Chapter 6
Conformational Studies of Cyclobutylmethyl Cations 1
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V. Prakash Reddy , G. K. Surya Prakash , and Golam Rasul 2
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Department of Chemistry, University of Missouri at Rolla, Rolla, M O 65409 Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, C A 90089
α,α-Dicyclopropylcyclobutylmethyl cation is a unique cyclobutylmethyl cation, which is stabilized by neighboring cyclopropyl groups. It is persistent up to -40 °C in superacidic medium. The primary and secondary cyclobutylmethyl cations, however, are still elusive species. α,α-Dicyclopropylcyclobutylmethyl cation adopts a bisected conformation as shown by the variable temperature dynamic C N M R spectroscopy, in conjunction with density functional theoretical (DFT) calculations. D F T calculations for α,αdimethylcyclobutylmethyl cation, and endo- and exo- αmethylcyclobutylmethyl cations show that these carbocations also exist as bisected conformers. Primary cyclobutylmethyl cation, on the other hand, spontaneously collapses to cyclopentyl cation at B3LYP/6-311+G* level. 13
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© 2007 American Chemical Society
Laali; Recent Developments in Carbocation and Onium Ion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Introduction Stabilization due to neighboring group participation of carbocationic intermediates in small ring compounds, especially those involving cyclopropylmethyl cations is well documented in the early solvolytic studies. The bent 'banana-shaped' cyclopropyl ring C-C bonds are sterically well positioned for the stabilization of the adjacent carbocationic center so that dramatic rate accelerations were observed in the solvolyses of cyclopropylmethyl tosylates. The structure of parent cyclopropylmethyl cation has been under intense scrutiny over many years and is now well established, by ultra-low temperature C N M R and high level ab-initio theoretical calculations among other techniques, that it exists as degenerately equilibrating, σdelocalized bicyclobutonium ions, having a pentacoordinated carbon, in equilibrium with a minor amount of the bisected cyclopropylmethyl cation (Figure l). "
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1 3
1
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Figurel. Equilibrating bicyclobutonium ions and bisected cyclopropylmethyl cations.
Although not as strained as the cyclopropylmethyl cation, the corresponding homologous cyclobutylmethyl cation is relatively strained, and the cyclobutyl ring exists as boat-shaped puckered conformation to relieve some of the ring strain. Thus cyclobutylmethyl cations are expected to show substantial neighboring group stabilization of the adjacent carbocationic centers. In this chapter, we outline our recent progress on C N M R and DFT theoretical studies on the nature of the cyclobutylmethyl cations. Prominent earlier work by others, based mostly on solvolytic studies, is also outlined from a historical perspective. 9
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Solvolytic Generation of Cyclobutylmethyl Cations Early solvolytic studies by Winstein and coworkers clearly established the anchimeric assistance provided by the neighboring cyclobutyl ring in the
Laali; Recent Developments in Carbocation and Onium Ion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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solvolysis of the cyclobutylmethyl cations. Thus, solvolysis of nopinyl brosylate, a bicyclic analogue of the parent cyclobutylmethyl cation resulted in the formation of the products resulting from ring-expansion rearrangements: 7,7dimethyl-2-norbomanol, 2,2-dimethyl-2-norbomanol, and 4,4-dimethyl-2norbomanol (Figure 2). Further, kinetics of acetolysis of 1-cyclobutylethyl brosylate and dicyclobutylmethyl brosylate showed rate accelerations attributable to the neighboring group participation of the cyclobutyl ring(s). The combined kinetic and product studies led to the postulation of a nonclassical σbridged cyclobutylmethyl cation as an intermediate in these reactions. 9
H C Downloaded by UNIV LAVAL on April 23, 2016 | http://pubs.acs.org Publication Date: July 7, 2007 | doi: 10.1021/bk-2007-0965.ch006
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X /
CH
H C
3
3
X /
CH
3
aqueous acetone -OBs"
H C^CH 3
3
OH
OH H
Figure 2. Product distribution from the solvolysis of nopinyl brosylate, a cyclobutylmethyl substrate.
Dauben and coworkers observed unusual rate accelerations and ringexpansion rearrangements in the solvolysis of bicyclo[2.2.0]hexane-l-methyl pnitrobenzoate, in agreement with Winstein's proposed nonclassical cyclobutylmethyl cations (Figure 3). Thus, the rate of the solvolysis of bicyclo[2.2.0]hexane-l-methyl p-nitrobenzoate is 7 χ 10 times faster than that of the corresponding extrapolated rate for neopentyl derivative. The lack of scrambling of the 0 label in the solvolysis of the 0-labeled bicyclo[2.2.0]hexane-l -methyl p-nitrobenzoate confirmed the involvement to a significant degree of σ-participation from the neighboring cyclobutyl ring. Brown-Hammett correlation of 1-aryl-1-cyclobutylethyl p-nitrobenzoate gave a p value of -3.94 which is more positive than that of a model compound, 1-aryl- 1,2-dimethylpropyl p-nitrobenzoate (p = -4.65), which indicates substantial neighboring group participation of the cyclobutyl ring even in tertiary carbocationic systems. Solvent effects on the kinetics of the solvolysis 10
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Laali; Recent Developments in Carbocation and Onium Ion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
109 OPNB
CH OPNB
OH
2
aqueous acetone 19%
81%
Figure 3. Products of solvolysis ofbicyclo[2.2.0] hexane-1-methyl p-nitrobenzoate.
of the cyclobutylmethyl systems also show evidence for the σ-bridged cyclobutylmethyl cationic system. Ring expansion rearrangement was also observed in the solvolysis of the endo- and exo- bicyclo[2.1.1]hexane-5-methyl tosylates. Interestingly, the exoisomer gives 96% of the rearranged product, presumably going through the nonclassical cyclobutylmethyl cation. The reaction of endo- isomer presumably involves predominantly the S 2 pathway (k ) (Figure 4).
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N
s
4% Figure 4. Products of acetolysis of bicyclo[2.1. lJhexane-5-methyl tosylates.
Gajewski has demonstrated stereochemical integrity (retention of configuration) in the ring expansion rearrangements in the solvolysis reactions of cis- or /ra«j-2-methylcyclobutylmethyl brosylates (Figure 5). Thus, acetolysis of /raws-2-methylcyclobutylmethyl brosylate gives predominantly /r
