Vinylketene intermediate

The thermal benzannulation of Group 6 carbene complexes with alkynes (the Dotz reaction) is highly developed and has been used extensively in synthesis [90,91]. It is thought to proceed through a chromium vinylketene intermediate generated by sequential insertion of the alkyne followed by carbon monoxide into the chromium-carbene-carbon double bond [92]. The realization that photodriven CO insertion into Z-dienylcarbene complexes should generate the same vinylketene intermediate led to the development of a photochemical variant of the Dotz reaction (Table 14). 

The cyclobutenone 70 is transformed to the r/4-vinylketene complex 72 with (t/5-indenyl)Co(PPh3)2 71. The vinylketene complex 72 undergoes cyclization with alkynes to produce the corresponding phenols 73. FeCl3 oxidation of the (2-phenylvinyl)ketene complex, however, leads to the naphthol 74. A catalytic synthesis of phenols via the vinylketene intermediates 72 is achieved by the use of Ni(COD)2 as a catalyst [36]. (Scheme 26)... 

There is mechanistic evidence to show that this formal (3 + 3) cycloaddition starts with attack of betaine-C at CI<2 of the three-ring (429) and leads to 2-pyrone formation either by a concerted process (429 — 428) or stepwise via cyclobutenone and 0-acyl vinylketene intermediates (430/431) depending on the leaving group ... 

Despite these important studies, most steps of the reaction mechanism are still only postulated. Therefore, we have decided to undertake a theoretical investigation of the Dotz reaction by discussing whether the reaction proceeds via a dissociative or an associative pathway in the initial steps of the process. We have also analyzed the central part of the reaction, the key issue being whether the reaction proceeds through a vinylketene intermediate (route A) or, instead, via a metallacycloheptadienone (route B). As will be seen, we came across a novel third pathway (route C) that turns out to be the best alternative from thermodynamic and kinetic points of view... 

Similarly, Yamashita has utilised free alcohols to trap the vinylketene intermediates formed in the reactions of aryl and heteroaromatic carbenes, resulting in >8, y-unsaturated esters.26,45 If a secondary amine is used in the... 

The methoxyketene 297, coordinated to Cr carbonyl, is formed from methoxy-carbene easily by insertion of CO under irradiation [90]. An ester is formed by the reaction of ketene with alcohol. The aminocarbene complex 298 was prepared from benzamide and converted to phenylalanine ester 300 under irradiation of sunlight in alcohol via ketene 299 [91]. The eight-membered lactone 304 was prepared in high yield by the reaction of the alkyne 301 having the OH group in a tether with Cr carbene without irradiation. The vinylcarbene 302 is formed at first and converted to the vinylketene intermediate 303 as expected. The keto lactone 304 is formed from 303 by intramolecular reaction with the OH group and hydrolysis [92],... 

A rhodium-catalyzed ring opening of the cyclobutenones 637 forms the r 4-vinylketene intermediates 638, which can dimerize to afford (/. )-6-vinyl-2//-pyran-2-oncs in excellent yield (Scheme 146) <2004AGE5369>. A ruthenium-mediated ring opening and dimerization of cyclobutenones forms a mixture of (E)- and (Z)-6-vinyl-2//-pyran-2-ones in high yield (Equation 257) <2004AGE5369>. 

R.L. Danheiser and co-workers generated a key vinylketene intermediate via tandem Wolff rearrangement-ketene-alkyne cycloaddition to utilize it in a photochemical aromatic annulation reaction Danheiser benzannulation) for the total synthesis of the phenalenone diterpene salvilenone. ... 

As to the underlying reaction mechanism, it appears that vacuum thermolysis of (545), (554) and (561) generates in each case, initially, a transient o-quinodimethane which cyclizes in the highly dilute gas phase. o-Quinoid vinylketene intermediate (562) could be regenerated in solution from the isolated benzocyclobutenone (563) on heating or irradiation and trapped by electron-poor dienophiles, e.g. (632) (Scheme 141). = Alternatively, benzocyclobutenones can be easily functionalized at the carbonyl group or at Ca, affording further o-quinodimethane precursors. 

In a general illustration of the Dotz reaction a terminal or internal alkyne reacts with a carbene 123 and one carbonyl ligand at a [Cr(CO)3] template in a formal [3 + 2+1] cycloaddition reaction producing a chromium-complexed naphthol (124) under mild reaction conditions via the vinylketene intermediate 125 (see Scheme 57). Terminal alkynes (R1C = CR2 R HjR H) react with total regioselectivity, while the regiocontrol in the reaction course of internal alkynes... 

An analogous vinylketene intermediate (127, see Schemes 57 and 59) as proposed for the Dotz reaction has been assumed in the so-called cyclobutenedione methodology [161]. The key intermediate is a 4-aryl or 4-alkenyl substituted 2-cyclobutenone (128) that can be obtained e.g. by the reaction of the 3-cyclo-butene-1,2-dione (129) with the appropriate lithium reagent or Stille coupling with 4-chloro-3-cyclobutenone. Thermal cyclobutenone ring opening to the vinylketene 130 followed by electrocyclization furnishes the highly substituted aromatic compound 131 (see Scheme 59). 

Cyclobutenones are fairly common side-products in the reaction of chromium arylalkoxy-carbenes with internal alkynes. As indicated in Scheme 5-1, the branch point in the formation of cyclobutenone versus naphthol products is believed to be vinylketene intermediate 4, which may undergo electrocyclic ring closure to 9, followed by reductive elimination to the product [7 a]. Cyclobutenone formation occurs only in the presence of internal or external ligands that can coordinate to unsaturated chromium species sufficiently well to prevent complexation to an internal n-system and thus divert the system toward 9. Depending on the alkyne and aryl substitution patterns and the reaction conditions, cyclobutenone formation can be made to predominate. Thus, solvents of good coordinating ability such as acetonitrile, o-OMe aryl substitution (which allows internal coordination to chromium), and bulky alkynyl substituents all favor cyclobutenone formation [Eq. (23)] [13]. In fact, the effect of solvent alone can be even more dramatic for the reaction partners in Eq. (21), a 0.5 M concentration of the carbene complex in acetonitrile gives instead a 78 % yield of cyclobutenone and only a combined 17 yield of quinone and indene products [9]. 

Danheiser and coworkers have developed a convenient synthetic approach to useful, highly substituted, aromatic compounds by the reaction of alkoxyacetylenes with cyclobutenone derivatives (equation 61) . The mechanism of this annulation is similar to the analogous reaction of siloxyacetylenes (equation 31, Section II.D.2) and the key step is a [2+2] cycloaddition of alkoxyacetylene with a vinylketene intermediate. ... 

A different approach to triquinane was made by Moore [107-110]. His synthetic route includes the intramolecular [2 + 2] cycloaddition of the vinylketene intermediate as shown in Fig. 4. If the bicyclo[3.2.0]heptenone from this reaction is designed to have an alkenyl-substituent at the bridgehead (i.e., 101 102 103), the next oxy-Cope rearrangement is satis-... 

Cascade reactions of Zincke aldehydes (159) yield (Z)-a,, y,5-unsaturated amides (160). A computational study has discounted previous mechanistic proposals and points to a vinylketene intermediate. ... 

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