Vinylketene complexes, synthesis

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)... 

Cycloaddition of the carbene chromium complexes 97 with CO incorporation provides a versatile method for naphthol synthesis, in which the metallacy-clic intermediates 99 are involved [47]. An alternative entry to 101 is achieved by metal carbonyl-catalyzed rearrangement of the cyclopropenes 98 via the same metalla-cyclobutenes 99 and vinylketene complexes 100 [52], Mo(CO)6 shows a higher activity than Cr(CO)6 and W(CO)6. The vinylketene complex 103 is formed by the regioselective ring cleavage of 1,3,3-trimethylcyelopropene 102 with an excess of Fe2(CO)9 [53]. (Scheme 35 and 36)... 

Among the metals investigated, manganese and vanadium have been the least explored in the synthesis of -(vinylketene) complexes, with only... 

The reactions between cyclopropenes and carbon monoxide in the presence of transition metals have been of some use in synthesis,93 and in 1978 Binger initiated a study of the reactions between metal carbonyls and cyclopropenes in order to elucidate the generality of these reactions.75 It was found that dicarbonyl 775-cyclopentadienyl(tetrahydrofuran)manganese(I) reacted with 3,3-dime thy Icy clopropene at 0°C to produce -(vinylketene) complex 81 in fair yield. The only other transition metal in Binger s study that was found to react with 3,3-dimethylcyclopropene in this manner was iron (see Section VI,B). 

Although the preparative chemistry of (vinylketene)cobalt(I) complexes is relatively limited in the literature, the methods used include all the major procedures that have been more widely exploited in the analogous chromium and iron systems. There are many similarities between the intermediates involved in the synthesis of vinylketene complexes of iron, chromium, and cobalt, but as the metal is varied the complexes containing analogous ligands often exhibit significant differences in stability and reactivity (see Sections II and VI). Comparison of such species has often been an important aim of the research in this area. The (vinylketene)cobalt(I) complexes have also been shown to be synthetically useful precursors to a variety of naphthols, 2-furanones, ce-pyrones, phenols,6,22,95 >8, y-unsaturated esters,51 and furans.51,96a... 

In 1986, Wulff reported the first synthesis of an -vinylketene complex from the reaction between a carbene and an alkyne51 the former had of course been postulated as key intermediates in analogous reactions of chromium8 and iron97 carbenes (see Sections II.B and VI.C, respectively). On treatment of the tricarbonylcobalt carbene (84) with 3-hexyne under mild conditions, the yf-vinylketene complex (85) and the y-keto unsaturated... 

This chemistry was developed with the specific intention of application to the synthesis of phenols. There had only been one previous example of the addition of alkynes to 774-vinylketene complexes yielding phenols,22 despite a cornucopia of other organic fragments having been isolated from such reactions (see Sections II, V, and VI). The best results were obtained on reaction of the 3-phenylvinylketene complex 114.a with several alkynes. 

In 1991, Park reported123 the first synthesis of iron alkynylcarbene complexes (184), involving the nucleophilic attack of a lithium acetylide on pentacarbonyl iron, followed by electrophilic quench. With such compounds in hand, he proceeded to investigate their reactivity123,124 and found that upon addition of cyclopentadiene to the alkynylcarbene complexes 184, the products formed were 774-vinylketene complexes (185). During column chromatography, some of these products (185.a and 185.b) were transformed into the tricarbonyl(norbornadiene)iron derivatives 186. Others (185.C and 185.d, not shown) were hydrolyzed as part of the workup procedure, to afford pure samples of the norbornadiene complexes 186.C and... 

The earliest alternative to cyclopropene insertion as a viable vinylketene synthesis was published by Hoffmann118 in 1972. Upon reaction of 206.a with diiron nonacarbonyl, the vinylketene complex 207 was isolated in low yield. The analogous bromide and iodide substrates formed the 77-allyl complexes 208, although a trace of 207 was isolated from the reaction of the bromide 206.b. 

Herrmann has reported127128 that irradiation of an ethereal mixture of iron pentacarbonyl and diphenylketene leads to the synthesis of the red vinylketene complex 209, containing the unusual feature of partial involve-... 

Having established a very effective method for the synthesis of tricar-bonyl(T74-vinylketene)iron(0) complexes, Thomas has subsequently undertaken the most comprehensive study on the reactivity of these complexes to date. The reactions of 221 with phosphoramidate anions,90134 coordinating ligands such as phosphines3 and isonitriles,69,87,89,135,142,143 a variety of nucleophiles,86,89135142 phosphonoacetate anions,88,89 alkynes,108,109,144,145 and al-kenes146,147 have ah been investigated. Crucially, Thomas has also developed a method138 for the kinetic resolution of the vinylketene complexes (221) that ultimately yields enantiomerically pure samples of the complex. This... 

Huffman MA, Liebeskind LS (1990) Insertion of (.eta.5-indeny)cobalt(I) into cyclobutenones the first synthesis of phenols from isolated vinylketene complexes. J Am Chem Soc 112... 

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). 

As part of a study of the reactions of metallacyclic y-ketovinyl complexes of molybdenum and tungsten with acetylenes, directed toward the synthesis of complexed -/-lactones, Stone has reported92 the isolation of several vinyl-ketene complexes. When complex 72 was heated with 2-butyne, one molecule of the alkyne was incorporated into the complex with concomitant carbonylation. X-ray analysis of the product (73) has shown unequivocally that the C-l to C-4 vinylketene fragment is bonded in a planar, rj4-configu-ration. In contrast to the thermal reaction, ultraviolet irradiation of 72 or 74 in the presence of 2-butyne affords the complexes 75 and 76, respectively, where the lone carbonyl remaining after alkyne insertion had been replaced by a third molecule of the alkyne. 

As mentioned earlier (Sections V,B and V,C), cobaltacyclobutenes have been proposed as intermediates in the synthesis of vinylketene cobalt complexes. The isolation of a suitable cobaltacyclobutene by O Connor96 (by... 

Since the conversion of 235 to 233 requires only heating, it is possible to synthesise the vinylketenimine complex directly from the vinylketene merely by extending the reaction time sufficiently.69,87,89 Note that 233.j and 233.k were obtained as 1 1 mixtures of diastereoisomers. The subsequent reactivity of the vinylketenimine complexes,87,89,135,143 particularly their utility in the asymmetric synthesis of quaternary carbon centers,143 has also been investigated, but is beyond the scope of this review. 

Further illustrative examples of the synthesis of five-membered carbo- or heterocycles by intermediate formation of vinylketene- or butadienylcarbene complexes are listed in Table 2.19. 

The ring opening of cyclobutenone derivatives leads to the formation of vinylketenes (equation 3). The potential of these compounds in the synthesis of complex molecules has been investigated in the 1980s by a number of research groups, most notably those of Moore, Liebeskind and Danheiser. An excellent review describing work in this area prior to 1986 is available. ... 

Cobalt complexes of vinylketene by reaction with alkynes in the presence of 2 moles of cyclooctadiene 100°C during 20 hours resulted in moderate yields of phenols (ref. 17). This procedure represents the first synthesis of alkylphenols by the insertion of (fi -indenyl)cobalt(l) into a cyclobutenone, the latter being prepared by analogy with a method for cpCo(PPh3)2. With symmetrical alkynes (R = = Et) the yield was 65%, although with unsymmetrical compounds the... 

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