Synthesis of Phenylacetylene Dendrimers Having Sterically Congested Diazo Units and Characterization of Their Photoproducts

Abstract

The compound (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,4,6‐tribromophenyl)diazomethane (1a), a precursor for persistent triplet carbene, was found to be stable enough to survive Sonogashira coupling reaction conditions to give (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,6‐dibromo‐4‐ethynylphenyl)diazomethane (2a). The diazomethane 2a was used as a starting compound for synthesis of phenylacetylene dendritic molecules having peripheral diazo groups by the convergent method. Thus, tridendrons with peripheral groups bearing 3 (3a), 6 (6a), and 12 (8a) diazo units were prepared, in which diazo groups were introduced so as to generate polycarbenes in a ferromagnetic fashion. In order to characterize the spin states of carbenes generated by the photolysis of these dendrimers, the magnetic susceptibility of the photoproducts was measured. Although the data for the photoproduct of the tris(diazo) compound were fitted with S = 2.5, those for the photoproducts of hexakis and dodecakis(diazo) compounds were fitted with S = 1.62, a value that is much smaller than the theoretically predicted value and even smaller than that of triscarbene. The results are interpreted in terms of the disjoint‐nondisjoint concept. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004

From Persistent Triplet Carbenes to Persistent High-Spin Polycarbenes

“…(2,6-Dibromo-4-ethynylphenyl)(4-t-butyl-2,6-dimethylphenyl)diazomethane (6) 32 was selected as a starting material to survive the chemical manipulations required to prepare the corresponding polymer. Treatment of 6 with [RhCl 2 (nbd)] (nbd = norbornadiene) complex in the presence of triethylamine as a cocatalyst in toluene for 1 day at room temperature followed by quenching by MeOH and filtration gave a red solid (Scheme 14).…”

“…The coupling reaction proceeded smoothly under mild conditions to form 1,3,5-tris{2-[4-(-diazo-4-t-butyl-2,6-dimethylbenzyl)-3,5-dibromophenyl]ethynyl}benzene (7-3N 2 ) (Scheme 5). 32 In order to obtain evidence concerning the spin states of the photoproducts from 7-3N 2 , the magnetic susceptibility of the photoproduct was measured. As noted previously, it is more difficult to obtain unequivocal evidence of the spin states for randomly oriented organic species from the conventional continuous-wave (CW) ESR spectroscopy as the spin states become higher.…”

“…The coupling took place very smoothly to afford phenylacetylene dendrimers having 6 (10-6N 2 ) and 12 (12-12N 2 ) diazo units. 32 SQUID measurements were made for the photoproducts from hexakis (10-6N 2 ) and dodecakis(diazo) compounds (12-12N 2 ). The experimental data (M=Ms vs H=T) were fitted best with Eq.…”

A simple and efficient synthesis of 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene from 1,3,5-triethynylbenzene

“…As one of the first generation of phenylacetylene dendrimers formed solely from 1,3,5-triethynylphenyl units, 1,3,5-tris[2-(3,5-diethynylphenyl)ethynyl]benzene (1) acts not only as a potential key building block in dendrimer synthesis but also as an important model molecule for investigation of the physical and chemical properties of these dendrimers [6][7][8][9]. Moore et al were first to explore the synthetic methodology and properties of phenylacetylene dendrimers.…”

“…The approaches often involved Sonogashira cross-coupling between a 1,3,5-triiodobenzene core and 3 equiv. ethynyl-terminated peripheral monomers with trialkylsilyl-protected ethynyl groups at the meta positions, and subsequent deprotection of the trialkylsilyl groups [6,8,10,11,16]. For example, by using an eight-step sequence, 1 was synthesized in an overall yield of only 19% [16].…”