Insertion ketene

Ketene Insertions. Ketenes insert into strongly polarized or polarizable single bonds, such as reactive carbon—halogen bonds, giving acid hahdes (7) and into active acid haUdes giving haUdes of p-ketoacids (8) (46). Phosgene [77-44-5] (47) and thiophosgene [463-71-8] (48) also react with ketenes. 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

In the presence of a double bond at a suitable position, the CO insertion is followed by alkene insertion. In the intramolecular reaction of 552, different products, 553 and 554, are obtained by the use of diflerent catalytic spe-cies[408,409]. Pd(dba)2 in the absence of Ph,P affords 554. PdCl2(Ph3P)3 affords the spiro p-keto ester 553. The carbonylation of o-methallylbenzyl chloride (555) produced the benzoannulated enol lactone 556 by CO, alkene. and CO insertions. In addition, the cyclobutanone derivative 558 was obtained as a byproduct via the cycloaddition of the ketene intermediate 557[4I0]. Another type of intramolecular enone formation is used for the formation of the heterocyclic compounds 559[4l I]. The carbonylation of the I-iodo-1,4-diene 560 produces the cyclopentenone 561 by CO. alkene. and CO insertions[409,4l2]. 

The alkylphenylacetyi chloride 843 and benzoyl chloride undergo decarbo-nylative cross-condensation to give the enone 845 in the presence of EtiNf723]. The reaction is e.xplained by the insertion of the ketene 844 into the Pd-aryl bond and, 3-elimination. To support this mechanism, o, d-unsaturuted ketones are obtained by the reaction of ketenes with aroyl chlorides[724]. 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

Ketenes can react in several ways with organometaUic compounds and complexes. They can add as ligands to coordinated metals forming stable ketene, ketenyl, and ketenyfldene complexes. Ketenes can be inserted into metal—hydride, metal—alkyl, metal—OR, and metal—NR2 bonds, react with metal—oxide complexes, and with coordinated Hgands. This chemistry has been reviewed (9,51). 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

Photolysis of ketene with light of wavelength 300-370 m/x produces the reactive intermediate carbene, which is capable of a variety of insertion and addition reactions. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Merlic et al. were the first to predict that exposing a dienylcarbene complex 126 to photolysis would lead to an ort/zo-substituted phenolic product 129 [74a]. This photochemical benzannulation reaction, which provides products complementary to the classical para-substituted phenol as benzannulation product, can be applied to (alkoxy- and aminocarbene)pentacarbonyl complexes [74]. A mechanism proposed for this photochemical reaction is shown in Scheme 54. Photo activation promotes CO insertion resulting in the chromium ketene in-... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

No CO-insertion products (metal-ketene complexes) were observed, even when specifically sought [9,10]. 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

Carboalkoxymethylenes, like acylmethylenes, undergo rearrangement to ketenes as well as the olefin addition and C—H insertion reactions characteristic of methylenes.<37> Thus the photolysis of ethyl diazoacetate in olefinic solvents leads to substantial yields of products, which can be rationalized in terms of a Wolff rearrangement of the carboethoxymethylene followed by cycloaddition of the resulting ethoxyketene to the olefin ... 

Table 2. Insertion reaction, phosphene and ketene rearrangements of phosphorylacylcarbenes (37)...

Wolff rearrangement of a-diazoketones to give ketenes or subsequent products is an often used synthetic procedure the scope and limitations of which are well established 13 390), so that only a few new features of this reaction need to be considered here. Concerning its catalytic version, one knows that copper, rhodium and palladium catalysts tend to suppress the rearrangement390). A recent case to the contrary is provided by the Rh2(OAc)4-catalyzed decomposition of ethyl -2-diazo-3-oxopent-4-enoates 404 from which the p,y-unsaturated esters 405 are ultimately obtained via a Wolff rearrangement 236). The Z-5-aryl-2-diazo-3-oxopent-4-enoates undergo intramolecular insertion into an aromatic C—H bond instead (see Sect. 4.1). 

At low temperatures cyclopropenones and enamines or ketene acetals were shown to yield 2-azonia-bicyclo(3,l, 0)hex-3-enolates-3 (371, X=0), which can be isomerized thermally to penta-2,4-diene amides(372, X=0). At elevated temperatures the amides were found to be the principal products arising from C-N-insertion 237) (insertion of the cyclopropenone three-carbon unit into the C-N bond of the enamine). These were accompanied in some cases by 3-aminoenones 373 arising from C-C-insertion 237) (insertion of the cyclopropenone into the C-C double bond of the enamine) and a-amino cyclopentenones 375 formed by Stevens rearrangement of the ylide 369 and cyclopentenones 374 ( condensation 237)). 

At the present, the most straightforward mechanism for the formation of J5 from 1 is via insertion of CO into the Th-C(acyl) bond to form a ketene (H, (eq. (4)) which subsequently dimerizes. Presumably, initial CO interaction could involve coordination either to the metal ion as shown or to the electrophilic vacant "carbene p atomic orbital. Considering the affinity of the Th(IV) ion for oxygenated ligands, interaction of the ketene oxygen atom with the metal ion seems reason-... 

PBu3) to eliminate ethylene (in agreement with the enhanced ethylene production in the presence of CO and PBu3) or be further reduced to ethane by an iron hydride or by LAH.— The ethylidene could insert a new CO forming a ketene with three carbons and extend the chain. The importance of coordination of A1H3 to... 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

Carbon monoxide inserts into the Sc-Ge bond of 123, leading to a scandium enedione diolate via a proposed ketene-type intermediate (Scheme 40).236... 

In the alkoxycarbonylation, the hydride mechanism initiates through the olefin insertion into a Pd - H bond, followed by the insertion of CO into the resulting Pd-alkyl bond with formation of an acyl intermediate, which undergoes nucleophilic attack of the alkanol to give the ester and the Pd - H+ species, which initiates the next catalytic cycle [35,40,57,118]. Alternatively, it has been proposed that a ketene intermediate forms from the acyl complex via /3-hydride elimination, followed by rapid addition of the alcohol [119]. In principle the alkyl intermediate may form also by protonation of the olefin coordinated to a Pd(0) complex [120,121]. 

An alternative end-game has the CO insert into the Cr=C bond of the allylidenechromium compound to give a Cr complex of a ketene. Electrocyclic ring closing of the ketene would then give the product. 

In addition to the ring opening of cyclopropenes noted above, vinylketene complexes 103 have been prepared by (1) ligand initiated carbonyl insertion of vinyl carbene complexes 104 and (2) benzoylation of ,/3-unsaturalcd acyl ferrates 105 (Scheme 20)114. X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an /j4-dicnc type complex (103) and an jj3-allyl r/1 acyl complex (106). The Fe-Cl distance (ca 1.92 A) is shorter than the Fe-C2, Fe-C3, or Fe-C4 distances (ca 2.1-2.2 A)113a-C. In addition, the C—C—O ketene array is not linear (bend angle ca 135°). 

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. 

In 1994, Thomas reported146,147 that alkenes also underwent an addition reaction with vinylketene complexes that differed crucially in the loss of the ketene carbonyl fragment. Complexes 252.a-252.d were isolated as yellow crystalline solids. Clearly this suggests that the process occurs by a mechanism different from the alkyne insertion, and this will be discussed... 

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