- Electrophilic cleavage of cyclopropylmethystannanes: An experimental comparison of σ-σ and σ-π conjugation
-
(Chemical Equation Presented) Cyclopropylmethyltrimethylstannanes undergo electrophilic cyclopropane cleavage in chloroform with simple inorganic electrophiles (H+, SO2, I2) in a homologous reaction to the SE′ cleavage of allylic stannanes. The σ-σ conjugation between the carbon-tin bond and cyclopropane orbitals observed spectroscopically in the parent cyclopropylmethyltrimethylstannane is responsible for a rate enhancement of ca. 102 toward iodinolysis, relative to comparable alkyl stannanes. This acceleration is considerably less, however, than the ca. 109-fold rate enhancement provided by the corresponding σ-π conjugation in allylic stannanes. Methanol-tin coordination appears to reduce the activating influence of the metal, promoting methyl cleavage over cyclopropane fission with acid and iodine. Decreased σ-σ conjugation can also explain the decreased reactivity of cyclopropyltriphenylstannane compared with its trimethyltin counterpart. Cyclopropylmethylstannanes do not undergo the synthetically useful addition of aldehydes under conditions that facilitate the corresponding reaction of allylic stannanes.
- Lucke, Andrew J.,Young, David J.
-
p. 3579 - 3583
(2007/10/03)
-
- Ring-opening of Some Radicals containing the Cyclopropylmethyl System
-
Mono- and bi-cyclic radicals containing the cyclopropylmethyl system are readily generated by interaction of the appropriate halides with triphenyl- or tributyl-stannane.Each radical studied underwent ring-opening by fission of the more substituted βγ-bond.In the case of the secondary radical (12b) the new double bond was formed preferentially in the trans-configuration.Rate constants, which cannot be determined with high accuracy by this method, lie in the range 1E7-3E8 s-1 at 25 deg C.When generated by the flow method in the e.s.r. cavity α-hydroxycyclopropylmethyl radicals undergo β-fission followed by 1,5-hydrogen atom t ransfer to afford enoxyl radicals.The latter reaction occurs more slowly in water than in non-polar solvents.The rigid hydroxynortricyclyl (43) undergoes preferential fission of the less substituted βγ-bond, possibly because of the dipolar nature of the transition state.
- Beckwith, Athelstan L. J.,Moad, Graeme
-
p. 1473 - 1482
(2007/10/02)
-