Thermally-Induced 1,2-Shifts To Convert Olefins to Carbenes: Does Silicon Do It? If So, Why Not Carbon?

Abstract

Thermal isomerization of olefins to carbenes via a 1,2-silyl shift was examined by both experiment and theory. No evidence of this rearrangement was found for acyclic vinylsilanes, nor could electronic assistance by silicon be identified in cis, trans isomerizations. Serendipitous synthesis of a 2,4-dimethylene-1,3-disilacyclobutane allowed a kinetic examination of its gas-phase, thermal ring expansion to a 2-methylene-1,3-disilacyclopentene. The Arrhenius parameters (log A = 12.48, Eact = 54.09 kcallmol) are the first to be reported for an olefin-to-carbene rearrangement. The analogous all-carbon system failed to ring expand. Ab initio calculations revealed that this was opposite to any predictions which would be made from ring strain considerations. Calculations showed that for silyl migration the transition state was late and was actually the carbene, while for carbon migration the TS was early and considerably higher in energy than the resulting carbene. The 2-methylene-1-silacyclobutane rearrangement (ref 5) was reexamined to find that reversible ring opening to a 1,4-diradical occurred at temperatures below those required to ring expand via a carbene TS. Disciplines Chemistry CommentsReprinted (adapted) Abstract: Thermal isomerization of olefins to carbenes via a 1,2-silyl shift was examined by both experiment and theory. No evidence of this rearrangement was found for acyclic vinylsilanes, nor could electronic assistance by silicon be identified in cis, trans isomerizations. Serendipitous synthesis of a 2,4-dimethylene-1,3-disilacyclobutane allowed a kinetic examination of its gas-phase, thermal ring expansion to a 2-methylene-1,3-disilacyclopentene.The Arrhenius parameters (log A = 12.48, Eact = 54.09 kcallmol) are the first to be reported for an olefin-to-carbene rearrangement. The analogous all-carbon system failed to ring expand. Ab initio calculations revealed that this was opposite to any predictions which would be made from ring strain considerations. Calculations showed that for silyl migration the transition state was late and was actually the carbene, while for carbon migration the TS was early and considerably higher in energy than the resulting carbene. The 2-methylene-1-silacyclobutane rearrangement (ref 5) was reexamined to find that reversible ring opening to a 1,4-diradical occurred at temperatures below those required to ring expand via a carbene TS.

On the Question of Cyclopropylidene Intermediates in Cyclopropene-to-Allene Rearrangements − Tetrakis(trimethylsilyl)cyclopropene, 3-Alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes, and Related Model Compounds

“…Bearing in mind that cyclopropylidenes such as 5 , when generated from a diazocyclopropane7 or a 1,1‐dihalocyclopropane via a carbenoid,8 are prone to undergo ring opening to allenes, we conceived the possibility that in a 1‐silyl‐substituted cyclopropene 2 , a fast 1,2‐silyl shift9,10 might occur in the first step leading to a cyclopropylidene 5 ,11 which could then rearrange to the observed allene 8 . An example of a 1,2‐silyl shift leading to a carbene has been reported by Barton et al,12 who found that heating of 1,1,3,3‐tetramethyl‐2,4‐dimethylene‐1,3‐disilacyclobutane results in its rearrangement by consecutive 1,2‐silyl and 1,2‐H shifts via 1,1,3,3‐tetramethyl‐2‐methylene‐1,3‐disilacyclopent‐4‐ylidene to 1,1,3,3‐tetramethyl‐2‐methylene‐1,3‐disilacyclopent‐4‐ene. Of course, an allene 8 would also result from a fast 1,2‐silyl shift in the vinylcarbene 4 .…”

A new mechanism of nucleophilic vinylic substitution and unexpected products in the reaction of chlorotrifluoroethylene with [Re(CO)5]Na and [CpFe(CO)2]K

“…In case of free carbenes the 1,2-shift of the b-substituent to the carbene centre is usually a rapid process [13]. The ease of this rearrangement increases for b-substituents forming weak bonds to carbon, such as silicon [14,15]. The same must be true for transition metal groups, Re(CO) 5 and Fe(CO) 2 Cp.…”

“…As for the b-element substituted carbenes we can find some data only for silicon. It was shown (at MP2 or B3LYP levels of theory) that bsilicon substituted carbenes represent no minima on potential energy surface rearranging to the corresponding alkenes without an activation barrier [14,15].…”

1,3‐Dipolarophile Character of an Extremely Bulky Phosphaalkyne Mes*C≡P (Mes* = 2,4,6‐tBu3C6H2) Leading to the Formation of 1,2,4‐Diazaphospholes with Unique Hydrogen Bonding Properties

“…15–17 In the 13 C NMR spectroscopic data of 9 , the H‐substituted carbon atom C(sp 2 )‐3 has a slightly smaller 1 J PC constant ( 1 J PC = 53.7 Hz, C H) than the C‐5 ( 1 J PC = 59.7 Hz, C Mes*), which might represent a 5‐substituted 1 H ‐1,2,4‐diazaphosphole 18–20. The exclusive formation of 9 without a 1,2,3‐diazaphosphole isomer indicates that regioselective [2+3] cycloaddition of 1 with the CN 2 moiety proceeds under kinetic control by the bulky Mes* group21 followed by subsequent silyl group migration13,14,16 to gain higher aromatic stabilization. The N–Si bond in 9 is sensitive to alcohol and moisture, and desilylation took place to afford 10 by addition of ethanol (Scheme ) 13,22.…”