Alkynes cyclodimerization

In 2006, Chatani and coworkers published a rhodium vinylidene-mediated alkyne cyclodimerization incorporating allylamine (Table 9.13) [26]. Although RhCl(PPh3)3 alone was a competent catalyst for the stereospecific formation of ( )-3-alkylidene-3,4-dihydro-2H-pyrroles (69), the addition of an ammonium salt helped to suppress undesired alkyne homocoupling. Under optimized conditions, a variety of functionalized alkynes underwent cyclodimerization with allyl- or crotylamine in good yield. 

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

An interesting cyclodimerization of internal alkynes is apparently proceeding via metallation in the propargylic position, followed by an insertion of a second alkyne molecule, intramolecular carbometallation and o-bond metathetical protonolysis (Fig. 24) [65]. The reaction is slow even at elevated temperatures and the insertion is not regioselective in case of unsymmetrical alkynes. More hindered substituents such as Bu are not tolerated as the reaction stops at the metallation step. The reaction is more of a fundamental interest, as the cyclization pattern is different from the most common [2 + 2] route of existing protocols of transition-metal (in particular cobalt)-catalyzed alkyne cyclodimerization, which are significantly more versatile and synthetically applicable [194],... 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

Cp2Ti=CH2 (Cp = cyclopentadienyl) is liberated from a titanacyclo-butane primer by thermal cleavage in the presence of an iminoborane (product IX) (81). Alkynes may also undergo [2 + 2]-cyclodimerizations with unsaturated polar molecules. Rather polar alkynes seem to be favorable, e.g., ethoxyethyne, which can react with hexafluoroacetone to give the rather unstable product X (78). 

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). 

Table 9.13 Rh(l)-Catalyzed cyclodimerization of alkynes with allylamine.

The complex Re(CO)5(FBF3) also reacts with but-2-yne or pent-2-yne via cyclodimerization of the alkyne to give complexes with a coordinated methylenecyclobutene derivative.7... 

By using the hypersensitive molecular mechanistic probe 2-(2-methoxy-3-phenylcy-clopropyl)-5-methylhexa-2,4-diene in the 2 + 2-photocycloaddition of [60]fullerene, it was shown that the reaction proceeds via a biradical and not a dipolar intermediate.6 Zirconium-induced cyclodimerization of heteroaryl-substituted alkynes produces tetrasubstituted cyclobutenes with high regio- and stereo-selectivity.7 The ruthenium-... 

A similar reaction applied to propargyl alcohols in place of terminal alkynes led to the one-step catalytic head-to-head cyclodimerization of propargyl alcohols and to the formation of alkylidenecyclobutene derivatives [90] (Eq. 69). It was shown that the reaction occurs via cyclobutadieneruthenium and cy-clobutenylruthenium intermediates, dehydration and carboxylate addition. 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

Many of the complexes discussed in the previous sections are catalysts for alkyne oligomerization. In fact, alkyne dimerization and trimerization (see Cyclodimerization -tri-merization Reactions) at a cobalt center is recognized as one of the most synthetically useful catalytic reactions mediated by a homogeneous transition metal complex. The cobalt complexes most useful and extensively studied are CpCoL2, where L is CO, alkene, diene, or phosphine. The complex types... 

Nb and Ta alkyne complexes are of interest in part because they have found use in organic synthesis and cyclooligermerization (see Cyclodimerization -tri-merization Reactions) of alkynes. 

Arene complexes are usually prepared by the following methods (i) metal vapor synthesis (see Metal Vapor Synthesis of Transition Metal Compounds).(ii) AEAIX3 reduction of a metal halide in the presence of the arene and (hi) alkyne cyclotrimerization (see Cyclodimerization -tri-merization Reactions). Synthetic procedmes to obtain r] -arene derivatives have been reviewed by Pampaloni and Calderazzo. Over the past ten years, studies in this... 

Not unexpectedly, more reactive ligands like alkynes undergo acid-promoted transformations such as the cyclodimerization in equation (4) see Oligomerization Polymerization by Homogeneous Catalysis). 

Cyclodimerization of disubstituted alkynes R C=CR" mediated by this type of precatalyst (Ln = La, Ce Y is unreactive) seems to be limited to alkynes bearing at least one a-methyl group (R ) and a small second alkyl group (eq. (6)) [97]. Already use of R" = Et, n-Pr results in formation of two isomers. For example, formation of l,2-dimethyl-3-ethylidenecyclobutene from excess 1- butyne is complete in ca. 10 h at 80 °C, giving a TOF of 2 h. Unprecedented propargylic metalation/alkyne insertion are the key steps of the proposed mechanistic cycle. 

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