a significant problem for the gas chromatography/mass spectrometry instruments on the Huygens probe. There should be hydrocarbon fragments, producing perhaps... 

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