Grob fragmentation

A Grob fragmentation, named for the British chemist Cyril A. Grob, is an elimination reaction taking place when an electrofuge and nucleofuge are situated in positions 1 and 3 on an aliphatic chain.[1][2][3] The reaction product is an electrofugal fragment (carbonium ion, acylium ion), an unsaturated fragment (alkene, alkyne, imine) and a nucleofugal fragment (leaving group such as tosyl, hydroxyl).

Grob fragmentation

Contents

History

An early instance of fragmentation is the dehydration of di(tert-butyl)carbinol yielding trimethylethylene and isobutylene, a reaction described in 1933 by Frank C. Whitmore.[4] This reaction proceeds by formation of a secondary carbocation followed by a rearrangement reaction to a more stable teriary carbocation and elimination of a t-butyl cation.

Fragmentation reaction Whitmore 1933

Albert Eschenmoser in 1952 investigated the base catalysed fragmentation of certain beta hydroxy ketones [5]

Fragmentation reaction Eschenmoser 1952

The original work published by Grob (1955) concerns the formation of 1,5-hexadiene from cis or trans 1,4-dibromocyclohexane by sodium metal. According to reviewers Prantz and Mulzer in 2010 the name Grob fragmentation was chosen in more or less glaring disregard of the earlier contributions.[6]

Grob fragmentation 1955

Reaction mechanism

The reaction mechanism varies with reactant and reaction conditions with the fragmentation taking place in a concerted reaction or taking place in two steps with a carbocationic intermediate when the nucleofuge leaves first or taking place in two steps with an anionic intermediate when the electrofuge leaves first. The carbanionic pathway is more common and is facilitated by the stability of the cation formed and the leaving group ability of the nucleofuge. With cyclic substrates the preferred mode of elimination is anti.

Scope

An example of a Grob-like fragmentation in organic synthesis is the expansion of the Wieland-Miescher ketone (1 in scheme 2) to Thapsigargin.[7][8]

Scheme 2. Grob-like fragmentation in Thapsigargin synthesis

Another example is an epoxy alcohol fragmentation reaction as part of the Holton Taxol total synthesis.

The Grob fragmentation has been applied in the synthesis of the fragrance muscenone (scheme 3) [9][10]

Scheme 3. (+)-(R,Z)-5-Muscenone synthesis

See also

References

  1. ^ Grob, C. A.; Baumann, W. (1955). "Die 1,4-Eliminierung unter Fragmentierung". Helvetica Chimica Acta 38 (3): 594. doi:10.1002/hlca.19550380306.  edit
  2. ^ Weyerstahl, P.; Marschall, H. Comp. Org. Syn. 1991, 6, 1044-1065. (Review)
  3. ^ Strategic Applications of Named Reactions in Organic Synthesis Laszlo Kurti, Barbara Czako Academic Press (March 4, 2005) ISBN 0-12-429785-4
  4. ^ Whitmore, F. C.; Stahly, E. E. (1933). Journal of the American Chemical Society 55 (10): 4153. doi:10.1021/ja01337a042.  edit
  5. ^ Eschenmoser, A.; Frey, A. (1952). "�ber die Spaltung des Mesylesters von 2-Methyl-2-oxymethyl-cyclopentanon mit Basen". Helvetica Chimica Acta 35 (5): 1660. doi:10.1002/hlca.19520350532.  edit
  6. ^ Prantz, K.; Mulzer, J. (2010). "Synthetic Applications of the Carbonyl Generating Grob Fragmentation". Chemical reviews 110 (6): 3741–3766. doi:10.1021/cr900386h. PMID 20163188.  edit
  7. ^ Ley, S.; Antonello, A.; Balskus, E.; Booth, D.; Christensen, S.; Cleator, E.; Gold, H.; Högenauer, K. et al. (2004). "Synthesis of the thapsigargins.". Proceedings of the National Academy of Sciences of the United States of America 101 (33): 12073–12078. Bibcode 2004PNAS..10112073L. doi:10.1073/pnas.0403300101. PMC 514437. PMID 15226504. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=514437.  edit
  8. ^ Reaction details: Organic reduction of Wieland-Miescher ketone with sodium borohydride to alcohol 2 followed by functionalizion to mesylate 3 with mesyl chloride in pyridine. Then reduction of enone to allyl alcohol 4 with tri-tert-butoxyaluminum hydride in tetrahydrofuran followed by hydroboration with borane in THF to borane 5 (only one substituent displayed for clarity). The Grob fragmentation to 6 takes place with sodium methoxide in methanol at reflux. A methoxide group attacks boron giving the borate complex which fragments. As each boron atom can hold three substrate molecules (R), the ultimate boron byproduct is trimethyl borate
  9. ^ Fehr, C.; Buzas, A.; Knopff, O.; De Saint Laumer, J. (2010). "(+)-(R,Z)-5-Muscenone and (-)-(R)-Muscone by Enantioselective Aldol Reaction and Grob Fragmentation". Chemistry (Weinheim an der Bergstrasse, Germany) 16 (8): 2487–2495. doi:10.1002/chem.200902774. PMID 20077541.  edit
  10. ^ reduction of 1 with NMe4BH(OAc)3 in AcOH/water gves 2, mesylation (pyridine / TsOH) gives 3, elimination (tert-butanol / Potassium tert-butoxide) gives 4