Strained Carbocyclic Systems

Strained Carbocyclic Systems.—A large number of examples of catalysis of valence isomerization of strained ring compounds by transition-metal complexes has been considered during the period covered by this Report. The most commonly used metal centres are rhodium and silver, and much of the interest in this field is concentrated on determining and accounting for the differences, in effectiveness and in the nature of the products, between rhodium catalysts and silver catalysts. Thus, for instance, in the presence of [RhCl(nor)]2 or [RhCl(cod)]2, cubane (1) isomerizes to the diolefin (2), but in the presence of silver perchlorate cubane isomerizes to cuneane (3).  

It has been suggested that the number of /-electrons on the metal may be important in determining the mechanism and products of isomerization, but the observation that the d cation palladium(u) also causes the isomerization of cubane to cuneane, like the silver(i) but unlike d rhodium(i) complexes, eliminates this idea. The redox properties of the respective complexes may be relevant - rhodium(i) is much readier to undergo oxidative addition than silver(i) or palladium(ii). One factor which may be of considerable importance is the relative strengths of metal-carbon bonds. It is energetically much more favourable to insert rhodium(i) into a carbon-carbon bond than to insert silver(i) into such a bond. Though the different courses of isomerizations catalysed by rhodium(i) and silver(i) complexes may be ascribed to the operation of a non-concerted mechanism for the former but 

After these general introductory sentences, we shall mention the numerous specific examples of isomerization that have been reported recently. The reactions detailed are grouped according to the type of organic compound isomerized rather than by the transition metal. 

The rate law for the isomerization of cubane (1) to cuneane (3) in the presence of silver perchlorate is  

Rate constants k have been determined for several substituted cubanes, and their values show that isomerization rates vary greatly with the nature of the substituents. Thus the derivative (4) isomerizes 17 000 times as rapidly as cubane itself. Logarithms of k values correlate linearly with substituent a values, The rate law for isomerization of seco-cubane (5) derivatives in the presence of silver(i) is exactly the same as that for isomerization of cubanes [equation (2) above]. The products of isomerization are tetra-cyclo[3,3,0,0 , 0 ]octanes (6). Copper bronze also forms an effective 

Strained Carbocyclic Systems.— The rearrangement of polycyclic organic compounds catalysed by Ag+ and other metals has been reviewed. The silver(i)-catalysed isomerization of bishomocubanes (1) (R =Me or CHgOAc, R =C02Me) could follow four mechanistic pathways to give the two isomeric snoutane diesters (2) and (3) but the latter is formed preferentially (82% when R =Me 65% when R = CHgOAc) in both cases, The isomerizations of bishomocubanes substituted at the 9,10-positions, the C-4 corner, at C-4 and C-3, and at C-2 and C-3, to the structurally related snoutane follow the second-order rate law 

Ag+-catalysed isomerization of 4-substituted homocubanes (5) to norsnoutanes (6) also proceeds via pre-equilibrium complex formation and follows second-order kinetics. In the case R= Me adherence to Michaelis-Menten kinetics could also be demonstrated. C-4 Substituents capable of resonance interaction in the cationic transition state promote deviations in the rate of reaction relative to substituents which exhibit inductive effects only. With R=Bu bond-switching is reduced in rate, presumably because of steric inhibition of Ag+ attack on the homocubane to give an intermediate analogous to (4). Placement of deuterium or CDg at C-4 produces only a minor inverse kinetic deuterium isotope effect (kH/kD=0.97) which implies that a completely free carbonium ion intermediate is not involved and so argues in favour of a delocalized species analogous to (4). 

The mode of rearrangement of 1,8-bishomocubanes (1) catalysed by Rh and Pd complexes depends, not only on the metal and the ligands attached, but also on the level of substitution at the C-4 and C-5 positions of the carbocyclic species. Both snoutanes and the exo- and e fo-diolefins (7) and (8) can be produced, the amount of the former increasing in the order [Rh2(nbd)2(Cl)2] [Pd(I)a(PPh3)2] [Pd(Cl)2-(PPh3)2l [Pd(Ci)2(PhCN)2] which is the order of decreasing softness of the metals. Steric effects are significant and since the endo- som r (8) of the diene is formed preferentially in reactions of 9,10-disubstituted compounds, attack by the metal from the direction syn to the 9,10-substituted position must occur in most cases. 

The isolation of (10) from the reaction of [Rh2(Cl)a(CO)J and 1,3-bishomocubane also provides evidence for such a mechanism since its formation probably involves oxidative addition followed by carbonyl insertion.  

The ease of the silver(i)-promoted rearrangement of dehydronoriceane to 2,4-ethenonoradamantane according to empirical force-field calculations cannot be rationalized in terms of strain release in the former and stabilization of a Agt-dehydronoriceane intermediate complex by hyperconjugative effects is considered to be responsible.  

Strained Carbocyclic Systems.— Addition of [Rh(CO)2Cl]2 to methanol solutions of tricyclo[4,l,0,0]heptane (1) (and similar compounds substituted at the bicyclobutane fragment) caused first a rapid fall then a slower rise in pH. The proportion of the products (2) and (3) was found to depend on the rate of addition of solutions of the hydrocarbon to [Rh(CO)2CI]2 or on the presence of NaHCOs, slower addition or NaHCOs favouring (3). These and i.r. results suggest the intermediate (4), from 

The irons- (32) and cw-2-methylcyclopropanes (33) are isomerized by trans-[NiCl2(PBu 3)2]-BuiaAlCl and [Ni(C2H4) P(o-tolyl)3 2]-HCl catalysts to yield mixtures of /ra f-2-methylpenta-l,3-diene and transjrans- and trans,cis-h xa.-2,4-dienes. It was found also that reaction rates and product distribution were markedly 

In the case of Rh catalysis, it is generally agreed that the reactive intermediate has a formal carbenoid structure (see tricycloheptane isomerization later). In the presence of [Rh(nor)Cl]2, (1 R = R = H or R = Me, 

As in the case of the bicyclobutanes, products of the isomerization of tricyclo[4,l,0,0]heptane (18) vary according to the catalyst. Thus, Table 1 

We have noted above the markedly different products obtained in the catalytic isomerization of bicyclobutanes and tricycloheptanes with Rh and Agk It is interesting, then, that the catalytic isomerization of (30) and (31) to give (32) in chloroform is promoted by both [Rh(CO)2Cl]2 and AgC104, 

The bicyclopropenyl compounds (46) (R = Me or OMe) are catalytically isomerized by silver(i) to (47) (R = Me or OMe) and (48) or (49). The different products are rationalized in terms of initial retrocarbene fission and an overall mechanism in which the bridgehead substituent R can dictate the 

The mercury(ii)-catalysed rearrangement of the cyclodeca-l,2,5,8-tetraene (51) in the presence of HOAc gives (52), (53), and an unassigned product. Transannular rearrangement via a mercury-carbene intermediate is favoured.  

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Unstable metallacycloalkane intermediates play an important role in a variety of transition-metal-catalyzed isomerizations or rearrangements of strained carbocyclic systems and olefin metathesis reactions ... 

It has been suggested that the strain of the [3.1.0] ring system plays a role in facilitating the reaction. In a carbocyclic system, the [3.1.0] ring system opens 200 times faster than the analogous [4.1.0] ring system. ... 

Because cation stabilization makes an important contribution, N-, S-, and O-substituents are often used to support C-C bond cleavage [109, 144]. C-C bond scission is readily achieved in strained carbocyclic [145] or heterocyclic [146] ring systems, with many examples stemming from cyclopropyl sulfides [147]. 

Facile cycloalkenylations of carbonyl groups have been carried out with cyclopropylphosphonium fluoroborates . Complex carbocyclic systems, such as the sesquiterpene a-cedrene, can be effectively constructed by cationic cyclization Epoxide cleavage which follows upon dissolving metal reduction of proximal cyclopropane rings makes possible the ready synthesis of functionalized strained ring compounds inaccessible by other methods... 

The 10-membered carbocyclic enediyne 3.437 and its N- and O-analogs, 3.438 and 3.439, respectively [218], undergo the Bergman cyclization at room temperature, in contrast to the thio-analog 3.440 [196], which is stable under these conditions. When the strained enediyne system is condensed within a ring, it becomes quite stable at normal temperature as in the case of the condensed 10-membered enediynes 3.441-3.443 (Scheme 3.6) [24, 219, 220]. 

A sequence of an inter- and an intramolecular Heck coupling has been used to construct the 26-membered carbocyclic compound 60 from an acyclic precursor 59, which presents half of the target molecule (Scheme 20). The first step of this twofold coupling is favored to occur inter- rather than intramolecularly, because the latter would lead to a highly strained 13-membered ring system with a biaryl unit and a frans-configmed double bond. In the cyclizing second step, polymerization is disfavored by the orientation of the two side arms in the 3- and 3 -positions of the initially formed 1,1 -biaryl derivative. 

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