Cyclopropyl carbenes

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Reactions of cyclopentyne with alkenes gives [2+2] cycloadduct with complete retention of stereochemistry (Scheme 24) [120], Laird and Gilbert observed the expected [2+2] cycloadduct along with the polycyclic adduct in the reaction of norbomyne with 2,3-dihydropyran (Scheme 24) [121], and located a cyclopropyl-carbene intermediate [122],... 

Some fragmentation reactions from cyclopropyl carbenes and aziridine derivatives are as follows ... 

Under suitable conditions, carbon atoms react with olefines to form allenes 46 presumably via intermediate cyclopropyl carbenes viz. 

Evidently a large part of the energy liberated in the approach of the carbon atom to ethylene will go into this normal mode — which is the one required for conversion of 30 to 31. Unless the interconversion of vibrational energy is incredibly efficient, one would then expect the initially formed 30 to be converted to 31 even at the lowest temperatures. The fact that allene is formed at -190° is not therefore surprising. On the other hand the existence of a large barrier between 30 and 31 would prohibit rearrangement of 30 if formed under milder conditions free cyclopropyl carbenes do not rearrange to allenes if formed by conventional methods in solution 49). 

Fig. 2.32. Possible mechanism of the formation of cyclopentenones and cycloheptadienones from alkoxy(cyclopropyl)carbene complexes [373],...

The formation of cycloheptadienones from alkoxy(cyclopropyl)carbene complexes and alkynes (Entry 5, Table 2.24) [388,389] proceeds essentially by the same mechanism as the Dotz benzannulation reaction (see Figure 2.32). The cyclopropyl group participates in the electrocyclic rearrangement as the equivalent of a vinyl group. 

Both thermal (120 °C at 0.005r) and photochemical decomposition of the tosylhy-drazone salt (21) were proposed to proceed by initial formation of the cyclopropyl-carbene followed by fragmentation to biradicals (22) and (23), which proceeded to 1- and 2-vinylnaphthalene and benzobarrelene (24). ... 

The assembly of highly functionalized cydohexene derivatives was achieved by Lee and Shin using a highly stereoselective tandem reaction. In this process, the nucleophilic partidpation of the O-Boc group appeared to intercept a carbocationic (or cyclopropyl carbene) gold intermediate [163]. 

A different strategy is utilized to prepare Fischer-type cyclopropylmethoxycarbenium complexes. [LM C=(OMe)(c-Pr)]+ by alkylation of cyclopropylcarbonyl complexes c-PrC(0)ML (vide supra). Thus, treating acyl complex FpCO(c-Pr), obtained from the reaction of NaFp [Fp = Cp(CO)2] and c-PrCOCI, with Me,0+BF4 in CH2C12 gives cyclopropylmethoxycarbenium fluoroborate complex salt [Fp=C(OMe)(c-Pr)]BF4 (equation 68) which can be further reduced to the corresponding Schrock-type cyclopropyl carbene complex (vide infra, Section II.C.2)142. 

The more bulky 1-ethoxycyclopropyl group also induced a complete Z-diastereoselec-tivity to the Michael reaction of the ethynyl carbene complex with dimethylamine (equation 179)246. This is due to the sterically favored arm-position acquired by the bulky group in the intermediate. Unlike the parent cyclopropyl carbene complex, which gave only... 

Cyclic diazocompounds are very unstable and must be prepared in situ at low temperature on warming the solution spontaneous decomposition takes place. Cyclopropyl carbene (from diazocyclopropane) adds stereospecifically to 2-butenes . Friedman and Schechter have examined the nature of the product distribution as a function of progressive ring enlargement... 

Singlet generated cyclopropyl carbenes, e.g. 253 (Z = CR), give cyclobutadienes by Wolff-like rearrangement whereas alkyne products most likely result from the triplet carbene 34 4.-347 nitrene analogue 253 (Z = N ) is thought to behave similarly but azacyclobutadiene is not isolable from the Sj reaction as it fragments to alkyne and nitrile. 

The structure of A, as deduced in Problem 31.4, was a trans-alkene in a seven-membered ring, i obviously formed from the cyclopropyl carbene released when nitrogen is lost from the c compound. 

Side products are observed in the case of both carbenes for example, 1-chlorocyclobutene was formed from chloro(cyclopropyl)carbene via 1,2-carbon shift (kinetic and activation parameters °° of this rearrangement have been studied), and products of 1,2-carbon and hydride shifts (kinetics of these reactions have been investigated ) for chloro(cyclo-butyl)carbene. The chloro(cyclobutyl)carbene is particularly prone to these rearrangements. 

An explanation for the discrepancy between the experimental and calculated selectivity indices of chloro(cyclopropyl)carbene has been made. ° ... 

Procedure A A solution of pentacarbonyl[methoxy(cyclopropyl)carbene]chromium(0) (0.276 g, 100 mmol) and an alkene (2.00 mmol) in THE (20 mL) was added dropwise via a syringe pump over a period of 5 h to refluxing THE (tOOmL) under N2. 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

Photochemical conversion of an amino(cyclopropyl)carbene (obtained from cyclopro-pyl(methoxy)carbene with optically pure D,i.-erythro amino alcohols) gave a cyclopropyl-substituted lactone, e.g. formation of 13, which upon hydrogenolysis gave a chiral cyclopropyl-glycine system.The reaction proceeds via a ketene complex. 

Experience has shown that cyclopentadiene annulatlon of 2,3-dimethylenebicyclo[2.2.2]octanes can be efficiently realized by means of the Skattebtfl procedure.7 However, the added strain in 2,3-dimethylenenorbornanes reroutes the rearrangement instead into vinylallene formation. This phenomenon has been attributed to an inability on the part of the torsionally-constrained empty carbene p orbital to interact with the flanking double bond. This structural inhibition 1s entirely alleviated by positioning the cyclopropyl carbene completely external to the norbornene ring as in the present example. The heightened conformational maneuverability of the carbenoid center 1s conducive to exclusive cyclopentadiene ring formation. 

Cyclopentenones have been also obtained from alkoxy(cyclopropyl)carbene complexes and alkynes via a cyclopentadienone intermediate. [85] The intramolecular version of this type of reaction has been studied intensively. 

Figure 13.26 The orbital overlap of the orbital basis of the coarctate transition state of the cyclopropyl carbene fragmentation is topologically equivalent to the % system of...

In contrast to 1,6-enynes with a terminal triple bond, substrates with an internal alkyne suffer cleavage of both multiple bonds (Scheme 4-22). In this case, the ring opening of the cyclopropyl carbene and 1,2-migration of the carbene fiagment are followed by a proton shift and removal of the cationic gold catalyst. 

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