Cyclopropene insertion

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. 

Formation of the cyclotrimer 7 can be understood to occur via cyclopropene insertion into a metal-carbon bond of the five-membered metallacycle 8 instead of direct reductive elimination of the dimer 4. 

Dirhodium compounds have proved to be highly efficient and effective catalysts for diverse reactions, which has led to their reputation as being important players in synthetic organic chemistry. A wide range of synthetically useful transformations, such as cyclopropanation, cyclopropenation, insertion into C-H and heteroatom-H bonds, cydoadditions, ylide formation and rearrangement, and other processes are mediated by the catalytic formation of dirhodium(II) carbenes via diazocarbonyl compounds [77],... 

Whereas the Rh2(OAc)4-catalyzed addition of diazoalkanes to propargyl alcohols readily gives the insertion of the carbene into the 0-H bond, with only a small amoimt of cyclopropenation of the resulting propargylic ether [54] the 2-diazopropane 59 reacts at 0 °C with l,l-diphenyl-2-propyn-l-ol 62a in dichloromethane and exclusively gives, after 10 h of reaction, only the adduct 63a isolated in 75% yield and corresponding to the regioselective 1,3-dipolar cycloaddition of the 2-diazopropane to the alkyne C - C bond (Scheme 15). 

Reaction of propargylic alcohols 229 with alkyl diazoacetates entails competition between O/H insertion and cyclopropenation. 

Under the catalytic action of Rh2(OAc)4, formation of a propargylic ether from a terminal alkyne (229, R1=H) is preferred as long as no steric hindrance by the adjacent group is felt162,218>. Otherwise, cyclopropenation may become the dominant reaction path [e.g. 229 (R1 = H, R2 = R3 = Me) and methyl diazoacetate 56% of cyclopropene, 36% of propargylic ether162)], in contrast to the situation with allylic alcohols, where O/H insertion is rather insensitive to steric influences. 

The ring expansion reaction of diaryl cyclopropenones and cyclopropene thiones occurring with pyridinium, sulfonium, and phosphonium enolate betaine 427268-270) is related to 1,3-dipolar cycloadditions. This process results in formation of 2-pyrones 428 by loss of pyridine (or sulfide or phosphine) and insertion of the remaining fragment C=C-0 to the C1(2)/C3 bond of the cyclopropenone ... 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

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°). 

Although the original synthesis of an 774-vinylketene complex1 (see Section VI,B) had used the method of metal insertion into a cyclopropene ring, it was more than 20 years before it was applied to metals such as... 

In a later paper by Weiss,68 the methodology was extended to a more complex cyclopropene, and an intermediate cobaltacyclobutene (103) was proposed. In an analogous insertion reaction with nonacarbonyldiiron, a vinylcarbene complex was isolated along with the expected vinylketene complex (see Section VI,B). However, no such vinylcarbene cobalt complex was isolated, even when cyclopentadienyl bis(ethene) cobalt was used in place of dicarbonylcyclopentadienyl cobalt, and the only product isolated was the vinylketene complex 104, represented here in the rf -allylacyl structure. 

It has already been seen that cobalt may insert into cyclopropenes to yield, after further insertion of a carbonyl ligand, -vinylketene complexes... 

In the example shown in Figure 4.4 either of these mechanisms leads to insertion of the alkyne into the C-Rh double bond of the initially formed acylcarbene rhodium complex. The resulting vinylcarbene complex undergoes intramolecular cyclopropanation of the 1-cyclohexenyl group to yield a highly reactive cyclopropene, which is trapped by diphenylisobenzofuran. 

The dichlorocarbene adduct 148 of 9-methoxyphenanthrene eliminates HCl instead of MeOH and forms a cyclopropene 152, Ring-opening produces the substituted vinylcarbene 154. The latter inserts intramolecularly into the methoxy group and, after elimination of HCl from 157, a phenanthrofuran 158 is obtained. The sequence is applicable to substituted furans and even to phenanthrocyclopen-tadienes. ... 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

The reaction of ketocarbenoids with alkynes is a direct method for the synthesis of functionalized cyclopropenes.1 122 123 Until quite recently copper catalysis was generally used and the reactions proceeded in fairly moderate yields, except with terminal alkynes, which failed to generate cyclopropenes due to competing C—H insertions.1 This limitation could be circumvented, however, by using trimethylsilyl derivatives. This approach is illustrated in the synthesis of (131), the unsaturated analog of 1-aminocy-clopropanecarboxylic acid, the biosynthetic precursor to ethylene in plants (Scheme 27).124 The initial... 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

Cycloaddition of the carbene derived from 205 to bis(trimethylsilyl)acetylene yields the expected cyclopropene in low yield both photochemically (20%) and under catalysis by copper triflate at 80 °C (10-13%)119. The latter version of the reaction is accompanied by [3 + 2] cycloaddition of the diazo compound to the alkyne, and the photochemical route yields a by-product which obviously comes from carbenic C,H insertion at a SiMe3 group of the alkyne. 

Unsaturated ethers. The efficient insertion of carboalkoxycarbenes into the O—H bond of alcohols catalyzed by Rh(II) acetate (5, 571-572) extends to reactions with unsaturated alcohols. For this reaction copper(II) triflate is usually comparable to rhodium(II) alkanoates. Insertion predominates over cyclopropanation in the case of ethylenic alcohols. In reactions with acetylenic alcohols, cyclopropenation can predominate over insertion because of steric effects, as in reactions of HC=CC(CH3)2OH where the insertion/addition ratio is 36 56. 

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