Acetylenes, conformers

PolyTCDU (X = C5H5) can be synthesized so that the red, high-temperature phase is stable at room temperature (12). Conversion to the blue phase can be achieved at low temperatures (at least partially) (13) or under a strain field (14). The optical properties of the red aTi blue phases of polyTCDU are nearly identical to those of polyETCD and polylUPDO (9,J ). The X-ray structure of polyTCDU (2 ) suggested that thTs polymer existed in the butatriene conformation, -(R)C C=C=C(R)-, rather than the commonly observed (and theoretically more stable) acetylenic conformation, =(R)C-C5C-C(R)=. This led to the suggestion that thermochromism involved an acetylenic-to-butatrienic conformational change (10). 

M. Leclerc and R. E. Prud homme, Theoretical analy- 23. sis of model compounds of substituted poly(acetylenes) conformation versus electronic properties,... 

The orbital phase theory was applied to the conformations of alkenes (a- and P-substituted enamines and vinyl ethers) [31] and alkynes [32], The conformational stabilities of acetylenic molecules are described here. 

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

A different synthetic access to a 1 -metallacyclopropene, which can be a versatile organometallic synthon, is displayed in Scheme 33. The mono-alkyne derivatives of W(IV)-calix[4]arene are easily accessible through the thermal displacement of cyclohexene from 32 using the appropriate acetylenes. The reaction led to complexes 34 and 172-174. The proposed 3-metallacyclopropene has been confirmed from the spectroscopic and the X-ray data. The H NMR data reveal a cone conformation of the calixarene with a four-fold symmetry, for which the... 

W=X is N=N or C=N, never if it is C=C. The ring stability of 3-furyl-carbenes conforms with this rule. Ring opening is again the main reaction in a biradical which is generated by extruding carbon dioxide from a lactone at 675°C and which then collapses to an acetylenic ketone.277... 

Fig. II. (a) Schematic representation of hydrocarbons adsorbed on the [111] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane (2) all-cis conformation of cij-l,3,5-hexatriene (3) transoid conformations of cis- and trans-1,3,5-hexatriene (S4). (b) Adsorption configurations of acetylene and ethylene found most probable according to LEED studies 141).

Overview. The proposed near-term research is divided into three areas, which will be pursued simultaneously. The first is the complete characterization of the current mercury-responsive fluorescent chemosensor system, including the measurement of fluorescence lifetimes, to discern the origin of the conformational control of fluorescence. The second is to develop two classes of substituted biaryl acetylenic fluorescent chemosensors, to move the observed fluorescence into the visible region and increase the magnitude of the fluorescence signal, which occurs upon conformational restriction. 

Fig. 4-6. X-ray analyses of [18]crown-6 complexes with dimethyl acetylene di-carboxylateU), malonodinitrilen), and benzenesulfonamide 13). In the latter case the crown is in a conformation intermediate between A and B (Fig. 3e in 8)...

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

A number of hydrocarbons, called alkynes or acetylenes, have triple bonds between carbon atoms.3 They conform to the general formula C H2 2 for one triple bond. 

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