2.3- Dimethylbutadiene, reaction with

The preparation of resin-bound nitroalkenes via a microwave-assisted Knoevenagel reaction of resin-bound nitroacetic acid with aryl and alkyl substituted aldehydes is reported. The potential of these resin-bound nitroalkenes for application in combinatorial chemistry is demonstrated by a Diels-Alder reaction with 2,3-dimethylbutadiene (Scheme 8.9). It is also used for one-pot three-component tandem [4+2]/[3+2] reactions with ethyl vinyl ether and styrene 46... 

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

Tab. 7.2 Diels-Alder reactions with 2,3-dimethylbutadiene (12) and isoprene (13) by use of the GS/MW process [27, 30, 31],...

Introduction of trimethylsilyl substituents attached directly to the ot-carbon atom of a-(benzotriazol-l-yl)alkyl thioethers provide new opportunities. Thus, treatment of lithiated monosubstituted a-(benzotriazol-l-yl)alkyl thioethers with chlorotrimethylsilane produces a-(trimethylsilyl)alkyl thioethers 837. In reactions with hexamethyl-disilathiane and cobalt dichloride, thioethers 837 are converted to thioacylsilanes 838 that can be trapped in a Diels-Alder reaction with 2,3-dimethylbutadiene to form 2-alkyl-4,5-dimethyl-2-trimethylsilyl-3,6-dihydro-27/-thiopyrans 839 (Scheme 133) <2000JOC9206>. 

Diels-Alder reactivity of 4-aza-6-nitrobenzofuroxan 235 has been studied via reactions with cyclopentadiene, cyclohexadiene, and 2,3-dimethylbutadiene this has led to three types of Diels-Alder adducts, namely the normal Diels-Alder adducts 234, 236, a Diels-Alder hetero-adduct 238, and the di-adducts 237, 239 arising from a minor dinitroso tautomer of compound 235 (Scheme 59) <1999CC1009, 2000JOC7391>. 

The reactions of 4-nitrobenzodifuroxan 242 with a series of common dienes, such as cyclopentadiene, cyclohexa-diene, isoprene, 2,3-dimethylbutadiene, and 1-acetoxybutadiene, with ethoxymethyleneacetylacetone were found to proceed very readily to afford stable cycloadducts, which are the result of highly stereoselective normal electron-demand (NED) Diels-Alder reactions. Due to the additional activation provided by the two adjacent furoxan rings, the nitroalkene double bond of compound 242 is also prone to undergo NED reactions with less reactive dienic structures, such as the enol form of ethoxymethyleneacetylacetone and the in situ generated 2-ethoxy-4-(2-furfur-yl)buta-l,3-diene <2004TL1037, 2005T8167>. 

Compound 64, after reaction with 2,3-dimethylbutadiene, produces a cyclic stannylene 66 in an oxidative addition step104. 

The germabenzene species 15 reacts in two different fashions with a variety of substrates to give cylcoaddition products (Scheme 7).30 With MesCNO and 2,3-dimethylbutadiene, 15 behaves similarly to a compound with a single Ge-C double bond, whereas in reactions with styrene and phenylacetylene, 15 behaves as a 1-germabuta-l,3-diene to give Diels-Alder-type adducts. The germanium-carbon doubly bonded species 23 reacts with nitriles in several different ways, including as a 1,2-dipolar species with Bu CN, as a 1,4-dipolar species with PhCN, and as a base with various /3-functionalized nitriles (Scheme 8). 

Later, Wender and co-workers showed that an intermolecular reaction with disubstituted alkynes and 2,3-dimethylbutadiene provided PKR products in high yields under similar conditions/... 

Isoxazoles with electron-withdrawing substituents readily undergo cycloaddition reaction as dienophiles. For example, the nitroisoxazoles (382) react with 2,3-dimethylbutadiene to form the adduct (383) (90JOC1227). Similar reaction with 4-nitro-2-phenyloxazole (384) leads to (386) possibly via initial adducts (385) (93CC978). 

Further evidence for the generation of silabenzene (46b) and 5-silafulvene (57) as intermediates comes from their reaction with 2,3-dimethylbutadiene and benzophenone,... 

In contrast to their sulfur and selenium analogs, telluroaldehydes 20 and telluroketones have not yet been isolated in the pure state and are known in the form of their complexes with transition-metal ions (see, for instance, 89JA8749). Therefore in the reaction with 2,3-dimethylbutadiene these were used in situ. To generate 20 two different approaches were... 

Scheme 5 Effect of pressure and catalyst in the reaction of dimethylbutadiene (12) with the indole 13 [28]...

Reactions with dienes and carbonyl compounds did either not occur below 150°C (e.g. 2,3-dimethylbutadiene, cyclopentadiene, tetrachloro-a-pyron acetaldehyde), or were accompanied by decomposition (2,3-dicarbomethoxybutadiene, hexachlorocyclopentadiene, 1,3-diphenylisobenzofuran acrolein ). However, 1,3-dipoles reacted readily to give well-defined addition products ... 

This opens the interesting question of whether upon replacing the hydrogen atoms with suitable substituents it would be possible to realize a linear disilyne with an Si=Si triple bond. As early as 1986 West and coworkers150,151 reported the preparation of a synthon of dimethyldisilyne, bis(7-silanorbornadiene) 163 which, at very high temperature, sets the desired molecule free, as demonstrated, for example, by trapping reactions with anthracene or 3,4-dimethylbutadiene to give the products 164 and 165, respectively (equation 42)152. 

Kinetic studies [205] have demonstrated that aromatic thioketones are highly dienophilic. Thiofluorenone exhibits an extremely high rate constant (5.12 x 10101/mol s) for the reaction with cyclopentadiene and an activation enthalpy as low as 8.2 kcal/mol with 2,3-dimethylbutadiene. 

The perfluorinated cyclopropenes (292, X = C(CF3)NH) may be prepared from (293) by 1,3-dipolar addition of diazomethane followed by desulphurisation with, triphenylphosphine to produce, (294), and then thermolysis. Addition of (292, X = C(CF3)0) to 2,3-dimethylbutadiene occurs predominantly in an endo-manner, the intermediate undergoing an intramolecular ene-reaction to produce (295). In the same way reaction with pyrrole leads to (296), in this case presumably by an intramolecular nucleophilic attack in the initially formed endo-adduct 237). 

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