Carboranes alkynes

C/o50-carboranes are the most numerous and the most stable of the carboranes. They are colourless volatile liquids or solids (depending on mol wt.) and can be prepared from an alkyne and a borane by pyrolysis, or by reaction in a silent electric discharge. This route, which generally gives mixtures, is particularly useful for small c/o50-carboranes (n = 5-7) and for some intermediate c/ow-carboranes (n = 8-11), e.g. 

A convenient route to three-carbon carboranes is the hydroboration of an alkyne with a preformed dicarbaborane. For example, reaction of ethyne (or propyne) with arachno-4,5-C2B7Hi3 (70) in hexane at 120°C gives a mixture of tri- and tetra-carbaboranes, e.g. (71), (72), (73), (74) in modest yield. Access to other... 

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

The vast majority of known carboranes synthesized in the literature are derivatives of twtfo-carborane, 1,2-C2B10H12. They were synthesized from the reaction of decaborane, B10H14, with suitable alkynes in the presence of a suitable... 

The nido-carborane anion, 7,8-C2B9H12, and derivatives are generally formed by deboronating ort o-carboranes with suitable bases (COMC (1982) 5.4.2.6.6) but a direct route to two derivatives of the anion has been developed in good yields from decaborane and polarized alkynes.78... 

The research field of ortho-carborane chemistry continues to expand rapidly mainly due to the commercial availability of ort/fo-carborane itself and the recent interest of these derivatives as agents for BNCT and other applications. Synthesis of these carboranes is still dominated by reactions of decaborane with various alkynes as shown in Table 19 (Figure 2). 3-Substituted derivatives are made by capping the /[Pg.71]

Table 19 Synthetic routes to c/oso-1, 2-C2BioH- 2 (ortho-carborane) derivatives from decaborane and alkynes...

Many derivatives of isomeric 2,3-dicarba-nido-hexaborane(8) are known, available from the reaction of pentaborane(9) with alkynes [59, 60]. In contrast, the carboranes of type 43 with non-adjacent carbon atoms are rare. 

A one pot route to nido-1,6-diiodo-tctracarba-nuio-hcxaborane derivatives is depicted in Scheme 3.2-32. After iodoboration of the alkynes with BI3 the dehaloge-nation of the alkenyl derivatives presumably leads to dimerization and rearrangement to give structurally characterized 1,6-diiodo-nido-carboranes 56d [81]. 

An extensive series of stannoles results from trialkylboranes and dialkynylstannanes, notably if the alkyne substituent is H, Bu or Me3Si (Scheme 199) (78JOM( 148)137). However, if it is methyl the isomeric l-bora-4-stannacyclohexadiene is formed along with the stan-nacyclopent-3-ene (Scheme 200) (78JOM(153)153>. The lability of the tin-carbon bond makes the compounds useful synthetic intermediates, notably in the preparation of mWo-carboranes (Scheme 201) (77JOM(132)213). 

A number of reactions of platinum(O) complexes have been discussed in the earlier sections of this chapter. These reactions include protonation reactions of PtL3 to give PtHLJ, oxidative addition of HX to give PtHXL, replacement with carboranes, alkenes and alkynes (L1) to give complexes of type PtLjL. The most studied complex of platinum(O) is Pt(PPh3)3 and in Scheme 10 are outlined examples of the numerous reactions which this compound will undergo. 

Continuing with the theme of motors, coupling the bromoalkyne 612 with p-carborane produces a phenylene ethynylene axle with two carborane wheels. The crowded alkene 613 was then coupled through alkyne units to two axles producing a motor car as a mixture of two pairs of enantiomers (Scheme 239). Photoisomerisation activates the unidirectional motor as above and the unstable state thermally reverts to the stable form. Fullerene units have also been introduced as the wheels though these were less effective than the carborane unit <20060L1713>. 

The polyalkyne precursors to the hydrocarbon-based unimolecular micelles 179 allowed the testing of chemical modification at specific sites within the interior of a cascade infrastructure. 180 Thus, treatment of the alkynes 153 or 154 with decaborane afforded excellent yields of the 1,2-dicarba-cfoso-dodecaboranes 181 (o-carboranes) or with Co2(CO)8 afforded the desired poly(dicobalt carbonyl) clusters. 182 Details are given in Chapter 8. 

This method is applicable to a wide variety of alkynes (e.g., R,R = H, alkyl, aryl, alkoxy, alkenyl, alkynyl), but reagents containing OH or COOH moieties degrade the boron cage and cannot be employed. However, carboranes containing these functional groups can be obtained via electrophilic substitution on the parent compound as outlined below. 

Figure 5-11 Insertion of an alkyne into the decaborane cage to form an icosahedral RR C2B10H10 carborane.

The larger nido and arachno cages are usually obtained via degradation (in one or more steps) of icosahedral carboranes, as previously mentioned, but the smaller members are typically prepared in reactions of alkynes with neutral boranes, as in the synthesis of 2,3-RR C2B4H8 (Fig. 5-14). However, the nido-2,4-QBJHf carbons apart ion can be obtained by removal of an apex boron from closo-2,4-QB5B7 (Fig. 5-10d).8... 

Interestingly, depending on the catalyst, the diboration of vinylboranes with B2cat2 leads via seqnential diboration and dehydroboration steps either to 1,1,1-triborylalkanes (20) or to 1,1,2-triborylalkanes (21). 1,1,1-Triborylalkanes, 1,1,2,2-, and 1,1,1,2-tetraborylethane species have been proposed as intermediates in the reaction of borylated alkynes with excess Et2BH ( hydride bath ) that eventnally leads to the formation of small carboranes. Indeed, isolated triboryhnethane species with alkyl or hahde snbstituents on boron have recently been shown to rearrange to small carboranes upon thermal treatment. ... 

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