Carbonium ions cyclopropylcarbinyl

One of the most characteristic properties of carbonium ions is their great tendency to undergo rearrangements. These rearrangements include 1,2-alkyl shifts, hydride shifts, cyclopropylcarbinyl rearrangements, Wagner-Meerwein rearrangements, and others. 

The Nature of the Intermediate in Carbonium-Ion Type Interconversion Reactions of Cyclobutyl, Cyclopropylcarbinyl and Allylcarbinyl Derivatives. J. Amer. chem. Soc. 73, 3542 (1951). 

Roberts suggested that a set of charge-delocalized, rapidly equilibrating carbonium ions, which he called bicyclobutotiium ions, areThe first-formed ions from all three systems, In Scheme 3 are shown the bicvclobntnninm inns fm-mad fmm the deamination of 14C-labeled cyclopropylcarbinyl amine (Equation 6.32). According to Roberts, there would be two equivalent first-formed rarhonium... 

Using stereospecifically labeled precursors, Huang and Poulter have provided convincing evidence that squalene and botryococcene arise from a common cyclopropylcarbinyl intermediate, presqualene pyrophosphate 40). Huang and Poulter propose that the relative binding position of NADPH and the cyclopropane intermediate will control the outcome with respect to the final carbon-carbon bond that is produced 40). This work provides not only strong evidence for the existence of carbonium ion intermediates, but also implicates them in a wide variety of enzyme-catalyzed carbon-carbon bond formation processes. 

Radatus, B., and B. Fraser-Reid Cyclopropylcarbinyl-oxo-carbonium Ions, Part V. Synthesis and Chemistry of Some Cyclopropyl Glucopyranosides. Can, J. Chem. 50, 2909 (1972). 

PouLTER has demonstrated that chrysanthemyl derivative 16-OPy I asymmetrically deuterated at the carbinyl position ionizes with 91 2% stereoselectivity from a conformation with the leaving group aligned antiparallel to the 1,3-cyclopropyl bond (68). Since rotation about the C1-C2 bond in the cyclopropylcarbinyl carbonium ion intermediate should be slow relative to nucleophilic capture, this selectivity is reflected in the position of the deuterium atom in the vinyl group of the two major products. The stereoselectivity is attributed to stabilization of the transition state for ionization by charge delocalization into the vinyl group which should be more favorable in the a/i/f-parallel alignment. Kinetic data also indicate that the double bond facilitates the ionization process. 

It is of interest to note that artemisia alcohol (18) produced in the hydrolysis of 16-OPy I" is essentially completely racemic (>98%) (57). Apparently (18) is formed by nucleophilic capture of the acyclic allylic carbonium ion (29) rather than direct attack in the 3 position of the chrysanthemyl carbonium ion (28). Nucleophilic substitution upon cyclopropylcarbinyl cations to give homoallyl products occurs with inversion of configuration 74—75). In the case of (28), however, position 3 is highly hindered by the adjacent gem dimethyl groups thus collapse to the allylicly stabilized (29) is faster than direct substitution. The formation of a small amount of cw-chrysanthemol (27, 0.25%) is taken to indicate that allylic ion (29) recyclizes, at least in part, back to (28) and its cis isomer (30). 

The biogenesis of the thujane family of monoterpenes involves a homoallylic cyclization of the 4-terpinenyl carbonium ion (36) to a cyclopropylcarbinyl cation. Tracer experiments seem to be in accord with this scheme (56, 100). 

Although the homoallylic- cyclopropylcarbinyl cyclization is well-precedented in carbonium ion chemistry (101, 102) there seem to be no reports of the direct cyclization of the tertiary 4-terpinenyl carbonium ion. However, deamination of cyclohex-3-enyl amine and solvolysis cyclo-hex-3-enyl tosylate gives exo- and n /o-bicyclo[3.1.0]hex-2-yl derivatives as 6—43% of the products resulting from nucleophilic capture (101, 103, 104). The modest yield of bicyclic products in these reactions apparently is the result of competing nucleophilic capture prior to cyclization and hydride shift to the 2-cyclohexenyl cation. More efficient cyclization occurs in the acetolysis of 2-bicyclo[2.2.2]oct-5-enyl tosylate (49) owing to the rigid boat-like conformation of the precursor (105). The high efficiency of the base-catalyzed cyclization of the epoxide of 4-iso-... 

The reaction of sabinene (50) and a-thujene with aqueous perchloric acid results in a retro-biogenetic ring opening to terpinen-4-ol (37-OH) and the corresponding dienes (107). The high optical purity (80—85%) of the terpinen-4-ol indicates that this product is formed mainly by direct nucleophilic attack upon an intermediate cyclopropylcarbinyl carbonium ion. 

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