Strain relief

Regarding AH, one bond is broken and a similar bond is formed in a linear product. The linear molecule is unstrained, but the magnitude of the strain relief upon ring opening depends on the ring size. 

In contrast to the cyclopropyl- and cyclobutylhalo- (and acetoxy)carbenes, the cyclopentylhalo- and acetoxycarbenes (91), and their corresponding benzo derivatives, the indanylcarbenes (94), prefer 1,2-H shifts over 1,2-C shifts Eqs. 35 and 36.115 In the absence of strain relief to drive ring expansion, as with the cyclobutylcarbenes, 17, where kc exceeds ku, the 1,2-H shift is preferred.115... 

The effects observed for the shorter chains are easily understood as a strain relief phenomenon. Indeed, as Illuminati et al. (1981) have noted, replacement of suitably placed methylenes of diethers [2] with oxygens to give the corresponding benzo-crown ethers [20] transforms two 1,4-CH- -0 and (x—1) 1,4-CH -HC into 1,4-0 -O interactions, and 2(x—1)... 

FIG. 32. The first 10 layers of copper deposition on Au(lOO) proceeds in a smooth layer-by-layer mode with the formation of b.c.c. copper. During the deposition of the eleventh layer, a striped structure appears due to the onset of strain relief via a shear transformation. (From Ref. 462.)... 

The past decade saw introduction of the low-energy electron microscope (LEEM) (12). It was expected potentially to be capable of moderately high resolution (6-mn) video-rate imaging of surfaces and interfaces. This would make it capable of studying dynamic processes at surfaces, thin-fibn growth, strain relief, etching, absorption. 

I found today that stiff wires from the battery will break off the card supply plug pins as shown - use better connector or drill the card tie in place. Have been letting it flap in the wind, pins are too tender to do much of that need the solder joints expoxied for strain relief. Is idiot resistant in case you get the plug in backwards -fried a chip at 3 a.m. once. 

The spherical shape causes a pyramidalization of the C atoms and, therefore, a large amount of strain energy. Addition chemistry is driven by the strain reKef introduced by the formation of almost strain-free sp -C atoms. At a certain degree of addition this strain relief mechanism has to compete with new strain-introducing processes such as the increasing introduction of eclipsing interactions and the formation of planar cyclohexane substructures. 

The ET photochemistry of (IR, 35)-(+)-c/i-chrysanthemol (c/i-127) proceeds via nucleophilic attack of the internal alcohol function on the vinyl group with simultaneous or rapid replacement of an isopropyl radical as an intramolecular leaving group, forming 128. This reaction is a mechanistic equivalent of an Sn2 reaction the mode of attack underscores the major role of strain relief in governing nucleophilic capture in radical cations. 

There often is a correlation between the strain relief in a reaction and the rate of thermolysis, but other factors may also be of importance. The Cg propellanes 16 and 17 have quite different reactivities. Whereas 16 undergoes thermal cleavage at 360 17 undergoes cleavage at 25 °C. ° Aside from the difference in... 

The large difference in reactivity between cyclopropanes and cyclobutanes toward most electrophiles arises because there is little strain relief in the rate-determining step. The cleavage of C-C bonds by transition metal species leads to metallocy-cloalkanes, and if cleavage proceeds to a significant extent in the rate-determining step, the reactivities of cyclopropanes and cyclobutanes should become more comparable. 

Another example is the indirect acetonylation of a 1,2,3,4-tetrahydropyridine derivative at the (3-position of the enamide system [99]. The reaction involves a double alkylation to furnish an acetylcyclopropane which undergoes fragmentation on acid treatment. It is noted that polarity alternation and ring strain relief allow the facile and regiospecific C-C bond cleavage to proceed, the intervention of a cyclopropane intermediate also leads to disjoint system. This is a fundamental strategy [100] which has been frequently employed to gain access to compounds with disjoint functionalities. 

The cyclopropylmethyl ion 11 is unusually stable and has a 14kcal/mol barrier to rotation about the cyclopropyl-carbocation bond.69 In contrast, the corresponding cyclobutylmcthyl ion 12 quickly rearranges to a cyclopentyl cation. Here, some strain relief occurs in the rearrangement, but this is opposed by the conversion of the stable tertiary carbocation to the less stable secondary ion. Although rearrangement is the normal process for cyclobutylmethyl cations, there is one case 13 in which rearrangement does not occur, and a small rate acceleration is observed.70... 

The fundamental types of thermally and photochemically induced intramolecular transformations are summarized in Scheme 1. All reactions of this class involve intermediates in which aromaticity is lost hence they are most common in the classes of less aromaticity, i.e. polyhetero rings, cationic rings, rings containing carbonyl groups. However, polysubstitution, especially by bulky groups, can also induce reactions by strain relief in transition states. Most of the reactions known are photochemical. 

Further examples in which pentacoordinate phosphorus intermediates are implicated are the reactions of phosphonium salts. The cyclic phosphonium salt (161) shows a rate enhancement of 1300 1 in alkaline hydrolysis relative to either its six-membered or acyclic analogues (65ACS931). Ring strain relief in going from (163) to (164) seems to account for the higher rate of formation of the favoured intermediate (163 see Scheme 19). 

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