E. Wissing

Influence of bulky substituents on the regioselective group-transfer reactions of diorganozinc compounds with N,N'-bis(2,6-di-isopropylphenyl)-1,4-diaza-1,3-butadiene

Abstract Diorganozinc compounds R 2 Zn (R=alkyl or aryl) react with N,N ′-bis(2,6-di-isopropylphenyl)-1,4-diaza-1,3-butadiene, (i-Pr 2 Ph)NCHCHpN(i-Pr 2 Ph) (i-Pr 2 Ph-DAB) to give thermally unstable 1:1 coordination complexes R 2 Zn(i-Pr2Ph-DAB), which subsequently undergo a slow regioselective alkyl or aryl group-transfer reaction from the zinc atom to an imine-nitrogen or a carbon atom of the NCCN system of the i-Pr 2 Ph-DAB ligand. In the case of R=methyl, n-propyl, n-butyl, s-butyl, neopentyl and benzyl, C-alkylation occurs with a subsequent 1,2- hydrogen shift in the amino-imino skeleton affording RZn[(i-Pr 2 Ph)N-CH 2 -CRN(i-Pr 2 Ph)], whereas for R=t- butyl the C-alkylated product t-BuZn[(i-Pr 2 Ph)NCH(t-Bu)CHN(i-Pr 2 Ph)] is stable. Surprisingly, diphenylzinc reacts with i-Pr 2 Ph-DAB exclusively to give the N-arylated product PhZn[(i-Pr 2 Ph)NCHCHN(Ph)(i-Pr 2 Ph)].

An unexpected 1,2-alkyl shift within a chelate bonded organoaluminium-enamine *

The organozinc-enamines EtZn[R’N-CH=CH-N(Et)R’] (R =‘Bu (la), ‘pent (2a), ‘Pr (3a)) react with Et,Al via a transmetallation reaction to the corresponding organoaluminium-enamines Et,Af[R’N-CH=CH-N(Et)R’l (R’ =‘Bu (1~1, ‘pent (2~)) and EtzZn. Prior to the transmetallation reaction initial 1: 1 zinc-enamine-trialkylaluminium adducts are formed. Compounds lc and tc are thermally unstable and rearrange via a 1,Zalkyl shift partly in combination with a hydrogen shift, to EtzAl[R’N=CH-C(H)R’-NEtl (R’ =‘Bu (la), ‘pent (2d)) (70%) and Et,Al[R’N-CH,CR’=NEtl (R’ =‘Bu (le), ‘pent (2e)) (30%). The driving force for the alltyl shift, which is restricted to tertiary alkyl groups, is release of steric strain in the five-membered NCCN aluminium chelate ring.

Condensation reactions of α-amino-zincenamines with aldehydes : application to indolizines

The zinc-enamine 1 is easily accessible via the reaction of Et2Zn with t-BuNCHCHNt-Bu and is reactive towards aldehydes. The condensation reaction of 1 with 2-pyridine-carboxaldehyde results in the thermally instable zinc-aldolate 2b that subsequently rearranges to the indolizine 4.

New developments in zinc-mediated organic synthesis

Results of current studies on mechanism of the reactions of diorganozinc compounds with 1,4-disubstituted-1,4-diaza-1,3-butadienes are discussed. Independent syntheses of some of the proposed cationic and neutral (radical) organozinc intermediates are described as well as the unique reactivity of these organozinc species which leads to selective synthesis of e.g.β-lactams, 2- and 3-pyrrolidinones and indolizines.

1-tert-Butyl-2-(tert-butylaminomethylidene)-3-pyrrolidinone, C13H24N2O

The eneamine moiety on the pyrrolidinone rig is coplanar to the carbonyl function, with an E configuration stabilized by an intramolecular N-H...O bond