Vinylcarbenes, cyclopropene

Compared to the parent system 3a, the barrier for formation of 3d is the highest in this series whereas the formation of 3b should be the most facile according to our computations. Although the reactions of carbenes la-c are initiated photochemically, the observed reactivity seems to be in line with the computed ground state properties. Thus, while methyl substitution in 3-and 5-position inhibits the vinylcarbene-cyclopropene rearrangement, methyl substitution in 2- and 6-position has the opposite effect. 

Among the methods at hand to synthesize cyclopropane derivatives, carbene addition to alkenes plays a prominent role 63). As a source of vinylcarbenes, cyclopropenes might be useful in this kind of approach. In 1963, Stechl was the first to observe a transition metal catalyzed cyclopropene-vinylcarbene rearrangement64). When treating 1,3,3-trimethylcyclopropene with copper salts, dimerization occurred to give 2,3,6,7-tetramethyl-octa-2,4,6-triene (9), the product from a formal recombination of the corresponding vinylcarbene (Eq. 8). 

Although the ring expansions [Eqs. (1)—(4)] have been represented above as one-step reactions, it is conceivable that a bicyclic intermediate is involved. This would form by a simple vinylcarbene-cyclopropene cyclization [Eqs. (11)—(12)]. 

The photochemical study of 3H-pyrazoles was carried out in the search for a route to cyclopropenyl tertiary alcohols. Irradiation of 63a in dry dichloromethane at 300 nm and at room temperature for 0.5 h led to the exclusive formation of the gem-dimethylcyclopropene 65 (Scheme 17). The formation of cyclopropene 65 arises from the loss of N2 and cycUzation of the vinylcarbene intermediate (III). 

Alkinyloxy)diazoacetic esters 11 give rise to product mixtures that could be separated only partially. The isolated products result from a tandem intramolecular cyclopropenation/cyclopropene —> vinylcarbene isomerization (12, 14) and from a twofold intermolecular (3+2)-cycloaddition of the intact diazo compound (13). 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

Allylic cations (180) were also generated by LFP of allenes (174) in TFE.86 Deuterium labels revealed that the cations 180 originate predominantly from vinylcarbenes (177), which are formed from 174 by way of a 1,2-H shift. Protonation at the central carbon of the photoexcited allenes87 is a minor reaction path with 174a,b,d. Vinylcarbenes are also known to arise in photolyses of cyclopropenes, 175 — 177.85bi88 However, LFP of 175 in protic media proved to be rather inefficient in generating allylic cations, presumably due to low quantum yields. 

Vinylcarbene is known to close to cyclopropene.59 The reverse reaction is also possible Triplet-propene-l,3-diyl (frans-T-33 ) can be generated from cyclopropene 32 by irradiation in a bromine-doped xenon matrix at 10 K 1-methylcyclopropene (34) yields triplet-2-butene-l,3-diyl (Iruns-T-SS ).60-62 The concentration of 35 under these conditions is high enough to be able to detect this diradical IR spectroscopically. The experiments suggest that even the parent vinyl carbene 33 is detectable.61,62 Calculations ((U)B3LYP/6-31G )61,62 not only allow the comparison of theoretical and experimental IR spectra but also... 

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. 

Fig. 3.29. Isomerization of cyclopropene complexes into vinylcarbene complexes [5831.

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Interestingly, copper(I) salts also catalyze the cyclopropene-vinylcarbene isomerization [681]. In this case the transient carbene complexes again show electrophilic behavior, behavior similar to that of the complexes formed from copper(I) salts and diazoalkanes or sulfonium ylides. 

The intermolecular reaction of alkynes with acylcarbene complexes normally yields cyclopropenes [587,1022,1060-1062]. Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result. In particular intramolecular cyclopropanations of alkynes, which would lead to highly strained bicyclic cyclopropenes, often yield rearrangement products of the latter. In many instances these products result from a transient vinylcarbene complex, which can be formed by two different mechanisms (Figure 4.3). 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

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. ... 

The synthesis of cycloproparenes resulting from formal fusion of a cyclopropene to furan and thiophene has been attempted with limited success. Reaction of the dichloro-oxabicyclohexane 180 (X = O) " afforded a cyclopropene 181 which ring-opened to a vinylcatbene 182, but the cycloproparene 183 was not produced. Similarly, the thia-analogue 180 (X = S) could not be converted to 184. The intermediate cyclopropenes and/or vinylcarbenes have been trapped. A cyclopropathiophene derivative 186 was generated, however, from 185. Although it was not isolable, it afforded a bis-adduct 187 when it was produced in the presence of isobenzofuran (45)." ... 

In the early syntheses of alkenyl alkylidene-mthenium catalysts, the first generation of Grubbs catalyst, it was observed that propargyl chloride could be a convenient source of the vinylcarbene initiator [53] with respect to the previous one arising from activation of cyclopropene [4] (Equation 8.3). In this synthesis the alkylidene hydrogen atom arises from the ruthenium hydride. 

The extension of this reaction to include the conversion of 3H-pyrazoles into cyclopropenes is established.86 3,3,5-Trimethyl-3/i-pyrazole (100 R = H), for example, on photolysis in pentane solution gives 1,3,3-trimethylcyclopropene (101), and an intermediate diazo-alkene (102) has been characterized. The proposed conversion of the diazoalkene into the cyclopropene (101) via a vinylcarbene has a... 

Functionalized cyclopropenes are viable synthetic intermediates whose applications [99.100] extend to a wide variety of carbocyclic and heterocyclic systems. However, advances in the synthesis of cyclopropenes, particularly through Rh(II) carboxylate—catalyzed decomposition of diazo esters in the presence of alkynes [100-102], has made available an array of stable 3-cyclopropenecarboxylate esters. Previously, copper catalysts provided low to moderate yields of cyclopropenes in reactions of diazo esters with disubstituted acetylenes [103], but the higher temperatures required for these carbenoid reactions often led to thermal or catalytic ring opening and products derived from vinylcarbene intermediates (104-107). 

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