Vinylcarbene-cyclopropene rearrange

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. 

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. 

Among the methods at hand to synthesize cyclopropane derivatives, carbene addition to alkenes plays a prominent role 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 rearrangement 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). 

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). 

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

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) ... 

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. 

On standing for 18 h at 20 °C in the absence of an alkene, cyclopropene (151, R = Et) rearranges to the vinylsulphine (154), which can be trapped by cycloaddition to diazopropane photolysis of (150, R = 4—MeC6H4) in a similar way leads to a high yield of (155), and an intermediate vinylcarbene (153, R = 4—MeC6H4) may be trapped by ethyl vinyl ether. In each case the intermediate vinyl carbene apparently... 

Photolysis of matrix isolated diazo(2-furyl)methane led to the aldehyde (79) by stereospecific rearrangement of the carbene (80). " The corresponding 3-furyl compound gave the (5-Z)-methylenecyclopropene (81) by ring closure of the initially formed vinylcarbene to give cyclopropene (82) followed by ring opening of the furan. 

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... 

In contrast, carbene-to-carbene rearrangements are often reversible vinylcarbene-to-cyclopropene interconversions. High temperatures are required for silylene rearrangements because ring-closure via C-H insertion is mildly exothermic and has a significant barrier, higher than that... 

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)). 

Not all vinylcarbenes will cyclize to the cyclopropene. For example, those with an acyl substituent on the carbene-bearing center preferentially undergo a Wolff rearrangement, but the most significant limitations to the scope of this synthetic route arise from the problems of making the required 3//-pyrazoles. Many routes exist, but as far as cyclopropene synthesis is concerned, the 3/f-pyrazoles have been made (1) by addition of diazoalkanes to alkynes (2) from dihydropyrazoles (3) from cyclization of a,/l-unsaturated tosylhydrazones and finally (4) by modification of the structure of other 37/-pyrazoles. 

The stereochemistry of the ring opening in the above cyclopropenes has also been analyzed at the semiempirical level.The three 3-alkylcyclopropenes 9 (R = Me, i-Pr, CgHn) all rearranged to vinylcarbenes 10 at ambient temperature, and these were trapped by electron-rich and electron-deficient alkenes. ... 

The photolytic conversion of 3,3-dimethyl-l-methylsulfanyl-2-trimethy]silylcyclopropene into 3-methyl-l-methylsulfanyl-l-trimethylsilylbuta-1,2-diene (2c) has been suggested to occur either through a vinylcarbene or a cyclopropylidene. However, a computational study of the rearrangement of tetrakis(trimethylsilyl)cyclopropene (3) to the corresponding allene 2j indicates that tetrakis(trimethylsilyl)cyclopropylidene (4) is not an intermediate. ... 

In a related reaction two molecules of cyclopropene 2 were converted into a cyclohexadiene 5 in a reaction which has been explained in terms of ring opening of one molecule of 2 to a vinylcarbene 3 which undergoes a 1,4-hydrogen shift to produce a diene 4 this then undergoes cycloaddition to the second molecule of 2 and the product undergoes cyclopropyl to allyl rearrangement. ... 

In a related reaction, generation of vinylcarbene 164 by photolysis of 3,3-dimethyl-3//-pyrazole 163 in the presence of furan affords a 1 2 mixture of cyclopropene 165 and Cope substrate 166, which slowly rearranges at room temperature to give 167897. 

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