The Ambivalent Nature of Halogenated Tropone Derivatives: Dihalocycloheptatriene vs. Halotropylium Halide

Abstract

Although dichlorocycloheptatriene and dibromocycloheptatriene have been known for a long time, their exact structure remained uncertain so far. In this computational and crystallographic study, we elucidate the nature of these compounds in solution and in the solid state. In the latter case, both compounds exist as halotropylium halides, which also feature Br–Br halogen bonds in the case of bromotropylium bromide. In solution, there is a highly dynamic equilibrium involving both the ionic halotropylium halide form and different isomers of the covalent dihalocycloheptatriene form. The presence of the 7,7‐dihalocycloheptatriene isomer, which until now has been the most commonly used representation in literature, can be excluded.

Hydroxytropylium chloride: the first crystal structure of an unfunctionalized hydroxytropylium ion

“…The seven-membered ring is close to being perfectly planar, with no C-C-C-C torsion angle deviating from 0/180 by more than 3 , thus supporting the idea of an aromatic tropylium system. The average of the C-C bond length [1.393 (3) Å ] is close to the values reported for halogen-substituted and unsubstituted tropylium ions (Jandl et al, 2017). However, in contrast to halotropylium ions, the C-C bond lengths of the hydroxytropylium ion show an alternation between the longer C1-C2, C3-C4, C5-C6 and C1-C7 bonds, and the shorter C2-C3, C4-C5 and C6-C7 bonds (Table 2) Computer programs: APEX2 (Bruker, 2013), SAINT (Bruker, 2012), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), ShelXle (Hü bschle et al, 2011), PLATON (Spek, 2009), Mercury (Macrae et al, 2006), enCIFer (Allen et al, 2004) and publCIF (Westrip, 2010).…”

“…Hydrogen-bond geometry (Å , ). (Jandl et al, 2017). Such a phenomenon is also possible for hydroxytropylium chloride in solution.…”

“…The only crystal structure so far features 1,6-methano [10]annulene-fused hydroxytropylium perchlorate [see part (b) in Scheme 1], but is of low quality (Zhang et al, 2008; deposited with the Cambridge Crystallographic Data Centre as CCDC deposition number 654623). In this article, we report the crystal structure of hydroxytropylium chloride, (I), which we obtained serendipitously in our structural studies of halotropylium halides (Jandl et al, 2017). Halotropylium halides are relevant as precursors to carbocyclic carbene complexes and chlorotropylium chloride, in particular, has recently found application as a halogenating agent and activator in Swern-type oxidations (Ö fele et al, 2009;Nguyen & Bekensir, 2014;Nguyen & Hall, 2014;Nguyen & Lyons, 2015).…”

Ring Contraction of Tropylium Ions into Benzenoid Derivatives

“…Thus, we decided to employ bromotropylium bromide 10a (Schemes b,c), which we previously used as halogenating or esterification/amidation reaction promoters. − After an extensive optimization study, we found that a stoichiometric amount of 10a could react with the organometallic reagents from aryl halides 3 or react directly with arenes 4 to form intermediates 11 . As the second bromine was able to dissociate off from the newly formed cycloheptatriene ring, 11 could serve as a synthetic equivalent of tropylium salts 1 / 8 , eliminating the need for a lengthy synthetic sequence. Subsequently, the in situ generated 11 was directly subjected to the oxidative ring contraction conditions developed in Table to form the phenylated products (Scheme b,c).…”