Carbenes vinylic

Photolysis of the tosylhydrazone sodium salt (94) in diglyme gave, as the major product, 1,3-diene (95) by [l,2]-vinyl shift in the intermediate carbene. Vinyl migration occurs with retention of configuration and is postulated to occur in the singlet manifold. 

Insertion reaction of a vinyl carbene (terminal acetylenes)... 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Dipoles without a double bond but with internal octet stabilization, referred to as the allyl anion type, are shown in Table 3. A third group, 1,3-dipoles without octet stabilization such as vinyl carbenes, iminonitrenes, etc., is known, but these are all highly reactive intermediates with only transient existence. Reference is made to this type where appropriate and in Table 4 (p. 146). 

Kinetic investigations of alkylchlorodiazirine thermolysis were carried out in the gas phase 70JCS(A)1916). Chloromethyldiazirine (232) decomposition follows first order kinetics giving nitrogen and vinyl chloride, easily interpretable as isomerization of a carbene. 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

Vinyl Triflalc Chemistry Unsaturated Cations and Carbenes... 

An interesting class ot covalent Inflates are vin l and ar>/ or heteroaryl Inflates Vinyl inflates are used for the direct solvolytic generation of vinyl cations and for the generation of unsaturated carbenes via the a-elimination process [66] A triflate ester of 2-hydroxypyridine can be used as a catalyst for the acylation of aromatic compounds with carboxylic acids [109] (equation 55)... 

Intermediate 37 can be transformed into ( )-thienamycin [( )-1)] through a sequence of reactions nearly identical to that presented in Scheme 3 (see 22— 1). Thus, exposure of /(-keto ester 37 to tosyl azide and triethylamine results in the facile formation of pure, crystalline diazo keto ester 4 in 65 % yield from 36 (see Scheme 5). Rhodium(n) acetate catalyzed decomposition of 4, followed by intramolecular insertion of the resultant carbene 3 into the proximal N-H bond, affords [3.2.0] bicyclic keto ester 2. Without purification, 2 is converted into enol phosphate 42 and thence into vinyl sulfide 23 (76% yield from 4).18 Finally, catalytic hydrogenation of 23 proceeds smoothly (90%) to afford ( )-thienamycin... 

Photolysis of the sulphinyl-3H-pyrazole 587 in ether or methylene chloride leads to the formation of a relatively stable carbene 588 that can be identified by physical methods. When the irradiation is performed in ethyl vinyl ether or in furan, the expected cyclopropanes are formed smoothly and stereospecifically683 (equation 374). 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

An unexpected varying regiochemistry in intramolecular benzannulation has also been observed in the synthesis of cyclophanes. As mentioned above, there are only two possible regiochemical outcomes in the benzannulation reaction, which differ in the direction of alkyne incorporation. / -Tethered vinyl-carbene chromium complexes undergo an intramolecular benzannulation reaction with incorporation of the tethered alkyne with normal regioselectivity to give meta-cyclophanes [28]. 

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]. 

Sulfur-stabilized ylides underwent photodriven reaction with chromium alkoxy-carbenes to produce 2-acyl vinyl ethers as E/Z mixtures with the E isomer predominating (Table 22) [ 121-123]. The reaction is thought to proceed by nucleophilic attack of the ylide carbon at the chromium carbene carbon followed by elimination of (CO)5CrSMe2. The same reaction occurred thermally, but at a reduced rate. Sulfilimines underwent a similar addition/elimination process to produce imidates or their hydrolysis products (Table 23) [ 124,125]. Again the reaction also proceeded thermally but much more slowly. Less basic sulfilimines having acyl or sulfonyl groups on nitrogen failed to react. 

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

More recently, such vinyl cations generated by the alkaline decomposition of 3-nitroso-2-oxazolidones have been trapped by halogens to give vinyl halides as products (108). It has been suggested that unsaturated carbenes, RjC=C , may be the intermediates in the basic decomposition of 132 (109). Indeed, when 132 (Ri=R2=CH3, R3=H) was treated with lithium ethoxide in the... 

In the proper system, under appropriate conditions, a new intermediate which is simultaneously a carbene and a carbonium ion (i.e., a carbenonium ion) may be envisioned. A particularly attractive system would be 263 and 264, as promotion of an electron from the tr system in vinyl cation 263 a onto the... 

Silylfiirans are available from acylsilane dicarbonyl confounds <96JOC1140>. Fhotocycloaddition of 35 with aUcenes leads cleanly to tetrasubstituted furans 38 in yields of 85%. A mechanism is proposed involving an alkyl propargyl biradical (as 36) that closes first to a vinyl carbene (as 37) and than to 38 <96JOC3388>. 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

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