Cyclo alkynes

Fig. 1. General methods for the synthesis and generation of angle strained cyclo alkynes...

The method has found some use in the generation and trapping of strained cyclo-alkynes (equation 72) . Stable cycloalkynes are also obtained, but the rearrangement is accompanied by competing side-reactions, which drastically reduce the yields of the cyclic acetylenes, as illustrated for bromomethylenecyclooctane... 

Cyclo-) Alkenes (Cyclo-) Alkynes (Cyclo-) Alkanes... 

Finally, cerium(IV) ammonium nitrate can serve as the radical source by itself, generating NO3 radicals If by photolysis. The addition of such radicals to cyclo-alkynes 20 initiates an interesting tandem reaction [12]. Transannular hydrogen atom abstraction by the vinyl radical 21 affords the intermediate 22, which undergoes a 5-exo cyclization to the radical 23. In the last step, the ketone 24 is formed by elimination of NO2 in moderate yield thus, the overall sequence can be described as a self-terminating radical reaction (Scheme 7). 

Hydroxy-THISs react with electron-deficient alkynes to give nonisol-able adducts that extrude carbonyl sulfide, affording pyrroles (23). Compound 16 (X = 0) seems particularly reactive (Scheme 16) (25). The cycloaddition to benzyne yields isoindoles in low- yield. Further cyclo-addition between isoindole and benzyne leads to an iminoanthracene as the main product (Scheme 17). The cycloadducts derived from electron-deficient alkenes are stable (23, 25) unless highly strained. Thus the two adducts, 18a (R = H, R = COOMe) and 18b (R = COOMe, R = H), formed from 7, both extrude furan and COS under the reaction conditions producing the pyrroles (19. R = H or COOMe) (Scheme 18). Similarly, the cycloadduct formed between 16 (X = 0) and dimethylfumarate... 

Monocyclic Aliphatic Hydrocarbons. Monocyclic aliphatic hydrocarbons (with no side chains) are named by prefixing cyclo- to the name of the corresponding open-chain hydrocarbon having the same number of carbon atoms as the ring. Radicals are formed as with the alkanes, alkenes, and alkynes. Examples ... 

The types of cycloadditions discovered for enamines range through a regular sequence starting with divalent addition to form a cyclopropane ring, followed by 1,2 addition (i) of an alkene or an alkyne to form a cyclo-cyclobutane or a cyclobutene, then 1,3-dipolar addition with the enamine the dipolarophile 4), and finally a Diels-Alder type of reaction (5) with the enamine the dienophile. 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

Table 7 [2+2] and [4+2] cyclo additions of cyclopentadienes 151 with alkynes (see Scheme 33)... 

Other examples of [2C+2S+1C0] cycloaddition reactions have been described by Herndon et al. by the use of chromium cyclopropyl(methoxy)carbenes. These complexes react with alkynes releasing ethene and forming cyclopenta-dienone derivatives, which evolve to cyclopentenone derivatives in the presence of chromium(O) and water [122] (Scheme 76). This reaction has been extended to intramolecular processes and also to the synthesis of some natural products [123]. These authors have also described another process involving a formal [2C+2S+1C0] cycloaddition reaction. Thus, the reaction of methyl and cyclo-propylcarbene complexes with phenylacetylene derivatives does not afford the expected benzannulated products, and several regioisomers of cyclopentenone derivatives are the only products isolated [124] (Scheme 76). 

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

A one-pot procedure [9] based on the cycloaddition of 4-aryl-2-silyloxybuta-dienes 7 and bisdiene 8 with alkynes, followed by oxidative aromatization of the cycloadducts, opened a route to polycyclic phenols without isolating the cyclo-hexadiene derivative intermediates (Scheme 2.5). 

Other intramolecular cyclizations can follow generation and fragmentation of cyclo-propylcarbinyl radicals. In the example below, the fragmented radical adds to the alkyne. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

Cyclopentane has the low chemical reactivity which is typical of saturated hydrocarbons, while 2-pentene is much more reactive. Similarly, ring structures containing double bonds, called cyclo-alkenes, can be shown to be isomeric with alkynes. 

It is interesting to compare the structure of the species (1 ) (X-ray diffraction study for PR3 = PMePh2) with those of the isolobal bridged alkyne complexes (1 ) (L = PR3 or L2 = cyclo-... 

On the basis of the infrared spectra, the alkenyl complexes are considered to have a structure of the type shown in Fig. 23a, with a bridging CO group. This structure is related to those of the rhodium cyclo-pentadienyl adducts Rh3(C5H5)3(alkyne)(CO) (137). In the rhodium... 

The intermolecular PKR of an allenic compound with an alkyne Co2(CO)6 complex takes place under very mild conditions when N-methylmorpholine oxide (NMO) is used as a promoter, giving rise to ( )-4-heptylidene-2,3-dipropyl-2-cyclo-pentenone in 69% yield (Scheme 16.48) [50-53],... 

It has already been seen (see Section II,B and V,E) that vinylketene complexes of cobalt6,95 and chromium22 react with alkynes to produce cyclo-pentadienones, indanones, and substituted phenols. It has been shown108,109,144,145 that similar products may be derived from the alkyne adducts 247. Indeed, when alkyl or aryl substituted alkynes were reacted with vinylketene complex 221.e, a mixture of organic and organometallic products was isolated. In the cases where the alkyne is attached to an aromatic substituent, the expected alkyne adduct (247.h and 247.i) is isolated in low yield. However, when the vinylketene complex was treated... 

Step 1 Find tbe root Identify the longest chain or ring in the hydrocarbon. If the hydrocarbon is an alkene or an alkyne, make sure that you include any multiple bonds in the main chain. Remember that the chain does not have to be in a straight line. Count the number of carbon atoms in the main chain to obtain the root. If it is a cyclic compound, add the prefix -cyclo- before the root. 

Diels-Alder reactions with alkynes as dienophiles have been known for a long time. Iminoboranes, however, will more readily cyclodimerize than react with dienes, and even the cyclodimers are generally superior to dienes in the competition for excess iminoborane. Among many attempts, therefore, only two reactions with iminoboranes as dienophiles have been successful, and in both of them the diene is cyclo-pentadiene [Eq. (55)] (9, 14). 

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