Bonding cyclopentenyl

In the case of cyclopentenyl carbamate in which a directive group is present at the homoallyl position, the cationic rhodium [Rh(diphos-4)]+ or iridium [Ir(PCy3)(py)(nbd)]+ catalyst cannot interact with the carbamate carbonyl, and thus approaches the double bond from the less-hindered side. This affords a cis-product preferentially, whereas with the chiral rhodium-duphos catalyst, directivity of the carbamate unit is observed (Table 21.7, entry 7). The presence of a hydroxyl group at the allyl position induced hydroxy-directive hydrogenation, and higher diastereoselectivity was obtained (entry 8) [44]. 

Heterocycles.—The phosphonium salt (59) is an effective three-carbon synthon, as demonstrated by its reaction with enolates of /9-keto-esters (Scheme 20) to give cyclopentenyl sulphides via an intramolecular Wittig reaction.63 Ylides are also intermediates in the synthesis of dihydrofurans (60) from the cyclopropylphos-phonium salt (61) and sodium carboxylates (Scheme 21).64 Cumulated ylides are very useful for the synthesis of heterocyclic compounds, e.g. (62), from molecules which contain both an acidic Y—H bond and a carbonyl or nitroso-function, as shown in Scheme 22.65... 

More recently Miron and Lee (1962) analysed the hydrocarbons removed from strong acid catalysts in some detail, and suggested unsaturated cyclic structures. These structures contain from one to five five-membered rings with various methyl and alkenyl substituents and a minimum of two double bonds per molecule. However, during their drowning procedures, as the acid is diluted, considerable polymerization occurs. This conclusion is based on work by Hodge (1963), who showed that cyclopentenyl cations are rapidly destroyed by alkylation at 10 m concentrations in 35% H2SO4. 

Bicyclic y-lactams.1 N-Allyltrichloroacetamides in which the double bond is associated with a cyclopentenyl or -hexenyl group undergo cyclization in the presence of CuCl in CH3CN or of Cl2Ru[P(C6H5)3]3 in QHg or C6H4(CH3)2. 

If both cr-bonds form at the same time, the reaction is pericyclic, but carbocations are capable of reacting with alkenes, and so the first bond may form to give the cyclopentenyl cation 2.51 in one step, with the second bond formed in a separate step 2.51 (arrows). The reaction remains a cycloaddition whether both bonds are formed at the same time or not, but it is pericyclic only if they are both formed in the same step, as is probable in this case. Fig. 2.6 shows a small selection of other reactions that may similarly be pericyclic. >... 

A ring-opening has also been seen, when the relief of strain in a cyclopropane makes it thermodynamically favourable—the cyclopentenyl anion 4.100 opens to the pentadienyl anion 4.101. This reaction had no option but to be disrotatory with the two hydrogen atoms moving outwards 4,98, since a trans double bond is impossible in the 6-membered ring. 

Eq. (3.145)].222,1043 They gave instead the cyclopentenyl cation. The lack of formation of bishomoaromatic ions from cyclopentenyl derivatives is mainly due to steric reasons. The planar cyclopentene skeleton has to bend into the chair conformation to achieve any significant overlap between the empty p orbital and the 7i-p lobe of the olefinic bond, which is sterically unfavorable. However, such conformation already exists in ions 581 and 582. 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1... 

When y-CH bonds are present in the R group of the alkynyliodonium ion, cyclopentenyl sulfones predominate. For example, the treatment of 5-phenyl-1-pentynyl(phenyl)-iodonium tetrafluoroborate with te/ra- -butylammonium benzenesulfinate in THF (i.e. homogeneous conditions) affords a moderate yield of l-phenylsulfonyl-3-phenylcy-clopentene and a low yield of the corresponding alkynyl sulfone (equation 51)32. With appropriately constructed alkynyliodonium ions, annulated cyclopentenyl sulfones are obtained (equations 52 and 53)32. 

In the simplest picture of optimum resonance-derived stabilization, the two carbons of the double bond, the amine nitrogen and all five atoms affixed to these three enamine atoms lie in a common plane. Much the same demand for planarity exists for the exocyclic isopropylide-necycloalkanes. For these hydrocarbons, one finds the rearrangement of 1-isopropylcyclohexene to isopropylidenecyclohexane is exothermic by 3.1 0.6 kJ mol-1 but that of 1-isopropylcyclo-pentene to isopropylidenecyclopentane is exothermic by 4.3 0.3 kJ mol -1 [D. B. Bigley and R. W. May, J. Chem. Soc. (B), 1761 (1970)]. If one ignores error bars, the 3.1 - (-4.3) s 7 kJ mol -1 net destabilization for the exocyclic double bond in the cyclopentene vs cyclohexene hydrocarbon case is essentially identical to the 51 — 45 6 kJ mol -1 for the cyclopentenyl and cyclohexenyl enamines. 

The ketone IR absorption of 3-methyl-2-cyclohexenone occurs near 1690 cm-1 because the double bond is next to the ketone group. The ketone IR absorption of 3-cyclopentenyl methyl ketone occurs near 1715 cm-1, the usual position for ketone absorption. 

Information on electron delocalization in the bicyclo[3.1.0]hexenyl cations is available from their reported NMR spectra Data obtained with a variety of systems point to a completely different charge delocalization pattern to that found with the homotropenylium ions. For example, Olah and colleagues have obtained the NMR spectrum of the parent ion"", 61, and compared this with those of 42 and 11. As can be seen from the data summarized in Scheme 18, the chemical shifts of the five-membered ring carbons of 61 resemble those of the cyclopentenyl cation. There is a considerable difference in chemical shifts, and hence charge distribution, at C(2), C(4) and C(3) of 61. There is no evidence for the fairly even charge distribution as is found for the homotropenylium and homocyclopropenium ions (see previous Sections III. A and III. B). It was also noted by Olah that the chemical shift of C(6) is consistent with large delocalization to this position, i.e. to conjugation of the allyl system of 61 with the external cyclopropyl bonds. 

The polysulfanes formed on reaction of DCPD with liquid sulfur have been studied by extraction of sulfur cement and analysis by LC, H-NMR, MS, and other techniques.The initial products are trisulfane and pentasulfane derived from DCPD by addition of S3 or S5 units to the norbomenyl double bond. These monomers are believed to further react with elemental sulfur to form low-molecular mass polymers (CS2 soluble), and on further heating form an insoluble material. The cyclopentenyl unsaturation of DCPD is much less reactive and is still present in the CS2-soluble products. endo-T>CP D reacts more slowly with liquid sulfur at 140 °C than eco-DCPD, while the cyclic trisulfanes of endo- and gxo-DCPD react at almost the same rate with liquid sulfur at 140°C. The stmctures of DCPD-S3, DCPD-S5, and the hkely stmcture of the low-molecular mass polymer, are shown in Figure 8. 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

Beyond the disrotatory or conrotatory stereochemical imperative which must accompany all Nazarov cyclizations there exists a secondary stereochemical feature. This feature arises because of the duality of allowed electrocyclization pathways. When the divinyl ketone is chiral the two pathways lead to dia-stereomers. The nature of the relationship between the newly created centers and preexisting centers depends upon the location of the cyclopentenone double bond. The placement of this double bond is established after the electrocyclization by proton loss from the cyclopentenyl cation (equation 5). Loss of H, H or in this instance generates three tautomeric products. The lack of control in this event is a drawback of the classical cyclization. Normally, the double bond occupies the most substituted position corresponding to a Saytzeff process. The issue of stereoselection with chiral divinyl ketones is iUustrated in Scheme 7. The sense of rotation is defined by clockwise (R) or counterclockwise (5) viewing down the C—O bond. Thus, depending on the placement of the double bond, the newly created center may be proximal or distal to the preexisting center. If = H the double bond must reside in a less substituted environment to establish stereoselectivity. 

In the cyclopentenyl series the relative stereoselection is variable. With a simple methyl substituent vicinal to the newly forming bond the selectivity is poor. This may be remedied with bulkier silyl groups (Scheme 19) but in unacceptable yields. In the synthesis of ( )-A -capnellene, Stille reports the formation of a single c/f, anrj,cjf-triquinane (16) in the final SDNC. By contrast in an approach to hirsutene, the SDNC produced triquinane (17) as a 2.7 1 mixture favoring the desired d.r,anr/,c i5 isomer. 

The other two double bonds in 7-23 have rearranged. The allyl double bond and the double bond in the new cyclopentenyl group are situated suitably for a [3,3] sigmatropic shift. 

The pericyclic process comes next and it is a Nazarov reaction (p. 962), a conrotator electrocyclic closure of a pentadienyl cation to give a cyclopentenyl cation. There is r stereochemistry and the only regiochemistry is the position of the double bond at the end of th -. reaction. Here it prefers the more substituted side of the ring. 

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