Alkenes, strained

Alkenes strained by twist or r-bond torsion, such as E-cyclooctene, exhibit much lower barriers due to relief of strain in the TS for the oxygen transfer step. While the epoxidation of symmetrically substituted alkenes normally involve a symmetrical approach to the TT-bond, the TSs for epoxidation of E-cyclooctene and E-l-methylcyclooctene exhibit highly asymmetric transition structures. The AAE = 3.3 kcalmol" for E- versus Z-cyclooctene is clearly a reflection of the relative SE of these two medium ring alkenes (16.4 vs 4.2 kcalmol ) ". The classical activation barrier (AE ) for the highly strained bicyclo[3.3.1]non-l-ene is also quite low (Table 10, Figure 26). In these twist-strain alkenes, the approach of the peracid deviates markedly from the idealized spiro approach suggesting fliat this part of the potential energy surface is quite soft. 

Triazines are reactive electron-deficient dienes in Diels-Alder reactions with inverse electron demand. They react with alkenes, strained double bonds, electron-rich and electron-deficient alkynes and C=N double bonds. In most cases it is found that the dienophile addition occurs across the 3- and 6-positions of the triazine ring, but ynamines can also add across the 2- and 5-positions. The reactions are still under active theoretical and practical investigation. 

If the released electrophile HSOs+ is not intercepted during the protodesulfonylation as in Figure 5.7, it reacts with the defunctionalized aromatic compound again. In this way an isomer of the original sulfonic acid may be obtained. The best-known example of such an isomerization is the conversion of naphthalene-1-sulfonic acid into naphthalene-2-sulfonic acid (Figure 5.8). Naphthalene-1-sulfonic acid is destabilized by the so-called peri-interaction, that is, the steric interaction between the Cs—H bond of the naphthalene and the substituent on Cl. The peri-interaction is thus a cis-alkene strain. Because naphthalene-2-sulfonic acid does not... 

Pericas and co-workers calculated the exothermicity of the cobaltacycle formation step and were able to correlate this with alkene strain (Table l).12... 

Figure 2.23 Azide cycloadditions with alkenes. Strained systems react extremely rapidly.

The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

The cyclohexadiene derivative 130 was obtained by the co-cyclization of DMAD with strained alkenes such as norbornene catalyzed by 75[63], However, the linear 2 1 adduct 131 of an alkene and DMAD was obtained selectively using bis(maleic anhydride)(norbornene)palladium (124)[64] as a cat-alyst[65], A similar reaction of allyl alcohol with DMAD is catalyzed by the catalyst 123 to give the linear adducts 132 and 133[66], Reaction of a vinyl ether with DMAD gives the cyclopentene derivatives 134 and 135 as 2 I adducts, and a cyclooctadiene derivative, although the selectivity is not high[67]. 

Hydroxy-THISs react with electron-deficient alkynes to give nonisol-able adducts that extrude carbonyl sulfide, affording pyrroles (23). Compound 16 (X = 0) seems particularly reactive (Scheme 16) (25). The cycloaddition to benzyne yields isoindoles in low- yield. Further cyclo-addition between isoindole and benzyne leads to an iminoanthracene as the main product (Scheme 17). The cycloadducts derived from electron-deficient alkenes are stable (23, 25) unless highly strained. Thus the two adducts, 18a (R = H, R = COOMe) and 18b (R = COOMe, R = H), formed from 7, both extrude furan and COS under the reaction conditions producing the pyrroles (19. R = H or COOMe) (Scheme 18). Similarly, the cycloadduct formed between 16 (X = 0) and dimethylfumarate... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

Bulky tert butyl groups are cis to one another on each side of the double bond and van der Waals strain destabilizes the alkene... 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

Allyl strain influences the conformation of Z-alkenes. A 4-substituted 2-alkene will prefer conformation C over D or E to minimize the steric interaction with the C—1 methyl group. ... 

Alkenes exhibit large strain energy when molecular geometry does not permit all the bonds to the two sp -hybridized carbons to be coplanar. An example that illustrates this point is -cycloheptene ... 

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

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