Structurally mapping alkyl and amide basicity in zincate chemistry: diversity in the synthesis of mixed sodium-zinc complexes and their applications in enolate formation

Pasquale Mastropierro, Zoe Livingstone, Stuart D. Robertson, Alan R. Kennedy, Eva Hevia

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

Abstract

While alkali metal zincates have shown promising utility in organic synthesis, frequently these compounds are prepared in situ, and their identities remain blurred. Herein, the synthesis of a new family of sodium zincates featuring the chelating silyl(bis)amido ligand {Ph2Si(NAr*)2}2− (Ar* = 2,6-diisopropylphenyl) is presented. Using a synchronized bimetallic approach, 2-fold deprotonation of Ph2Si(NHAr*)2 (1) by mixed-metal base NaZn(HMDS)2R (2) [R = CH2SiMe3; HMDS = N(SiMe3)2] afforded the solvent-separated ion pair alkyl zincate [{Ph2Si(NAr*)2Zn(R)}−{Na(THF)6}+] (3), showing a clear preference for 2 to
react as bis(amide) base via its two Zn−N bonds rather than reacting using its alkyl group. Alternatively, a stepwise approach using two single-metal bases sequentially, NaR and Zn(HMDS)2 affords the tris(amido) zincate [{Ph2Si(NAr*)2Zn(HMDS)}{Na-(THF)6}+] (6). Formation of 6 takes place by initial monosodiation of 1, furnishing the amido-amine [{Ph2Si(NHAr*)(NAr*)-Na}2] (5), which in turn undergoes a co-complexation and deprotozincation step with Zn(HMDS)2. Using 2,4,6-trimethylacetophenone (7) as a case study, the ability of sodium zincates 3 and 6 to access zinc enolates was investigated,
affording sodium zincates [{(THF)NaZnR[OC(==CH2)Mes]2}2] (8) and [{(THF)NaZn(OC(==CH2)Mes)3}2] (9), respectively. These studies revealed that the chelating silyl(bis)amide {Ph2Si(NAr*)2}2− far from being an innocent spectator is an effective base for the deprotonation of 7, showing an unexpected superior kinetic basicity than the CH2SiMe3 alkyl group when part of sodium
heteroleptic zincate 3. The bimetallic constitution of enolates 8 and 9 contrasts with that of all-sodium [{(THF)Na(OC(==CH2)Mes)}4] (10) obtained by reacting the homoleptic alkylzincate NaZnR3 (4) with 7, with the concomitant elimination of ZnR2. Revealing the divergent behavior of Mg versus Zn in these bimetallic systems, reaction of 7 with the magnesium analog of 3, [{Ph2Si(NAr*)2Mg(R)}−{Na(THF)6}+] (11), produces magnesiate enolate [{Ph2Si(NAr*)2Mg(O(==CH2)Mes)(THF)}{Na-(THF)5}+] (12), where the chelating silyl(bis)amide ligand is retained and metalation of the ketone is actioned by the alkyl group.
Original languageEnglish
Pages (from-to)4273-4281
Number of pages9
JournalOrganometallics
Volume39
Issue number23
Early online date9 Jul 2020
DOIs
Publication statusPublished - 14 Dec 2020

Keywords

  • alkali metal zincates
  • organic synthesis
  • sodium zincates

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