Bridgehead olefin

Several explanations have been put forth in attempts to reconcile the differences in stereoselectivity between sterically hindered and nonhindered cyclohexanones, and general agreement has not yet been achieved. One suggestion is that equatorial attack by the reagent is opposed by a developing torsional interaction in the transition state between the entering group and the axial substituents at C(2) and C(6). 

Organometallic reagents that are more sterically demanding than sodium borohydride or lithium aluminum hydride add to cyclohexanones with observed stereoselectivities reflecting a balance between steric approach control and developing torsional interactions in the transition state. Steric approach control will be very 

Another system in which the effects of steric resistance to reagent approach have been studied systematically is the bicyclo[2.2.1]heptene ring system. The stereochemistry of a number of reactions has been studied with the parent hydrocar- 

These are just a few examples of a very general phenomenon. In the absence of overriding electronic and torsional effects, it is generally observed that reagents prefer the most unhindered pathway to the site of reaction. This generalization is an extremely useful one on which to base predictions of the stereochemistry of organic reactions. 

Allinger, S. J. Angyal, and G. A. Morrison, Conformational Analysis, Interscience, New York, 1965. 

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Chemical reactivity differences may be calculated if for the transition state of a rate-determining step of a reaction a structural model can be given which is describable by a force field with known constants. We give only two examples. Schleyer and coworkers were able to interpret quantitatively a multitude of carbonium-ion reactivities (63, 111) in this way. Adams and Kovacic studied the pyrolysis of 3-homoadamantylacetate (I) at 550 °C and considered as transition state models the two bridgehead olefins II and III (112). From kinetic data they estimated II to be about 2 kcal mole-1 more favourable than III. 

SOME ASPECTS OF THE CARBENE BRIDGEHEAD-OLEFIN CARBENE REARRANGEMENT... 

The carbene route to bridgehead olefins is a well established reaction and has been developed to a major method for the generation of bridgehead alkenes. The field has been recently reviewed by one of the main contributors to this area.1 The reversed reaction, the formation of carbenes from distorted olefins, has also been known for a long time. Distortion of the tt bond in alkenes is easily affected by photoexitation. In connection with those reactions, carbene chemistry has been observed and the field has also been reviewed some years ago.2... 

Only little information is available on carbene formation by rearrangement of bridgehead olefins, generated in thermal reactions. A prominent example is the rearrangement of bridgehead olefin 2, obtained as short-lived intermediate from bromosilane 1 by reaction with potassium fluoride in DMSO at 110°C. Carbene... 

The first example of a carbene bridgehead-olefin carbene rearrangement comes from the laboratory of Eaton.7 Cubylphenyldiazomethane (9), when photolyzed or heated in ethanol, afforded a mixture of ethers 10 and 11. The reaction follows the route shown in Scheme 2. It has been reviewed,1 and the exiting chemistry of the parent system 12 — 13 — 14, elucidated by Jones and... 

Some Aspects of the Carbene Bridgehead-Olefin Carbene Rearrangement 271... 

As our experimental investigations started by generating carbenes of type 24, 25, and 26, we will as well include results of calculations of these carbenes and their conversion to the corresponding bridgehead olefins. 

For carbenes 25 and 26 1,2-shifts of bonds a and b will lead to different bridgehead olefins, 17 and 27 for 26, and 1-norbomene 16 and bicy-clo[3.1.1]hept-l-ene (30) for 25, whereas carbene 24a will only give alkene 15. In accordance with experimental observations,19 calculations show that the more strained bond will migrate preferentially. 

Figure 3. Energy profile for the rearrangement of bridgehead olefins 16 and 30 to car-benes 31 and 32, based on calculations at the B3LYP/6-311G(d,p)//B3LYP/6-31G(d) level of theory.18...

The calculations indicate that only the most highly strained bridgehead olefins, specifically of the rrans-cyclopentene type, are candidates for a rearrangement to carbenes. Only at high temperature is there a chance for tams-cyclohexene analogues to undergo such a conversion. 

According to the results of our calculations, carbenes of type 52 are ideal models for the carbene 52—bridgehead olefin 53-carbene 54 rearrangement. The numbers of the C-atoms of formula 52 have been retained in formulas 53 and 54 to show, which bonds are broken and which C atoms migrate. 

Whereas the subsequent rearrangement of bridgehead olefin 83 can lead only to 82, the isomeric olefin 84 could afford either carbene 82 (route a) or carbene 85. Obviously, further experiments were necessary to find out the reason for the selectivity of product formation. 

The interpretation of the reaction cascade starting with carbenoids of type 73 and subsequently proceeding with carbenes 52 and bridgehead olefins 53 and ending with carbenes 54 would gain substantial support, if we could generate the carbene of type 52 by a different method and then show that similar products were obtained as via the oiganometallic route. 

Most significant is the formation of 92 in all thermolysis reactions of 91. This result is consistent with the sequence 91 -> diazoalkane of type 94 — carbene 52 - bridgehead olefin 53 - carbene 54 — H shift to afford 92. Formation of olefin 93 is best interpreted by H shift from the methyl group of carbene 52 (X=/-Bu, Y=Me) to the carbonic carbon, whereas 95 is formed by insertion of the carbenic center of 52 (X=Y=r-Bu) into the C-H bond of the r-Bu group. 

Summing up these results, it has been shown that the carbene bridgehead olefin-carbene rearrangement is also observed when diazoalkanes are the precursors for the generation of the bicyclo[l.l.l]pent-l-ylcarbenes of type 52. 

Carbenes do not usually undergo intermolecular C-C bond insertions [1172], Intramolecular 1,2-insertions into C-C bonds are, however, frequently observed [976], and have, e.g., been used for the synthesis of strained bridgehead olefins [1173]. Further examples are listed in Table 4.10. 

CONTENTS List of Contributors. Introduction to the Series An Editor s Forward, Albert Padwa. Preface, Randolph P. Thummel. Cyclooctatetraenes Conformational and ii-Elec-tronic Dynamics Within Polyolefinic [8] Annulene Frameworks, Leo A. Paquette. A Compilation and Analysis of Structural Data of Distorted Bridgehead Olefins and Amides, Timothy G. Lease and Kenneth J. Shea. Nonplanarity and Aromaticity in Polycyclic Benzenoid Hydrocarbons, William C. Herndon and Paul C. Nowak. The Dewar Furan Story, Ronald N. Warrener. Author Index. Subject Index. 

The close relationship 44a b) between bridgehead olefins and ( )-cycloaikenes had been discussed solely with regard to their strained characteristics, but, a careful examination revealed that this relationship should be extended to their chiral natures. 

The oxy-Cope rearrangement can be used to obtain bicyclic bridgehead olefins from spirocyclic precursors (equation II).2-3... 

Whereas FMO theory correctly predicts the regioselectivity for cycloadditions in simple alkyl-substituted olefinic systems,51,58 extension of similar calculations for cycloadducts (7a,b-lla,b)120 predicts the formation of regioisomer a, although, except in the case of 7 and 8, the b isomer is the predominant one. The differences between prediction and experiment in stereoselectivity have been attributed primarily to double bond rehybridization arising from double bond distortion in bridgehead olefins,142 which also explains their enhanced reactivity.96,120 Also double-bond deformation that will alter the normal mixing of alkyl substituent orbitals with localized rc-bond orbitals may explain the unexpected formation of 8b.120 Attempts to explain the formation of the b isomers, based on a two-step diradical mechanism, also have failed.120... 

The thermolysis of triazoline adducts from other polycyclic bridgehead olefins is analogous to that of the norbornene-azide adducts (Scheme 164) and affords a route for the synthesis of various aziridine ring sys-... 

V. BREDT S RULE, BRIDGEHEAD OLEFINS AND CYCLIC SPECIES... 

The first species may be compared with the normal (i.e. non-bridgehead) olefin bicyclo[3.3.1]non-2-ene, 22d. The former is less stable than the latter by some 80 kJmol-1 even though the second species has a —CH=CH— linkage and the first has a >C=CH— linkage. (For calibration, one may compare the relative stability of the hardly strained monocyclic olefins our primary archive shows that of the isomeric 3- and 1-methylcyclopentenes, the latter is more stable69 than the former by 11.2 1.0 kJmol-1.)... 

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