Routes from alkene precursors

Potentially, phosphaalkenes can be precursors to phosphinidenes in the same manner that carbenes can be formed from alkenes. This latter metathesis route deserves more attention in light of the recognition that stable carbenes can be in equilibrium with their dimers [11]. However, a discussion on phosphaalkenes is outside the scope of the present survey. 

Two routes have been shown to produce sulfanyl- and selanyldifluoromethyl-phosphonates (342). Generation of phosphonodifluoromethyl radicals (343) from such precursors and their addition reactions with alkenes represents a... 

Fluorothiiranes are also prepared by cyclization of open-chain precursors. Treatment of sulfenyl chloride 92 with the triethylamine-boron triiluoride complex gave thiirane 30 in 68% yield. Tris(diethylamino)phosphine transformed disulfide 94, prepared from alkene 93 and sulfur monochloride, into thiirane 95. An indirect route from a chlorodisulfide that has the advantage of utilizing both halves of the molecule is exemplified by the transformation of 96 into thiirane 97. ... 

Functionalized olefins are important building blocks for organic synthesis. Catalytic olefin CM is a convenient route to functionalized and higher olefins from simple alkene precursors. One of the most appealing facets of this... 

Another intramolecular cyclization of amine 58 is the last step in the pyrrolo[3,2-fiflpyrimidine analogue 59 synthesis. Intermediate 58 formed in situ from nitro precursor 57 by reduction with tin(II) chloride is a key step of this version of Leimgruber-Batcho synthesis leading to formation of 59 finally. This simple three-step route to 59 started from the coupling of electron-poor dichloronitropyrimidine 54 with p-fluoroenamine 55 to form alkene 56. Regioselective nucleophilic... 

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. 

In 1998 it was revealed that allenylidene-ruthenium complexes, arising simply from propargylic alcohols, were efficient precursors for alkene metathesis [12], This discovery first initiated a renaissance in allenylidene metal complexes as possible alkene metathesis precursors, then it was observed and demonstrated that allenylidene-ruthenium complexes rearranged into indenylidene-ruthenium intermediates that are actually the real catalyst precursors. The synthesis of indenylidene-metal complexes and their efficient use in alkene metathesis are now under development. The interest in finding a convenient source of easy to make alkene metathesis initiators is currently leading to investigation of other routes to initiators from propargylic derivatives. 

From the three direct [2 + l]-cycIoaddition routes, path a employing electronrich olefins and acceptor-substituted carbenes is the most efficient one, since the alkenes can be synthesized from carbonyl compounds or other precursors and the carbenes are produced from easily available diazo alkanes. Therefore this very flexible mode to construct donor-acceptor substituted cyclopropanes is by far the most frequently used route. 

There has been a number of developments in the use of salicylaldehydes as precursors of both chromenes and chromans. Alkenes activated by acyl, formyl, nitrile and phenylsulfonyl groups react with 2-hydroxybenzaldehydes and 2-hydroxy-1-naphthaldehyde under Bayliss-Hillman conditions to yield 3-substituted chromenes via the in situ dehydration of the initially formed chroman-4-ol <02JCS(P1)1318>. In like manner, P-nitrostyrenes yield 2- and 2,2-substituted derivatives of 3-nitrochromenes <02H(57)1033>. A simple route to 2-phenyl-2H-chromenes starting from salicylaldehyde and utilising a Pd(0)-catalysed cyclisation of an allylic acetate has been described <02SC3667>. 

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