Cyclopropenation, carbene synthesis, diazo

Fluorodiazirines have also been used extensively as carbene photoprecursors for cryogenic matrix isolation " and laser flash photolysis studies. Photolysis of fluorophenoxydiazirine has made observation of the corresponding carbene directly observable in yet another fashion encapsulated in a hemicarcer-and. Diazirines have been used for studies in solution of carbene philicities as well as for synthesis of cyclopropanes and cyclopropenes.Of course, diazo compounds can also serve as carbene photoprecursors, but for carbenes such as difluorocarbene, the diazo precursor is either nonexistent or extremely unstable. 

The ruthenium-catalyzed addition of diazo compounds to alkynes has led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoalkane and one of alkyne [106] (Eqs. 82,83). The stereoselective formation of these conjugated dienes results from the selective creation of two C=C double bonds rather than leading to the cyclopropene derivative. This is expected to be due to the possibility for the C5Me5RuCl moiety to accomodate two cis carbene ligands. 

The most common source of a,j5-unsaturated carbenes for cyclopropene synthesis is from the decomposition of 3H-pyrazoles which are, in turn, available from the addition of a diazo compound to an alkyne. On photolysis these compounds suffer ring cleavage to an a>diazoalkene prior to nitrogen loss and carbene formation. In this way cyclopropene 27 is available from either 25 or 26 by way of common diazo and carbene intermediates. ... 

Cycloaddition of 1,3-dipolarophiles to alkynes for the synthesis of diazo compounds can also be applied to reaction of diazoalkanes with alkynes (2-91). 2-Diazopropane and 1,2-diarylethynes readily form 3//-pyrazoles (2.229). These pyrazoles isomerize photochemically to the 4-diazo-2-methyl-3,4-diarylbutenes (2.230), i.e., to a vinyldiazo compound (Pincock et al., 1973 Arnold et al., 1976 Leigh and Arnold, 1979). Some cyclopropene (2.231) is formed in a consecutive dediazoniation, i. e., by cyclization of the carbene formed. The method is not useful for unsymmetrically substituted alkynes because these cycloadditions are not regiospecific. It is, however, applicable to the synthesis of diazoalkenes with alicyclic... 

The metal-carbenoid intermediate has been widely applied in organic synthesis for cycloaddition, cyclopropanation, and selective C-H bond insertion [245, 246]. The traditional methods to prepare metal carbenoids are from diazo compounds, and the recent reports have shown the feasibility to generate metal carbenes or carbenoids in situ from some precursors, such as alkynes [247] and cyclopropenes [248]. With great efforts, the metal-carbenoid chemistry was esteemed as one efiftcient redox-neutral C(sp )-H bond functionalization protocol (Scheme 2.42). Herein, we list several key reviews in this topic to readers for extending reading [249-254]. 

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