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The synthesis of polyethynyl aromatics as starting materials for the preparation of highly cross-linked organic solids containing high atom fractions of carbon is described. Treatment of bromo-and iodoaromatic compounds with (trimethylsily1)acetylene (TMSA) in the presence of palladium(0) and copper(1) in amine solvents yields (trimethylsily1)ethynyl-substituted aromatics. The TMS protecting groups can be removed by hydrolysis with mild base. Compounds prepared by using this technique include 1,3-diethynylbenzene, 2,5-diethynylthiophene, 1,3-diethynyltetrafluorobenzene, 1,4-diethynyltetrafluorobenzene, 2-ethynylthiazole, 2,4-diethynylthiazole, 2,7-diethynylnaphthalene, hexakis((trimethylsilyl)ethynyl)benzene, tetraethynylthiophene, 2,5-bis((trimethylsilyl)ethynyl)-3,4-bis(3-hydroxy-3-methyl-l-butynyl)thiophene, 2,5-diethyny1-3,4-bis(3-hydroxy-3-methyl-l-b~-tynyl)thiophene, 2,5-bis(4-(2-thienyl)butadiyny1)-3.4-bis(3-hydroxy-3-methyl-l-butynyl)thiophene, and 2,5-bis-(4-(2-thienyl)butadiynyl)-3,4-diethynylthiophene. IntroductionWe are engaged in a project aimed a t the preparation of organic solids containing a high density of strong, directed carbon-carbon bonds. Our objective in this project is to relate the molecular-level structure of the solids to their macroscopic physical properties and to determine if it is possible to replicate in these materials some of the remarkable range of properties exhibited by pure allotropes of carbon (diamond: hardness, high thermal conductivity) (graphite: electrical conductivity, l~bricity).~ We expect high-carbon, densely cross-linked organic solids to display high thermal stability and a high degree of hardness. Our strategy for the preparation of these materials involves the initial syntheses of low molecular weight poly(diacetylenes); these polymers are then molded into desired shapes and processed thermally, photochemically, and/or catalytically to yield the final, highly cross-linked organic solids.This paper details the preparation of several monomeric polyacetylenic aromatic compounds. Certain of these monomers are the precursors to the intermediate polymers; others were too reactive and unstable to be useful in polymerizations but are of interest in their own right as unusual ethynylated compounds. We will discuss the polymerization of these monomers and materials processing of the resulting polymers in later papersa4The commercial development of organic solids from acetylenic precursors dates from the 1960s when Hay showed that the oxidative coupling of l,&diethynylbenzene (1) yielded a soluble p~l y m e r .~ The molecular weight and solubility of this polymer could be controlled by using mixtures of 1 and phenylacetylene (2) (as an end-capping group) in the polymerization. This procedure yielded phenyl-terminated poly(diethyny1benzene) oligomers 3 that could be further processed a t high temperatures to form carbon films and fibers. Research at General Electric expanded this type of chemistry to include a wide range of acetylenic aromatic monomem6 More recently, re-

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crux of the issue, the specific manner in which the trans double bonds are positioned within the taxane framework, was deduced through the combined adaptation of one-bond *H/13C COSY correlation and NOE experiments.13 Several of the more diagnostic measurements are given in the illustrations.Molecular mechanics calculations (MODEL version KS 2.96)14 provided indication that 9 might be as much as 3 kcal/mol more thermodynamically stable than 10. Indeed, heating 10 in C6D6 at 67 °C resulted in its unidirectional conversion to 9, k¡ = 2.58 X 10"4 s'1, / j ^2 = 45 min. The relative ease of this diastereomeric interconversion places 10 closer to franr-cyclononene (t[/2 = 4 min at 0 eC)3 23than to frans-cyclooctene (f1/2 = 122 h at 132.7 °C)lc in the intrinsic ability of these molecules to overcome their internal rotational barriers. The thermal stability of 10 is, however, more than adequate to allow for its individual utilization in synthesis, thus providing a new dimension to this area of chemistry.Both 9 and 10 undergo osmylation to give diols 11a (60%) and 12a (84%), respectively, by electrophilic attack from the only available direction external to the ring. Subsequent acetylation provided lib and 12b efficiently.In summary, this study has established the feasibility of controlling stereochemistry by means of rotationally restrictive nonbonded interactions and laid the groundwork for possible entry to various taxanes including 1 by this useful tactic. Our current efforts are focused on these applications.Acknowledgment. We are grateful to the Bristol-Myers Squibb Company for their financial support, to Dr. Dirk Friedrich for his assistance with the NMR experiments, and to Eugene Hickey for the molecular modeling studies. (12) (a) Chapman, O. L.

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The convergent synthesis of a series of monodisperse dendrimers based upon symmetrically substituted benzenetricarboxylic acid esters is described. These materials consist of 4,10,22, and 46 benzene rings connected symmetrically and have molecular diameters of up to 45 Á. The synthesis proceeds in a stepwise convergent manner, building dendritic arms, three of which are subsequently attached to a molecular core. The critical intermediate for the dendrimer arm synthesis is 5-(tert-butyldimethylsiloxy)isophthaloyl dichloride, obtainable in three steps from 5-hydroxyisophthalic acid. Reaction of the diacid chloride with phenol, followed by removal of the silyl protecting group, gives a new substituted phenol. Two moles of the latter are further reacted with 5-(tert-butyldimethylsiloxy)isophthaloyl dichloride. This process is repeated several times. The dendrimer arms formed by these reactions are coupled to 1,3,5-benzenetricarbonyl trichloride yielding dendrimers. Kinetic results suggest that the rate of reaction of the first dendrimer arm with the core is independent of the size of the dendrimer arm. These materials are stable up to 5000 C under Na and are highly soluble in typical organic solvents. Possible applications for these materials include molecular weight standards, polymer rheology modifiers, or molecular inclusion hosts.

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