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Synlett 2003(8): 1210-1212
DOI: 10.1055/s-2003-39915
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Novel Diradical-Mediated Ring Opening Reactions

Georg Rüedi*a, Matthias Nagelb, Hans-Jürgen Hansena
a Organisch-chemisches Institut der Universität, Winterthurerstrasse 190, 8057 Zürich, Switzerland
Fax: +41(1)6356812; e-Mail: georg@access.unizh.ch;
b Swiss Federal Laboratories for Materials Testing and Research (EMPA), Überlandstrasse 138, 8600 Dübendorf, Switzerland
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Publication History

Received 15 April 2003
Publication Date:
11 June 2003 (online)

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Abstract

Thermo-isomerization of medium- and large-ring 1-phenylcycloalkanols 1e-i at around 650 °C in a flow reactor system delivers the open-chain phenones 4e-i. A reaction mechanism via an open-chain diradical intermediate, followed by an intramolecular hydrogen transfer is proposed. Thermo-isomerization of α-substituted 1-phenylcyclododecanols 6 proceeds regioselectively under the formation of ω-substituted phenones 7. Furthermore, this novel hydrogen transfer reaction provides a new access to ω-substituted fatty acids.

Key words

thermal ring opening - dynamic gas phase thermo-isomerization - diradical intermediates - radical hydrogen transfer - regioselectivity in homolytic cleavage

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4

1-Phenylcycloalkanols 1 were synthesized from the corresponding cycloalkanones by addition of phenyl lithium or phenyl magnesium bromide according to ref. 9.

7

Deuterium content was determined by mass spectroscopy.

10

Deuterium content was determined by 1H NMR spectroscopy, chemical shifts were determined by 2H NMR spectroscopy.

11

2,2,12,12,12-d 5-4g: 1H NMR (300 MHz, CDCl3): δ
(ppm) = 7.95 (dt, J = 7.0 Hz, 1.2 Hz, 2 H), 7.55 (tt, J = 7.3 Hz, 1.3 Hz, 1 H), (tt, J = 7.2 Hz, 1.3 Hz, 2 H), 1.72 (t, J = 7.1 Hz, 2 H), 1.59-1.26 (m, 14 H). 13C NMR (75 MHz, CDCl3): δ (ppm) = 200.47 (C), 137.04 (C), 132.71 (CH), 128.42 (CH), 127.95 (CH), 31.71 (CH2), 29.49 (CH2), 29.37 (CH2), 29.27 (CH2), 29.22 (CH2), 29.21 (CH2), 29.20 (CH2), 24.23 (CH2), 22.42 (CH2). IR (CHCl3): 3063 (vw), 2929 (vs), 2856 (vs), 1679 (s), 1599 (w), 1581 (w), 1467 (w), 1448 (m), 1272 (s)cm-1. MS m/z (rel intensity) 266 (12), 265 (20), 135 (9), 121 (100), 105 (78).

12

Intermolecular processes can be excluded under DGPTI-conditions.