Thermal Rearrangement of Silylmethyl Acetates

Abstract

( 9 b ) , 63798-1 1-8; ( 9 c ) , 63798-03-8; ( 9 d ) , 63798-04-9: (9e), 63798-05-0; cycloheptatriene, 544-25-2 (2) and ( 3 ) , respectively. HeatingWe attempt to interpret the mechanism of this smooth valence i~omerization [~] with the aid of the following findings: 1 ) Crossover experiments prove that both the methyl and also the acetoxy groups undergo intramolecular migration to an extent exceeding 95%.2) The rate of rearrangement is practically independent of solvent polarity: k,,, for ( I a)+ ( 3 a ) : 1.0 (benzene); 0.9 (acetonitrile); 1.3 (propylene carbonate).3) The activation parameters for 4) The trifluoroacetate rearranges faster than the acetate: k(tbi/k(iai= 564.5) The fluorene derivative (1 c) is stable under the reaction conditions; only on raising the temperature (230°C) does slow decomposition set in. The entropy of activation for (Ia)+(3a) and the unexpectedly high thermal stability of ( I c ) can be rationalized neither by a concerted dyotropic process nor by a two-step free-radical mechanism. An ionic mechanism involving contact ion pairs (7) and (8) also appears unlikely on account of the lack of solvent dependence. Without precedent, and yet in accord with our findings, is a rate-determining intramolecular migration of the nucleofugal group to the silicon to form an inverted ylide ( 9 ) , which transforms via a fast 1,2 shift into (3). The differing behavior of ( 4 a ) and ( 4 b ) could be the result of stereoelectronic factors at pentavalent silicon. The results of solvent studies could be interpreted by assuming the formal positive charge at the cationic center of ( 9 ) to be delocalized as a result of hyperconjugation with pentacoordinate silicon. The question whether negatively charged hypervalent groups can stabilize carbenium ions has not previously been posed in general terms. However, it is known that compounds of pentacoordinate silicon are stable only if electronegative ligands effect a d-orbital contraction171. In this connection, carbenium ions are ideal substituents. I R 3 (9) Two-step dyotropic mechanisms forming inverted ylides are therefore conceivable for related rearrangernent~l~, ']-this working hypothesis is currently being tested.

Unsaturated carbene ligands: (.eta.2-vinyl)-, (vinylcarbene)-, and (.eta.4-butadienyl)tungsten dithiocarbamate complexes

“…;4-C4Ph4H)W=S (0.8 g, 50%). 4H NMR (CD2C12): 5 7.42 (d, Ph), 7.22 (t, Ph), 7.1-6.8 (m, Ph), 6.55 (d, Ph), 4.09 (s, CflPh, 14% 183W, d, Jwh = 15 Hz), 3.7-3.6, 3.4 (m, 4 H, CH2CH3), 1.23,1.12 (t,3 H each,CH2CH3).…”

“….433 (7) A.Table IV. Selected Bond Angles (deg) for (Me2NCS2)3W(n2-C4Ph4H)S(l)-W-S(2)67.38 (6) S(4)-W-C(ll)89.2 (2) S(l)-W-S(3)70.77 (6) S(5)-W-S(6) 68.49(5) S(l)-W-S(4)137.92 (6) S(5)-W-C(10) 86.5 (2) …”

Dyotropic Reactions: Mechanisms and Synthetic Applications

“…The thermal quantitative exchange of acetate and methyl groups in 60 is closely related to the above transformations (Scheme 24). 47 Mechanistic studies indicate first-order kinetics with activation barriers of ca. 34 kcal/mol and ∆S q ca.…”