Vinylcarbene from cyclopropenes

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... 

Definitive evidence for vinylcarbene intervention during cyclopropene thermolyses stems from the identification of cyclopropane 202 as a primary product from cyclopropene 200 (equation 69). Product 202 can only come from an intramolecular C-H... 

Cyclopropenes 2, prepared by treatment of a trihalocyclopropane 1 with alkyllithium (see Section 2.A. 1.1.1 for the generation of cyclopropenes from cyclopropenes by elimination), ring open under appropriate conditions to generate vinylcarbenes 3. Such intermediates bearing halogen substituents on the divalent carbon can be trapped by carbene scavengers such as alkenes. Optimized conditions even permit these transformations of trihalocyclopropanes to be carried out as one-pot procedures. 

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. 

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. 

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. 

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). 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

The position of the alkyl substituent in the product indicates that cyclisation occurs with rearrangement of the double bond, ie., by 1,1-elimination and formal formation and cyclisation of a vinylcarbene. Although the overall yields are not always good, the reagents are readily available and large quantities of the simple alkylcyclopropenes can be produced. 1,2-Dimethylcyclopropene has been prepared in a similar process by treatment of methallyl chloride with two equivalents of phenyl lithium, followed by quenching with methyl iodide presumably, the initial reaction leads to 1-methylcyclopropene which is converted in situ to the 2-lithio-species 10). The elimination of HBr from brominated alkylidenemalonates also leads to cyclopropenes, though in low yield U) ... 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

As might be expected from their inherent strain, many cyclopropenes undergo rearrangement, dimerisation or even polymerisation under relatively mild conditions. The conditions required for reaction are, however, very variable and some cyclo-propenes, such as 3,3-dimethylcyclopropene, are stable at relatively high temperature (150 °C in this case). Three main reactions are described below — the ene-reaction, [2+ 2]-dimerisation, and rearrangement to vinylcarbenes. 

The rearrangement of a vinylcarbene to a cyclopropene is well known and provides a viable method for the synthesis of compounds whose substituents range from simple to complex. For example, base-induced a-elimination of HCl from allyl chloride provides a straightforward" synthesis of cyclopropene. Despite the availability of the parent... 

As discussed earlier (Section II.B) the ring closure of vinylcarbenes provides a viable and synthetically useful route to cyclopropenes. These and the above-mentioned results suggest that the cyclopropene-vinylcarbene rearrangement is a reversible process. Elegant evidence for this comes from the thermolysis of optically active 1,3-diethylcyclo-propene (209). Thus cleavage of the more substituted ring bond would give the achiral... 

Further support for the intervention of vinylcarbenes in such reactions stems from the cyclopropenes 239 which provide diene products, the proportions of which reflect the facility of the aryl ring to stabilize the developing carbenic centre. The formation of... 

This ring cleavage is, in fact, reversible 39), and many cyclopropenes have been synthesized via vinylcarbenes generated from various precursors. 

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). 

In the meantime thermal65 and metal catalyzed66,67> rearrangements of cyclopropenes have been detected as convenient methods for the preparation of vinylcyclo-propanes via formal [2+ l]-cycloadditions of vinylcarbenes to alkenes (Eq. 9) (for an alternative entrance starting from allylidene dichloride or 1,3-dichloropropene, see Ref. 68)). 

These results have been established for both titanium and zirconium. When 3,3-dimethylcy-clopropene is added to an ethereal solution of bis( j -cyclopentadienyl)bis(trimethylphos-phane)titanium (24), a 1 2 mixture of the // -cyclopropene complex 25 and the vinylcarbene complex 26 is obtained in 71% yield. On the other hand, if the same starting complex is slowly added to a solution of 3,3-dimethylcyclopropene, the tricyclic compound 27 is the only isolable reaction product. 27 can also be obtained from reaction of the w-complex 25 with additional 3,3-dimethylcyclopropene at 0"C. 

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