METAL-INDUCED REDUCTIVE CLEAVAGE REACTIONS - AN EXPERIMENTAL AND THEORETICAL (MNDO) STUDY ON THE STEREOCHEMICAL PUZZLE OF BIRCH AND VINYLOGOUS BIRCH PROCESSES

The stereochemical puzzle posed by the lithium-promoted Birch and viny logous Birch reductive cleavage of unsaturated benzyl ethers (BICLE; t akes place with retention of configuration of the sensitive (2) Delta double bond) and the corresponding cinnamyl analogs (VIBICLE; gives ri se to ca. 2.5:1 E:Z mixtures) has been approached by experimental and theoretical means. NMR experiments indicate that the pi-type organolit hium compounds resulting from these reactions do not form observable m ixed aggregates with the lithium silyloxide species generated alongsid e in the reaction and do not undergo observable isomerization at the t emperature of operation. A simplified model for contact, solvent-separ ated, and isolated ion pairs has allowed us to evaluate these complex reactions in great detail from a theoretical viewpoint, using the MNDO semiempirical method. Relevant features that come out from these comp rehensive studies, for which we have employed lithium naphthalenide (L iNaph) or lithium benzenide (LiBenz) as promoters, are as follows: (1) the lowest energy routes for cleavage are those involving contact ion pairs (CIPs) in which the lithium counterion plays a key role by acti ng as a handle (Lewis acid) to which the leaving group (-)OR adheres p rior to detachment; (2) the different haptomeric structures which resi de (local minima) in the potential hypersurface of either the so-calle d radical anion or the dianion routes show that haptomeric activation is key to understanding cleavage of the C-O bond which, eventually, ta kes place as a syn beta elimination of LiOR; and (3) reductive cleavag e of unsaturated benzyl ethers (BICLE) involves transient cation/anion radicals which undergo cleavage and subsequent reduction to the final organolithium with retention of configuration, in accordance with exp eriment, whereas that of vinylogous cinnamyl ethers (VIBICLE) involve transient dianion/dication species resulting from long-lived cation/an ion radicals. In good qualitative agreement with experiment, MNDO find s two diastereomeric routes (Delta Delta G = 0.2 kcal/mol) for cleava ge of(appropriately substituted) cinnamyl ethers, but only one for cle avage of the unsaturated benzyl analogs.