Germylenes cycloadditions

Oxadative addition reactions of germylenes are a versatile method for the formation of germanium-pnictogen bonds.147-156 Reaction of germylenes with diazabutadienes yields [4+ 1]-cycloaddition products (Equation (115)),148... 

Reaction of the digermene 57 with diazomethane or trimethylsilyldiazomethane yields the azadigermiranes 58 via a [2+l]-cycloaddition (Equation (119)).158 Coupling of a cyclic carbene and a cyclic germylene furnishes the zwitter-ionic species 59 (Equation (120)).159... 

Attempts to trap the digermene 74 in solution with benzil afforded only the cycloadduct 109 because of the [1 + 4] cycloaddition with germylene 10863 [Eq. (26)]. Another proof of the formation of germylene 108 in solution from 74 was given by cryoscopic studies.63... 

The other important type of reaction leading to the formation of two bonds between a metal and four-atom fragment is addition of carbenoids, i.e. germylenes and stannylenes, to a-dicarbonyl and related compounds. The process can be classified as redox (4 + 1) cycloaddition since the dicarbonyl fragment is reduced while the carbenoid is oxidized. Formally the simplest example of such reactions is the interaction of methylglyoxal with activated tin metal to give a cyclic stannylene, Sn(II) enediolate (see Scheme 34) <83CL1825>. 

The reaction of l,3-bis(trimethylsilyl)-l,3-diaza-2-germa(II)-indane (365) with trimethylsilyl azide to give (366) may seem to be reminiscent of several preceding transformations but, as was shown with open-chain germylenes, the process is not a cycloaddition and probably involves nitrene intermediates <89CB245>. 

In addition to carbenes and nitrenes, organometallic species such as germylenes 107 have been utilized in [2 + 1] cycloadditions and, upon reaction with thioketones, afforded the corresponding thiagermiranes which are very stable and do not decompose even when heated to their melting point438 (equation 124). 

The cycloaddition reaction of singlet GeX2 (X = F, Cl) with formaldehyde was studied employing the HF/6-311+G theory.106 The reaction proceeds in two steps barrierless formation of an intermediate complex followed by rate-determining isomerization to form the product. The results were compared with those from other cycloadditions of germylenes and silylenes. 

Cycloaddition reaction of phosphaalkyne 42 and bis(2-/-butyl-4,5,6-trimethylphenyl)germylene 43 leads to germadiphosphacyclobutene 24 in 59% yield (Equation 5) <2001CC215, 2002JOM(646)39>. 

M-M bonds in polysilanes and polygermanes as well as silylgermanes are readily cleaved by photolysis to generate reactive silylenes and germylenes which are trapped with butadienes affording the cyclic products 155 and 156 <20000M3232, 2002JOM(649)25>. In the case of 156, the replacement of At by less bulky substituents leads to small yields and the cycloaddition is accompanied by the formation of many side products. 

Cycloaddition reactions are common routes to the formation of four- to six-membered ring heterocycles. Germylenes/ germenes, and metal germylene complexes " have been utihzed extensively for these reactions (Figure 7). 

Germylenes form Lewis base complexes with the lone pair of electrons they also form Lewis acid complexes with reagents possessing lone pairs of electrons . In solution, these adducts act as intermediates in the formation of insertion products or in cycloaddition reactions. The extreme manifestation is observed when Me2Ge is combined with a reactive diene in the presence of 18-crown-6. Only the polymerized product, (Me2Ge) , is ultimately isolated. Complexes of germylenes with heteroatom-containing substrates have been characterized at low temperatures . 

Summary Monoaminosilylenes and bis(diethylamino)silylene are formed by thermolysis of amino-substituted disilanes and characterized by trapping with dienes. In the case of MeSiNMe2, cycloaddition reactions were extended to 1,4-diheterodienes, unsaturated ketones and imines allowing an easy synthetic access to functionally substituted unsaturated silicon heterocycles. The syntheses of an isolable, unsymmetric diaminosilylene and of related germylenes and stannylenes are described. The results are presented and discussed in relation to theoretical work on stabilization of donor-substituted silylenes and recent work on isolable diaminosilylenes done by or in cooperation with other groups. 

Silagermenylidene (181) has been synthesized by reductive dehalogenation of a diaryl substituted dichlorosilane in the presence of an NHC-stabilized dichloro germanium and isolated in 8% yield, representing higher analogue of vinylidene which has so far only been detected trapped in matrix. Its formal [2+2] cycloaddition with phenylacetylene has been carried out and afforded germylene (182). ... 

Theoretical study of cycloadditions of formaldehyde with cyclopropyhdene, dimethyl-silylene carbene, and dichloro-germylene carbene has been conducted at MP2/6-311-t-G and MP2/6-31G levels of theory two competitive dominant pathways have been identified in each case. Likewise four competitive dominant pathways have been found for reaction of dichlorogermylene silylene and acetaldehyde. ... 

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