Allylic Sites Research Articles

The Reactivity of t-Butyldimethylsilyl Ethers Towards Some Stabilised Carbanions

Abstract The t-butyldimethylsilyl (TBDMS) protection1–3 of alcoholic and phenolic hydroxy groups has attained much importance of late. Certain shortcomings, however, of this protective method have recently come to light. These include acyl migration4 during the fluoride anion mediated desilylation of acylated/silylated glycerols, and cleavage of TBDMS groups under catalytic hydrogenation conditions intended for benzyl ether cleavage4. Further, aryl anions of trimethylsilyl protected phenols have been shown5 to rearrange into o-trimethylsilyl-phenolates; similar reaction of a TBDMS protected phenol has been observed6 in this laboratory. The more labile trimethylsilyloxy group has also been reported7 to act as a leaving group, if present at an allylic or benzylic site, towards a variety of nucleophiles (H−, alkyl Grignards, malonate anion). However, benzyl TBDMS ether is stable towards ethylmagnesiumbromide under reflux6.

Capricious Stereoselectivities of Alkenylpotassium Formation under Kinetic and Thermodynamic Control Oxygen‐Triggered Configurational Equilibration of an Allyl Type Organometallic Compound

Abstract(Z)‐1‐Alkylated propenes undergo metallation at an allylic site more rapidly than do their (E)‐isomers, no matter in which configuration the resulting organometallic is thermodynamically more stable. A striking illustration is given by (Z)‐4,4‐dimethyl‐2‐pentenyl potassium which is formed 15 times faster than its (E)‐isomer, although the latter is clearly favored under equilibrium conditions. The configurational reorganization of alkenylpotassium compounds in tetrahydrofuran solution is very slow. At least in one case, however, traces of oxygen efficiently catalyse the Z/E equilibration.

Nuclear magnetic resonance studies. XXX.Hydrogen–deuterium exchange in bicyclo[3.2.1] and -[2.2.2]octenes through allylic and vinylic anions

Bicyclo[3.2.1]oct-2-ene undergoes hydrogen–deuterium exchange through allyl and vinyl carbanionic intermediates under strongly basic conditions, tert-BuO−/tert-BuOD) at 185 °C. Only allylic exchange had been observed in previous studies with tert-BuO−/DMSO. Since the 2-tert-butoxybicyclo[3.2.1]octanes are stable under these conditions an addition–elimination sequence cannot account for exchange at C-3. The relative rates of exchange at C-2, -3, and -4, and the stereoselectivity at the allylic site were determined by 2H nmr. For comparison, the rate of olefinic exchange in bicyclo[2.2.2]octene was also measured.

Mechanism of the thermal stabilization of poly(vinyl chloride) with metal carboxylates and epoxy plasticizers

AbstractThe chemical reactions responsible for the retardation of thermal discoloration in poly(vinyl chloride) (PVC) stabilized with a combination of an epoxy plasticizer and a heavy metal soap mixture of Group IIa and IIb metals have been studied. Allylic chlorides (a mixture of 4‐chlorohexene‐2 and 2‐chlorohexene‐3) are used as prototypes for the degrading segment of the polymer chain. The results confirm earlier reports that, when a mixture of a covalent and ionic metal soap is used as the stabilizer, the covalent moiety (e.g., Cd and Zn soaps) functions to esterify the allylic site of the degrading PVC model. A synergistic effect displayed by the ionic soap (e.g., Ca or Ba) is caused by a transfer of carboxylate ligands from the ionic soap to the depleting covalent species, which has been largely converted to the corresponding chloride. When an epoxy plasticizer model (cyclohexene oxide) is used in conjunction with the metal soap stabilizer, the preferred reaction is esterification. After a considerable build‐up of ester, an α‐chloroether, 2‐hexenyl 4‐(2‐chlorocyclohexyl) ether, is formed by the reaction of cyclohexene oxide with the PVC model. This reaction was found to be catalyzed by cadmium chloride. The esterification and etherification reactions provide an explanation for the synergism observed in the stabilization of PVC containing a combination of an epoxy plasticizer with a covalent and an ionic metal soap.