Alkyne-vinylidene rearrangement

Alkylidenecyclopentene-l,4-diones (cf. 12, 130).2 The cobaltacyclopente-nedione (2) formed on reaction of a cyclobutenedione with 1, when complexed with dimethylglyoxime, reacts with 1-alkynes to form 5-alkylidenecyclopentene-1,4-diones (3). The reaction involves an alkyne-vinylidene rearrangement. Ben-zoquinones are usually formed in minor amounts. 

Often, selectivity for a vinylidene-mediated pathway is heavily dictated by substrate structure. It is especially true in the case of hetero-atom substituted alkynes that Jt-alkyne/vinylidene rearrangement is driven by a reduction in steric interactions at the metal center. 

Figure 5.48 Alkyne-vinylidene rearrangements E = H, SiR3, SnR3, SR, SeR...

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

As a supporting evidence, it is well-known that the electron-rich 0 6-arene)Ru complex of terminal alkyne 428 rearranges easily by the treatment with NaPR, of the )/ -vinylidenc complex 429, which is a strongly electrophilic carbene complex. Attack of ROH on the carbene carbon generates the the alkoxycarbene complex 431 via 430 [166]. Formation of ketone 427 by attack of the allylic alcohol is understanable by this mechanism. Formation of Ru-vinylidene complex 429 from the terminal alkyne has been proposed as the intermediate 432 of the reaction of terminal alkyne, amine and CO2 to form the vinyl carbamate 433 [167,168]. 

The tungsten carbene complex [(CO)5W=CHAr] (Ar = Ph, p-tol) reacts with 2-butyne to give stilbenes and [(CO)5W(MeC=CMe)]. Terminal alkynes, such as Bu C=CH, are however polymerized. Vinylidene complexes are proposed as intermediates since the acetylene-vinylidene rearrangement has ample precedence in stoichiometric reactions (Scheme 26). 

Cr dissociation, alkyne binding, rearrangement to the vinylidene, nucleophilic attack on the vinylidene by OH2, rearrangement to a PhCH2COIr intermediate, from which a elimination gives the product. 

Stepwise double alkyne to vinylidene tautomerization is the key step responsible for the formation of the 77 -butadienyl iridium(in) complex [Ir K -0,C -O=G(Me)CH=CPh (77 -PhCH=CHC=CHPh)(PPh3)2]SbF6 579 346 proposed mechanism, which is illustrated in Scheme 82, involves an alkyne to vinylidene rearrangement (I —> II) followed by a hydride insertion (II — III), a second alkyne to vinylidene rearrangement (III — V), and a migratory insertion of the vinyl to the vinylidene (V — VI) resulting in the G-C bond formation. 

This pathway involves cycloisomerization of rj -metal—alkyne complex to a vinylidene complex (Scheme 4) [19], An example is shown in Scheme 5 [20], The initial T -Mo—alkyne complex rearranges to vinylidene—Mo complex intermediate that midergoes an intramolecular nucleophilic attack of the hydroxy oxygen to give cyclic anionic intermediate, protonation of which yields 2,3-dihydrofuran. 

A sub-set of these reactions is provided by the redox rearrangements of several complexes whicdi have been extensively studied by Cormelly and coworkers [140]. Oxidation of the rj -alkyne complexes M(r] -Me3SiC2SiMe3)(CO)2(ri-arene) (M = Cr, Mo) results in formation of the vinylidene cations [M =C=C(SiMe3)2 (CO)2(ri-arene)]. ... 

The proposed reaction mechanism is shown in Scheme 9.15. Starting from the phenyl-rhodium complex 87, alkyne rearrangement is expected to furnish the phenyl-vinylidene complex 88. Migration of a phenyl ligand onto the vinylidene moiety of 88 must occur such that the vinyl Rh-C bond and the enone tether of the resultant complex (89) attain a cis-relationship to one another. Intramolecular conjugate... 

Insertion can also be carried out on the C-H bonds of heteroaromatics. Masahiro Murakami of Kyoto University has described (J. Am. Chem. Soc. 2003,125,4720) a Ru catalyst that will effect rearrangement of a silyl alkyne such as 10 into the vinylidene carbene. The intermediate Ru carbene complex is then electrophilic enough to insert into the aromatic C-H bond. The insertion is highly regioselective. The Au and the Ru alkylidene insertions are geometrically complementary, as Ru gives the E-alkcne. 

This is further indicated in the reactions of 3-butyn-l-ol with [Fe( /2-CH2=CMe2)(CO)2( -C5H5)]+, which afford a mixture of dihydrofuran complex (64) and the oxacyclopentylidene complex (65) (84). The formation of these two derivatives involves a common tp-alkyne intermediate, which either forms 64 directly by internal nucleophilic attack of the oxygen on the complexed C=C triple bond, or rearranges to the vinylidene. This forms 65 by a similar attack of the hydroxy group on the a-carbon, followed... 

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

Addition of internal alkynes to (t)5-C5H5)(PR3)2RuCI does not lead to the formation of the corresponding disubstituted vinylidene (68). The failure of this reaction could reflect the relative difficulty of a 1,2-alkyl shift for internal alkynes as compared to the 1,2-proton shift for the successful rearrangement of terminal alkynes (Scheme 9). Alternatively, if the deprotonation-reprotonation route is important in the rearrangement of terminal alkynes (vide supra), then clearly internal alkynes would not undergo a similar isomerization. 

Figure 5.38. In recent times these have included the rearrangement of terminal alkynes in combination with late transition metal hydrido complexes, presumably via vinylidene intermediates (see below).

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