Alkenylidenes

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Two alkenylidene derivatives 149 and 563 have been reacted with several electron-deficient alkenes (Table 45) [36, 149]. 

The same arsenal of preparative methods has been applied successfully for the corresponding dinuclear derivatives of ethyne HC CH and dialkynes HC C-X-C CH, where X can be virtually any spacer unit.50-52,54 55 57 61 62 71 76-83 As mentioned in the introduction to this chapter, ethyne is readily converted into polymeric explosive AuC=CAu and its complexes (L)AuC=CAu(L), of which the families with L = R3P84 and L = RNC are particularly large (Chapter 7). With the unit X in (L)AuC=CXC=CAu(L) being an alkylidene spacer, flexible complexes are obtained, while with alkenylidene, alkynylidene, or arylidene units,57 rigid molecules (L)AuC=CXC=GA11(L) are generated. Specific examples are presented below in the context with the structural patterns of extended systems. 

The ruthenium compounds described above show a distinctly lower metathetic activity than the molybdenum alkenylidene complex 24 developed by Schrock et al. (Fig. 4, see also the chapter by R.R. Schrock, this volume) [18], which is another standard catalyst for any type of olefin metathesis reaction. However, they... 

Titanacyclobutanes also serve as useful synthetic intermediates the titanacycle 43, prepared by the intramolecular reaction of the alkenylidene complex 44, affords the a-dike-tone 45 and the other functionalized cyclic compounds by further transformations (Scheme 14.20) [35]. 

As noted above, titanocene-alkylidenes can be prepared using various methods and starting materials. Like the methylidene complex, higher alkylidene complexes are useful for the transformation of carbonyl compounds to highly substituted olefins. Ketones and aldehydes are converted into substituted allenes by treatment with titanocene-alkenylidenes prepared by olefin metathesis between titanocene-methylidene and substituted allenes (see Scheme 14.7) [17]. Titanocene-alkenylidene complexes can also be prepared from... 

The formation of highly substituted titanacyclobutenes utilizing titanocene-alkylidenes has been investigated (Scheme 14.33). Alkylidenetitanacyclobutenes 76 are produced by the reaction of titanocene-alkenylidene complexes with alkynes [76]. The alkenylcydopro-pane 77 can be synthesized by thermolysis of dicydopropyltitanocene in the presence of diphenylacetylene, which is assumed to proceed through formation of the titanacyclobutene 78 [25c],... 

Alkenylidene cyclopropanes react readily with 246 to yield 1,4-diazo-bicyclo[3,3,0]oxtanes, whereas methylidene cyclopropane reacts only very slowly with 246 to yield a 2 + 2 cycloadduct (73AJ1553). Compound 246 also reacts with 5-methylfuran-2(3//)-one in an acyl-ene reaction to yield 7-acetyl-6,7-dihydro-2-phenyl-2.ff-pyrazolo[I,2-a]-l,2,4,-triazol-l,3, 5-trione [80JCS(P1)843]. 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

The metal vinylidene intermediates discussed elsewhere in this chapter are limited to a single carbon-substituent on account of the 1,2-migration process by which they form from terminal alkynes. Alkenylidenes—vinylidenes bearing two carbon-substituents—are formed by nucleophilic addition of the (i-carbon of a metal acetylide to an electrophile (Scheme 9.16) [30]. 

When the electrophile is an alkyl halide, a C—C a-bond is forged thus, alkenylidene formation is irreversible. Vinylidene formation by 1,2-migration, on the other hand, is generally reversible. Because of this contrast, alkenylidenes can offer access to new catalytic reaction manifolds, in addition to unique molecular architecture. 

The Lee group originated rhodium alkenylidene-mediated catalysis by combining acetylide/alkenylidene interconversion with known metal vinylidene functionalization reactions [31], Thus, the first all-intramolecular three-component coupling between alkyl iodides, alkynes, and olefins was realized (Scheme 9.17). Prior to their work, such tandem reaction sequences required several distinct chemical operations. The optimized reaction conditions are identical to those of their original two-component cycloisomerization of enynes (see Section 9.2.2, Equation 9.1) except for the addition of an external base (Et3N). Various substituted [4.3.0]-bicyclononene derivatives were synthesized under mild conditions. Oxacycles and azacycles were also formed. The use of DMF as a solvent proved essential reactions in THF afforded only enyne cycloisomerization products, leaving the alkyl iodide moiety intact. 

Double cyclization of iodoenynes is proposed to occur through a Rh(I)-acetylide intermediate 106, which is in equilibrium with vinylidene lOS (Scheme 9.18). Organic base deprotonates the metal center in the course of nucleophilic displacement and removes HI from the reaction medium. Once alkenylidene complex 107 is generated, it undergoes [2 + 2]-cycloaddition and subsequent breakdown to release cycloisomerized product 110 in the same fashion as that discussed previously (Scheme 9.4). Deuterium labeling studies support this mechanism. 

Lee and coworkers went on to show that the concept of alkenylidene formation and functionalization by a single catalyst can be applied to other transformations. Under conditions similar to those reported by Trost and coworkers for vmylidene-mediated catalytic intramolecular hydroalkoxylation (see Section 9.2.3), alcohol 111 was transformed into a mixture of enol ethers with moderate selectivity for three-component coupling (Equation 9.9). 

Like alcohols, arenes can attack the electrophilic a-position of metal vinylidenes (see Section 9.4.6). Substrate IIS was transformed into tetracycle 117 in high yield, presumably via 6it-electrocyclization and subsequent rearomatization (Equation 9.10). To date, no intermolecular examples of metal alkenylidene-mediated catalysis have come to light. The extension of Lee s alkylative approach to catalysis by other metals may prove fmitfiil in this regard. 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R2 = Ac, Bz no reaction with R2 = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRj was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-alkenylidene-1,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRj = SiPh2t-Bu, a formal intramolecular [3+2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

The development of new procedures in this area in the last decade seems scarce. 3, 3-Disubstituted alkenylidenes are generated from 2,2-disubstituted 1,1-dibromocyclo-propanes under phase-transfer-catalysis (PTC) conditions and added to a variety of electron-rich alkenes to give vinylidenecyclopropanes in good yields (equation 85)143. 

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