Driving Force 2 - Ring Strain

The driving force for ring expansion of 17-hydroxy-20-keto steroids probably comes from relief of strain accompanying conversion of trans-fused hydrindanes to tra/75-fused decalins. The greater susceptibility to rearrangement of 17/ -hydroxy-20-ketones as compared to the 17a-hydroxy-20-... 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

The transformation of the porphyrin intermediate 4 into a chlorin can be achieved after introduction of a C — C double bond into the 15-propanoate side chain of 4 to yield 5. The cyclization of 5 with participation of the 15-acrylic ester side chain under acidic conditions gives the chlorin 6 which is then transformed in a multistep reaction sequence into chlorophyll a. The driving force of chlorin formation from the porphyrin is believed to be the relief of steric strain at the sterically overcrowded porphyrin periphery which gives the desired trans arrangement of the propanoate side chain and the methyl group in the reduced ring. The total... 

When the bicyclic thiirene oxide 180164 is dissolved in excess furan, a single crystalline endo-cycloadduct (182) is formed stereospecifically (equation 71)164. This is the first propellane containing the thiirane oxide moiety. Clearly, the driving force for its formation is the release of the ring strain of the starting fused-ring system 180. In contrast, 18a did not react with furan even under forcing conditions. 

The mechanisms of these reactions are not completely understood, although relief of strain undoubtedly supplies the driving force. The reactions are thermally forbidden by the orbital-symmetry rules, and the role of the catalyst is to provide low-energy pathways so that the reactions can take place. The type 1 reactions are the reverse of the catalyzed [2 + 2] ring closures discussed at 15-61. The following... 

ROMP is without doubt the most important incarnation of olefin metathesis in polymer chemistry [98]. Preconditions enabling this process involve a strained cyclic olefinic monomer and a suitable initiator. The driving force in ROMP is the release of ring strain, rendering the last step in the catalytic cycle irreversible (Scheme 3.6). The synthesis of well-defined polymers of complex architectures such as multi-functionaUsed block-copolymers is enabled by living polymerisation, one of the main benefits of ROMP [92, 98]. 

The formation of the linear polymer from the cyclic monomer requires a decrease of the free energy. Because usually entropy is lost during polymerization, the main driving force for the ring-opening process is the release of the angular strain upon conversion of the cycles to linear macromolecules. Thus, a majority of three- and four-membered rings can be readily and quantitatively converted into polymers. 

The coordinated macrocycle readily reacts with alkoxide ions to yield products of type (71) (Taylor, Urbach Busch, 1969). In so doing additional flexibility is imparted to the ring which may reduce ring strain and, in part, provide a driving force for the reaction. Thus the coordinated imine carbons appear predisposed to attack by such nucleophiles. Based on this knowledge, elegant template syntheses of three-dimensional derivatives have been performed. The syntheses involved the reaction of [M(taab)]2+ (M = Ni, Cu) with the dialkoxide ions derived from bis(2-hydroxyethyl)sulphide or bis(2-hydroxyethyl)methylamine (Katovic, Taylor Busch, 1969). The products were demonstrated to be monomeric square-pyramidal complexes of type (72). The condensation... 

The inhibitory effects of polar functional groups are not nearly as pronounced when the substituent is attached to a strained cycloalkene, where the release of ring strain provides a significant driving force for its metathesis. The norbornene ring system polymerizes easily by ring opening thus, numerous functionalized polymers have been prepared by the sequence depicted in Eq. (61). Many of these polymers hold some potential for commercialization and hence the bulk of this work is reported in the patent literature. 

The cleavage of the cyclopropyl ring with its relase of the total strain is a powerful driving force. Electrophilic attack on the electron rich ring does provide one approach as shown in Eq. 13). The lack of selectivity in the cleavage of one of the... 

Whereas lactone annulation invokes a relief of strain of the four membered ring by migration of the ring bond to an electron deficient oxygen, a similar migration to an election deficient carbon creates a cyclopentanone synthesis (Eq. 73). The release of approximately 84 kJ/mole (20 kcal/mole) provides a strong driving force. Thus, the 1,1-cyclobutanone annulation of ketones translates into a 1,1-cyclopentanone annulation. 

The transformation from the radical cation to arenium ion is proposed to proceed in the manner shown in Figure 12. Here, the release of strain, which is calculated to be 42.9 kcal/mol at the B3LYP/6-31G(d) level, accompanying the contraction of the central six-membered ring to the five-membered ring, is considered to be an important driving force for this rearrangement. 

The driving force for the reaction is relief of the ring strain and the reaction is therefore irreversible and only works for strained alkenes. 

Apparently, the driving force for the ring opening is the relief of the strain in the spiro system and the formation of the stable carbonate double bond. The double ring opening is probably a concerted process from the initial radical addition product to the open-chain radical. Even though the spiro compound XI is an allyl monomer, it does copolymerize with a wide variety of comonomers. 

Somfai enhanced the driving force of some amide enolate aza-Claisen rearrangements by choosing vinylaziridines as reactants [24]. The additional loss of ring strain offered the advantage of running most of the reactions at room temperature to synthesize unsaturated chiral azepinones. Various substitution... 

A number of complexes, mainly of Ag(l) but also Rh(l) and Pd(ll), can catalyze the rearrangement and degradation of a wide variety of highly strained polycyclics, which may not react in the absence of catalysts. The products vary with the catalyst, the concentration of which features in the rate law. Again, the relief of ring strain which is a driving force for the reaction, appears aided by metal complexing. " ... 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

On the contrary, a-lithiated epoxides have found wide application in syntheses . The existence of this type of intermediate as well as its carbenoid character became obvious from a transannular reaction of cyclooctene oxide 89 observed by Cope and coworkers. Thus, deuterium-labeling studies revealed that the lithiated epoxide 90 is formed upon treatment of the oxirane 89 with bases like lithium diethylamide. Then, a transannular C—H insertion occurs and the bicyclic carbinol 92 forms after protonation (equation 51). This result can be interpreted as a C—H insertion reaction of the lithium carbenoid 90 itself. On the other hand, this transformation could proceed via the a-alkoxy carbene 91. In both cases, the release of strain due to the opening of the oxirane ring is a significant driving force of the reaction. 

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