Vinylidenes Alkenylidenes

Vinylidenes, C=CHR, have been generated in the gas phase, but rearrange extremely rapidly to the alkynes, HC=CR. 

If there is an analogy between alkenes/alkynes and alkylidenes/alkyli-dynes, then the metal-based analogue of an allene would be a vinylidene (alkenylidene), LnM=C=CR2, with cumulenated M=C and C=C double bonds. Such complexes are well established and generally arise directly 

The 7c-acidity of vinylidene ligands also points towards reactions with nucleophiles at Ca. This will be particularly true if the metal centre is positively charged or co-ligated by other strong 7t-acids, which competitively compromise M— Ca retrodonation. In the case of monobasic 

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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. 

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. 

The unsaturated hydride Mn2(li-H)2(CO)6(lt-dppm) reacts with RCMHH (R = H, Ph, Bu , COOMe) to form either hydrido-alkenyls Mn2(M.-H)(ji2. n. l -CR=CH2XCO)6(ii-dppm) (R = H, Ph), or alkenylidene species 2(tt-H)(il2,tl3,Ti2-CR=CHBuO(CO)6(M.- pm) and (OC)3Mn(ji-dppm) (i-C=CHCOOMe Mn(CO)3. The photochemical reactions of Mn2(CO)8(p.-dppm) with alkynes results in vinylidene complexes. 35 The photochemical reaction between Mn2(CO)io and MeCsCNEt2 affords the ynamine complex 86. This isomeiizes into an aminoallene complex on warming. 36 The corresponding riienium complex Re2(CXD)8(p.-MeC2NMe2) is transformed into three complexes on thermolysis these are the (dimethylamino)allenyl Re2(CO)7(p-T 3-... 

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