Conformations cyclohexenes

Step a The predominant cyclohexene conformer places (i) the /-Bu equatorial and (ii) the allylic-Me pseudoaxial to minimize A1,3 strain, syn-Addition of the H-B bond at the least, after oxidation, to B. 

In cyclohexene itself, the barrier to ring inversion is certciinly low at 5.3 kcal/mol m.ns) and it appears that the preferred conformation is a half-chair 39 in equilibrium with its mirror image conformation 40. There have been calculations of the energies of cyclohexene conformations by Bucourt and Hainaut and by AUinger and his colleagues as well as suggestions on the basis of experimental evidence These... 

Step a. 5y -Addition of the H-B bond at the sterically less hindered face. To minimize A strain, the favored cyclohexene conformer has the r-Bu group equatorial and the allylic-Me group pseudoaxial. 

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

In the conformationally biased cyclohexene ring, the two dioxolane oxygens must assume fixed axial and equatorial positions. Coordination of the zinc carb-... 

Another reaction unique to conjugated dienes is the Diels-Alder cycloaddition. Conjugated dienes react with electron-poor aikenes (dienophiles) in a single step through a cyclic transition slate to yield a cyclohexene product. The reaction is stereospecific, meaning that only a single product stereoisomer is formed, and can occur only if the diene is able to adopt an s-cis conformation. 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

A potential major benefit of a cyclohexene-based template was the capacity to mimic more closely the putative transition state sialosyl cation by the placement of a double bond in the correct position. While the ring conformation in Neu5Ac2en 4 is very similar to the sialosyl cation transition state 5, these two stracmres are essentially isomers due to the fact that the double bond is either between the ring oxygen and C-2 in 5 or C-2 and C-3 in 4. The synthesis and biological evaluation of... 

In the case of the cyclohexene adduct (XIV), the most favorable conformation should be that in which the bulky thallium substituent is equatorial ... 

The reason for the trend toward boat TSs in cyclic systems is the introduction of additional steric factors. For example, addition of methyl and isopropenyl substituents leads to a TS in which the cyclohexene ring adopts a boat conformation, whereas the TS is chairlike. 

That the high degree of torsional and other types of strain inherent in the triplet states or trans conformers of cyclohexene and cycloheptene may be responsible for their photochemical behavior is suggested by the reactions of compound (50), a moderately twisted olefin according to molecular models. Compound (50) quantitatively yields bicyclo[3.3.1]non-l-yl acetate (51) within 15 sec after being dissolved in glacial acetic acid(83> ... 

A different synthetic access to a 1 -metallacyclopropene, which can be a versatile organometallic synthon, is displayed in Scheme 33. The mono-alkyne derivatives of W(IV)-calix[4]arene are easily accessible through the thermal displacement of cyclohexene from 32 using the appropriate acetylenes. The reaction led to complexes 34 and 172-174. The proposed 3-metallacyclopropene has been confirmed from the spectroscopic and the X-ray data. The H NMR data reveal a cone conformation of the calixarene with a four-fold symmetry, for which the... 

In complex 95, an unusual conformation of an organic molecule is present as shown in Fig. 43 (215). The substituents in cyclohexene rings generally assume equatorial positions. In 95, they are axial. The container molecules can also stabilize resonance extremes. Complex 96, bearing a p-nitrophenolate in the quinoid aci-form is an example (228). Again, these unusual coordination... 

P. W. Rabideau (Ed.) The Conformational Analysis of Cyclohexenes, Cyclohexadienes and Related Hydroaromatic Compounds, VCH, New York, 1989. 

A study of the catalytic oxidation of six stereoisomers of 5-cyclohexene-l,2,3,4-tetrol revealed448 that, in all cases, an allylic hydroxyl group is selectively attacked, and a consideration of their favored conformations suggested that quasi-axial groups are selectively dehydrogenated before those that are quasi-equatorial. 

The conformation A is a half-chair, in which all atoms, except for the C-6 atom, are virtually in a single plane, from which the C-6 atom deviates due to rotation about the N-O(l) bond. The C(3)-N(2)-0(l)-C(6) dihedral angle 01 characterizes the degree of rotation. The N(2)-C(3)-C(4)-C(5) dihedral angle 02 corresponds to rotation about the C(3)-C(4) bond, which can occur independently of the first process and leads to a deviation of the C-5 atom from the plane of the nitronate fragment in the direction opposite to the C-6 atom. For the conformation A, 01=24.3° and 02 = 6.5°,that is, A = /01 / - /02/, which characterizes the difference in the degree of rotation about two bonds, is 17.8° (for cyclohexene, A=0). It should be noted that the results of these calculations are... 

FIGURE 47. Three-dimensional structure of the cyclohexene ring of retinal in bR as determined by 2H NMR, relative to the membrane surface in the x-y plane. Analysis of the orientations of the three deuterium labeled methyl groups on the puckered ring (skew around C1-C6) indicates that the chromophore has a 6s-trans conformation around the C6-C7 bond. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

Recent studies on iodination and iodochlorination of bicyclo[4.3.0]nona-3,7-diene (78) and its derivatives have shown that the reactions depend strongly on the presence and position of the methyl groups on the cyclohexene double bond and on reaction conditions. When I2 reacted with c/s-bicyclo[4.3.0]nona-3,7-diene the only product was endo-d-fcw-S-diiodobrexane84. The addition of IC1 or IBr to the same diene gives exclusively unrearranged products in an identical conformation, with the substituents in the six-membered ring in fraws-diequatorial position (equation 78)85. 

With a variety of substituted cyclohexene oxides, e.g., the 1-methyl and 3-bromo derivatives, Bellucci et al. demonstrated that the substrates in the catalytic site adopted the conformation shown in Fig. 10.29 for 10.5 (3,4M... 

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