Small-ring stabilization

As an example of small ring stabilization, consider the reaction of either cobalt carbonyl or the mixed nitroso iron carbonyl Fe(CO)3NO with triphenyl-3-bromocyclopropene in acetonitrile at room temperature, [6-57]. Either the metal or the ligand can be altered with equivalent results. 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done 74PMH(6)199, p. 235). The CRSEs change from oxetane... 

Fragmentation reactions are particularly common in small rings. Relief of strain and the gain in stability in forming certain common fragments (such as N2, CO2), as felt in the... 

Perfluocoalkyl groups thermodynamically destabilize double bonds and small rings, but they can kineiically stabilize highly stramed molecules [75]. This remarkable perfluoroalkyl effect has made possible the isolation of stmctures that are uncommon m hydrocarbon chemistry, especially valence-bond isomers of aromatics and heteroaromatics such as 1, 2, and 3 [108],... 

The efficacy of RF in stabilizing small rings is well illustrated by the fact that the azete from trifluoro-l,2,3-triazine is considerably more reactive. Trapping experiments were unsuccessful and a polymer was isolated at room temperature. The dimer (41) forms an observable anion with CsF, which confirmed the endo structure [87CC1699 90JCS(P1)975, 90JCS(P1)983], In contrast, trifluoro-l,2,4,-triazine is resistant to vapor phase photoysis and flow pyrolysis [87JCS(P1) 1251]. 

Small-ring formation is also seen in the reaction of IKCOXPPh3)2Cl with [M(RNCXHXNR )ln (M = Cu, Ag R = Me, Et, i-Pr, t-Bu, cyclohexyl n = 2, 4 R = p-MeCftH4) to give complexes (III) where the Ir—M bond is stabilized by a bridging formamido grouper... 

Observed geometrical features or stabilities of small ring molecules containing an endocyclic double bond have been explained in terms of the Ji-a interactions [137, 138]. The n relaxation is predicted to stabilize such small ring unsaturated molecules of heavy atoms recently prepared [139-142],... 

Once we leave the realm of small-ring propellanes5 and post-1975 reports41,42 , it appears that it should be possible to convert suitably disposed sets of dispiran moieties into their respective propellane counterparts 39,40) because of the relative stabilities of the two families. By the same token, there are plenty of examples of acid-catalyzed rearrangement of propellanes, through which systems of yet greater stability may be obtained 5 41,42). 

The construction of cyclopentanones can take several avenues based upon the concepts evolving from use of small ring conjunctive reagents. A very simple one is a-cuparenone (183) which can be thought to derive by a ring expansion of a cyclobutyl carbinyl system as in 184 (Scheme 8) 110,1U). If X. is an anion stabilizing... 

When small rings are fused to other rings, the cis ring fusion is almost always much more stable than the trans ring fusion. The opposite is true only for saturated 6-6 or larger ring systems. (Make models to confirm this.) The order of stability of the three possible products shown above is cis-6-5 > trans-1 A > trans- 8-3. 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done (74PMH(6)199, p. 235). The CRSEs change "from oxetane (106 kJ mol-1) by -11 kJ mol-1 to oxelan-2-one (95 kJ mol-1) (corrected for electronic effects) and 4-methyleneoxetan-2-one (95 kJ mol" ). In contrast, an increase of 10 kJ mol 1 over the value for cyclobutane (111 kJ mol-1) is observed on going to both methylenecyclobutane and l,3-bis(methylene)cyclobutane. 

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