2-Bicyclo hexyl systems

Paquette and coworkers have carried out a solvolytic study of a tricyclic system in which a 3,4-ethano group is attached to the 2-bicyclo[3.1.0]hexyl system. This was done to assess the importance of cyclopropyl versus cyclobutyl neighboring group involvement. Some relative rate data are given in Scheme 10 for 3,5-dinitrobenzoate hydrolyses in 80 % aqueous acetone at 115 °C. Product results are also shown. Deuterium labeling revealed the lack of any cyclopropylcarbinyl-cyclopropylcarbinyl cation rearrangements. Also, the results showed that cyclopropyl participation predominates over initial involvement of the cyclobutyl group. 

A study closely related to the previous except that the 2-bicyclo[3.1.0]hexyl system also contained a 3-spirocyclopropane as in 67 or a spirocyclopropane on the bridging methylene as in 68 was carried out by Ipaktschi and coworkers The rates and products... 

It is finally of interest to consider the solvolytic chemistry of 3,4-unsaturated and 3,4-benzo 2-bicyclo[3.1.0]hexyl systems. These are of special interest because they are sources of homologs of the antiaromatic cyclopentadienyl and inden-l-yl cations and as such should be destabilized antihomoaromatic systems. 

Solvolytic studies in the 3,4-benzo -2-bicyclo[3.1.0]hexyl system have been quite. The endo- and exo-3,5-dinitrobenzoates 73 and 74 have been found... 

Studies of the rates and products of hydrolysis of ewJo-and exo-2-bicyclo[4.1.0]heptyl derivatives 75 and 76 were first reported in preliminary form by Goering and Rubenstein and later in more detail by othersThe relative rates below for reactions in 80% aqueous acetone at 80°C reveal that as with the 2-bicyclo[3.1.0]hexyl system (fc[4.1.0] = ca.l50/c[3.1.0]) reaction rates are essentially independent of leaving group geometry. This indicates that both exo and endo isomers can ionize through bisected... 

Since the major reviews covering the parent 3-bicyclo[3.1.0]hexyl system appeared in the mid-1970s, several other papers dealing with the system have appeared. In one of these, Mjoberg and coworkers have concluded from analysis of far infrared and microwave data that the bicyclo[3.1.0]hexyl system exists in a boat form in its most stable conformation, and that the chair form in which cyclopropyl participation takes place must be vibrationally highly excited. Also, Walkowicz " has reported some further investigations of the influence of alkyl substituents on the chemistry of 3-bicyclo[3.1.0]hexyl tosylates. 

Wilt and coworkers have made further studies of aryl participation in solvolysis of the rrans-3-bicyclo[3.1.0]hexyl systems 121 and 122. Neither underwent significant long-range aryl migration. 

Brown et al. (1977) applied the tool of increasing electron demand to this bicyclic system with the solvolysis of a series of 2-aryl-2-bicyclo[2.1.1]hexyl p-nitrobenzoates in 80% aqueous acetone. The excellent log k rj a+ relationship, gave a p+ of —4.31, a value more negative than for the related 2-aryl-2-e rfo-norbornyl system where a-participation is believed to be absent. Also no support for u-participation in secondary bicyclo[2.l.l]hexyl derivatives could be found. 

In other studies, Clark and Smale have investigated the effects of a-phenyl substitution in the solvolyses of endo- and exo-bicyclo[3.1.0]hexyl through [5.1.0]octyl chlorides. For the endo derivatives, where the solvolyses proceed by a favored, concerted disrotatory mechanism in the parent systems, replacement of hydrogen by phenyl gives a relatively small rate enhancement (kph/ H = 1 )- other hand with the exo[3.1.0]... 

Of all of the parent 2-bicyclo[n.l.Ojalkyl systems, the 2-bicyclo[3.1.0]hexyl has been studied with the greatest number of different leaving groups and solvents and in the most laboratories. Thus, the chlorides have been hydrolyzed in 70% aqueous acetone at 250C142, amines have been deaminated in 70% aqueous acetone at 25 the... 

In an investigation stemming from a deamination study in the same system, Kirmse and Alberti examined the solvolyses of the 1-isobutoxy exo and endo-2-bicyclo[3.1.0]hexyl 3,5-dinitrobenzoates (60 and 61) in 70 % aqueous acetone. No kinetics were measured, and the only product which could be identified in low yield was bicyclo[2.1.1 ]hexane-5-one (62). 

Several tricyclic systems which are not usually treated as 2-bicyclo[3.1.0]hexyl derivatives, but which should be included in this section are as follows. TTie solvolyses of the exo and endo tricyclo[3.3.0.0 ]oct-3-yl p-nitrobenzoates (63 and 64) were examined by Johnson and coworkers in 70% aqueous acetone at 100°C. The relative rates and products of reaction are given in Scheme 12. It is seen that with the exception of higher endo-product selectivity, the behavior of this system is similar to that for the parent system. 

Another place in which a 2-bicyclo[3.1.0]hexyl moiety is incorporated within a tricyclic system is with the tricyclo[3.2.0.0r ]hept-3-yl derivatives 65 and 66 studied by... 

The nortricyclyl system 69 has been of interest for many years because even though it is formally a 2-bicyclo[3.1.0]hexyl derivative, it has a surprisingly low reactivity. This... 

As part of their key study of the reactivities of geometrically constrained cyclopropylcar-binyl systems, Rhodes and DiFate examined the solvolysis of the 2,4-trimethylene bridged 2-bicyclo[3.1.0]hexyl 3,5-dinitrobenzoate (71) in 50% ethanol. There is no other... 

The first studies in the 2-bicyclo[5.1.0]octyl system were reported by Cope and coworkers . However, they did not measure any reaction kinetics, and product studies were done under conditions where the initial products were not stable. Later in 1969, Friedrich and Wight studied the rates and products of hydrolyses of the endo- and exo-2-bicyclo[5.1.0]octyl 3,5-dinitrobenzoates 87 and 88 in 80% acetone. In contrast to the behavior seen with the [3.1.0]hexyl and [4.1.0]heptyl systems where rates and products are essentially independent of leaving group stereochemistry, the em/o-2-bicyclo[5.1.0]octyl 3,5-dinitrobenzoate (87) reacts 24 times faster than does the exo isomer (88). Both are. 

The proposal that cyclopropyl participation in solvolysis of the cis-3-bicyclo[3.1.0]hexyl tosylate (108) proceeds via a trishomocyclopropenyl cation intermediate (109) has been firmly established by product, deuterium scrambling and salt effect studies However, because the cyclopropyl participation in this system requires an unfavorable conformation, it is not apparent from simple kinetic studies. Thus, the cis-3-bicyclo[3.1.0]hexyl tosylate (108) has an acetolysis rate which is only nine times faster than that of the non-participating trans isomer, 110. However, when the 3-bicyclo[3.1.0]hexyl moiety is incorporated into a tricyclic or tetracyclic structure which locks it into a chair form, dramatically large rate enhancements are found. Some examples from the literature which are provided in the review by Haywood-Farmer are given below. These... 

Because of the observation that cyclopropyl participation is of major importance in the soIvolysis of 3-bicyclo[3.1.0]hexyl and related systems, a large number of investigations looking for evidence of cyclopropyl participation in other cyclopropylethyl systems have also been carried out. Most of the earlier work in this area has already been covered in detail in the reviews by Haywood-Farmer, Banciu and Barkhash . Thus, for the present review, the earlier work will just be summarized and any new information presented. 

The thermotropic behavior of liquid crystalline polynorbornenes also reach their limiting values at 50 repeat units or less [22, 182, 188-190]. For example, Fig. 9 demonstrates that the glass and nematic-isotropic transitions of both terminally and laterally attached systems level off at 25-50 repeat units, and correspond to the transition temperatures of the infinite molecular weight polymers. The same is true of the crystalline melting and smectic-isotropic transition temperatures of poly ( )-endo, exo-5,6-di [ -[4 -(4"-methoxyphenyl)-phenoxy]hexyl]carbonyl bicyclo[2.2.1]-hept-2-ene) [190]. 

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