Hydrogermylation

Interestingly, hydrogermylation chemistry has been applied to Ge nanowires, providing an organic passivating layer on the nano wire [103]. Hydride-terminated Ge nanowires can be fabricated by etching Ge nanowires in HF. Unlike the surfaces of bulk Ge [13,121], H-terminated Ge nanowires are only stable for a few minutes in dry air [103]. However, the organic-terminated nano wires exhibit much better stability. 

Trialkylgermanes add to carbon-carbon triple bonds in the presence of transition metal catalyst [41]. A review on fhe addition-i-reaction of Ge-H functional organo-germane compounds R GeH4 to unsaturated compounds (alkenes, alkynes, ketones, aldehydes) has been published [42]. 

Digermylation of alkenes and alkynes with l,2-dichloro-l,l,2,2-tetramethylger-mane proceeded in the presence of palladium catalysts [53]. 

Terminal acetylenes react with (Ph3Ge)2Cu(CN)Li or (i t(Ge)2Cu(SMe2)Li to afford 

The germyl anion has been employed in conjunction with a metal salt [58]. It was 

On treatment of an f/,/7-urisaturated ketone with the cuprate (11) in the pres- 

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The hydrogermylation of phenylacetylene has been shown to proceed with retention at germanium12. Addition of the germyl hydride 5-3 in the presence of a Pt or Rh catalyst led to the adduct 5-23 as the major product. The enantiomer 5-23 was prepared by addition of tran.s-/3-styryllithium to chlorogermane 5 -7 prepared as shown in Scheme 10. The isopropyl analogues of 5-23 and 5-23 were similarly prepared from hydride 5-612. 

Hydrogermylations by these hydrides were found to proceed with retention of stereochemistry at Ge (equation 14)32. 

An important synthesis method of organogermanes is the hydrogermylation of olefinic and acetylenic derivatives in the presence of a catalyst. Examples of such reactions are listed below38 ... 

Some other reactions of hydrogermylation of allenes (and acetylenes) are given below ... 

The allylic germanes and stannanes obtained are useful synthons. The hydrogermylation of acetylenes can also be performed with dimethyl(2-thiophenyl)germane58. This compound can be prepared quantitatively by the Grignard reaction of Cl2GeMe2 with 2-bromothiophene (Scheme 8). 

Hydroplumbation of unsaturated compounds is more difficult than hydrogermylation and hydrostannylation. Actually, olefins are unreactive towards HPbBu3, except when they are activated. Under such conditions they are successfully subjected to hydroplumbation in ether at 0°C184 ... 

Hydrostannylation, as a counterpart to hydrogermylation for tin, using HSnCl3, is not of common use in organotin chemistry. In contrast, R3SnH is very commonly used (see Section I.B). 

Tri(2-furyl)germane63,127 130 has been shown to be a useful hydrogermylation reagent for alkenes,127,128 dienes,129 alkynes,129,130 and carbonyl species (Equations (97) and (98), Table 17).127 It has been employed to selectively hydrogermylate the double bond of a,/3-unsaturated carbonyl compounds (Equation (99)).127 This species can also be used to affect the hydrogermylcarbonylation of alkynes in the presence of CO (Equation (100)).130... 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

These reactions are important because they play a prominent role in hydrosilyla-tion and hydrogermylation processes. 

The stereochemistry is assumed to involve retention of configuration. This assumption is very plausible, since retention has already been observed for Pt insertion in a Si-H bond, as well as for homogeneous and heterogeneous hydro-silylation and hydrogermylation . Moreover, the Si-H bond is known to react generally with retention of configuration . Finally, in a parent reaction of SiH with C02(C0)g, X-ray structural analysis reveals that the reaction proceeds with retention of configuration (see Sect. 2.2.2). 

A range of addition reactions of (TMS)3GeH with alkynes, alkenes, ketones, azines, and quinones has been studied using EPR. In addition, synthetic studies of hydrogermylation of alkynes have shown that the reaction proceeds regio- and stereo-selectively, whereas reactions with alkenes do not take place (presumably owing to the reversibility of the germyl radical addition) (Scheme 29). 

In a manner similar to silylformylation, germylformylation of 1-alkynes catalyzed by 12 proceeds with Bu aGeH under CO pressure (20 atm). However, it is difficult to completely suppress hydrogermylation of 1-alkynes as the side-reaction. ... 

The wet chemical functionalization of Ge surfaces has not been as well studied as that of Si surfaces [13,20], despite the fact that the first report on Ge functionalization was made by more than 40 years ago [105]. Three methods of wet chemical functionalization have been reported on Ge surfaces to date (a) Grignard reactions on chloride-terminated surfaces, (b) hydrogermylation reactions on... 

Figure 5.12. High resolution images of Ge nanowires from Korgel and coworkers. Image (a) shows an untreated Ge nanowire with a native germanium suboxide layer. Image (b) shows a Ge nanowire with a covalently bonded hexyl monolayer attached using the hydrogermylation reaction. An abrupt surface is observed. Figure adapted with permission from Ref. [103]. Copyright 2004 American Chemical Society.

In addition to Grignard chemistry and hydrogermylation, for which analogs also exist for silicon, there is another type of functionalization chemistry that has been carried out on germanium surfaces that of alkanethiol attachment to H-terminated Ge. Alkanethiols are well known for their use in self-assembled monolayers (SAMs) on gold surfaces. 

Unlike either the Grignard or hydrogermylation reactions, which create a Ge—C bond in the organic attachment to the surface, the alkanethiol reaction creates a Ge—S—C bond configuration upon functionalization. Various 1-alkanethiols react with the hydride-terminated Ge(lll) surface at room temperature, with a well-ordered monolayer resulting from 1 day of exposure to a solution of alkanethiol in... 

Transition metal-catalyzed hydrogermylation of unsaturated substrates has been known for many years,14 but has generated little interest in comparison to hydrosilylation. However, a few instances of transition metal-catalyzed hydrogermylation have been reported, including reactions of al-kynes,15 allenes,12d and dienes.16... 

The only report of catalytic germylformylation in the literature at this time is also effected by the zwitterionic rhodium(I) complex (l,5-COD)Rh1 (T76PhBPh3)-.127 The synthesis of (Z)-3-germylalk-2-enals in good yield is always accompanied by a minor amount of hydrogermylation [Eq. (52)]. 

In fact, additions of tributylgermyl radical and tributyltin radical to activated alkenes occur at about the same rate (see refs, 38 and 101). This addition reaction is probably more readily reversible in the case of tin (because a weaker bond is formed) and therefore hydrostannylation is a less serious problem than hydrogermylation. Thus, very reactive precursors (preferably iodides) are required as precursors if germanium hydride is used with an electron deficient alkene but this is not because the germanium radical is less reactive towards halides than the tin radical. 

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