HIGHLIGHTS
because some carbons electronically communicate with the enamine nitrogen, while the rest resonate with the iminium form. In terms of frontier orbitals (though I have not performed the calculations), this probably means the LUMOs and HOMOs have large coefficients on alternating carbons. Dual electrophilic and nucleophilic behavior of cyanines has been demonstrated in several papers. Thus over five years ago, at
Harvard, Zhuang was one of the inventors of STORM super resolution) and discovered that the nucleophile
TCEP adds to Cy5 (1) and some other dyes to form a non-fluorescent adduct, and that this could be reverted to the parent Cy5 system by UV radiation (Zhuang et al; J. Am. Chem. Soc. 2013, 135, 1197). This allows for cycling between light and dark states, as required for STORM. For electrophiles, in 2019, a group from Beijing showed that halide and pseudohalide electrophiles add to Cy5 dyes at other sites, eg (3) to (4); readers should note the different site of attack for (1) and (3) (Hu et al, Org. Lett., doi: acs.
orglett.9b00404). In that same paper, this group
reported similar transformations on Cy7 dyes (not shown). That actually followed work in
2018 by a group from Northwest University, Xi’an, China, that proved Cy3 dyes could be mono- and disubstituted with electrophiles as illustrated in the generic diagrams (5) to (6) (for the mono-substituted products) (Org. Lett., 2018, 20, 60−63). Collectively, these transformations will enable many synthetic routes to cyanine dyes.
Organic chemistry
G. RICHARD STEPHENSON University of East Anglia, UK
Combining metal and Lewis acid catalysis to control cyclisation reactions An unusual cyclopropane ring- opening reaction proceeds with a diazoketone, under combined rhodium and Lewis acid mediated catalysis (M. Petzold, P. G. Jones, D. B. Werz; Angew. Chem. Int. Ed., 2019, 58, 6225).
The cyclopropanes used in this study are activated with two ester substituents, and the aryl diazoketone also contains an ester group, which is essential to allow the formation of the unusual oxacylic ylide (1). It is proposed that this ylide then reacts with the cyclopropane. The reaction has been optimised using 2mole% of Rh2 of Sc(OTf )3
(OAc)4 and 1mole% of Yb(OTf )3
in dichloromethane to produce (2) diastereoselectively in a 97% yield.
Mechanochemistry of polymers containing strained-ring subunits An even more strained, and far more unusual building block, the benzo- fused tetracyclic ladder-like structure (3) has been employed to make a polymer with truly unusual properties (J. Yang, M. Horst, J. A. H. Romaniuk, Z. Jin, L. Cegelski, Y. Xia; J. Am. Chem. Soc., 2019, 141, 6479). Although not available yet in high
yield, over 2g of (3) was prepared using a classic tricarbonyliron-based
OMe O
O O
OMe N2
O N2 MeO MeO MeO , 20mole%
Scheme 1 The diastereoselective formation of (2) is helped by using both scandium and ytterbium triflates as the Lewis acid component
Scheme 1
Scheme 2 (i): ROMP using the Grubbs type III catalyst; (ii): ultrasound
Scheme 1 (i) n (i) 3 Scheme 2 3 Scheme 2 42 06 | 2019 N O (ii) n 2 n (ii) n
MeO2C MeO2C
MeO
MeO2C MeO2C 2
MeO Rh2(OAc)4 Rh2(OAc)4 1 CO2Me CO2Me 1 CO2Me CO2Me OH O H O OMe OMe H OH 20:1 dr O MeO OO O
approach. It was then polymerised by ring-opening metathesis (Grubbs III catalysis). The resulting polymer still retains much of the ring-strain present in the monomer, because only one ring is used to drive the polymerisation reaction. Sonication promotes a solid state
reaction in which the polymer is transformed into a poly(o-phenylene- hexatrienylene). The change from the non-conjugated initial polymer, to the conjugated final product, causes a colour change from colourless to yellow, as a response to the application of physical force, ultrasound in this case.
20:1 dr
Shape-changing crystals – another rare mechanochemical phenonemon A quite normal looking diyne (4) forms crystals that have far from normal chemical physical properties, a phenomenon called mechanochromism. Fluorescent green crystals of (4), when subjected
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