Silicon compounds cycloaddition

The synthesis of iminosilanes has opened up a new field of silicon-nitrogen chemistry. These compounds have an interesting chemistry. As shown, many addition and cycloaddition reactions are possible. Many new and surprising results are likely in the future. 

Comparable compounds have recently been described by Weidenbruch et al., starting from cyclotrisilanes as a source for di-organylsilylenes [10]. Whether these complexes are formed by electron donation from the nitrogen lone pairs to silicon with subsequent electron transfer from silicon to the bipyridyl system or by a [4+1]cycloaddition process involving the diazadiene fragment is still uncertain. 

Several compounds containing Tt-bonds show reactions with 1 which most likely proceed via [2+1] or [4+1] cycloaddition processes, but no detailed mechanistic studies have been performed so far. Not unexpectedly, the electron-rich species 1 preferentially reacts with electron-poor substrates, and ring-strained or dipolar intermediates rearrange or react further to more stable products in a sometimes rather complicated and surprising fashion. In a few cases even the pentamethylcyclopentadienyl substituents at silicon are involved in the reaction pathways. 

Most of the reactions of silicon or germanium organic compounds proceed photochemically via a radical mechanism or via a cycloaddition mechanism (Chapters 4 and 6). There are few examples of nucleophilic addition of RSi or RGe to Cgo [118,119]. Reaction of silyUithium derivatives RjSili or germyllithium derivatives RjGeLi with different alkyl- and aryl-substituents R yields mainly the 1,2-adduct 28 or the 1,16-adduct 29 1,4-addition and dimerization of two fullerene-units was also found as a minor pathway. One example is given in Scheme 3.15. 

AIkynyl(diisopropylsilyl)oxy-diazoacetates (295) undergo intramolecular 1,3-dipolar cycloaddition in good yield when R = H (isolation of silver pyrazolide 296 was possible) and R, R = Me,Me or (CH2)s, but no reaction occurred when R = H, R = H or R =Me (340). The silicon substimtion is apparently crucial. Replacement of the Si(i-Pr)2 in 295 (R = R = H) by Si(f-Bu)2 allowed an uncatalyzed intramolecular [3 + 2] cycloaddition to take place [xylene, 140-160 °C, 11% yield (340)], while Ag(I) catalysis led to decomposition. A diazoacetic acid (2-propyn-l-yl)oxysilyl ester also produced a bicyclic pyrazole, but in low yield. On the other hand, the same diazo compound 295, which reacted intm-molecularly under silver ion catalysis, underwent dimerization by an /nfermolecular... 

Salts 782 and 783 having chloro or triflate substituents at silicon could also be prepared by partial metal-halogen exchange347. The elimination of MX (X = Cl, OTf M = Li, K) from these compounds to give silanediimines did not occur, but may lead to substitution products like 784 (equation 261). Addition reactions and a [2 + 2] cycloaddition are reported (equation 262 and 263). 

Phospha- and arsasilenes undergo [2 + n] cycloadditions (n = 1-4), thermal decomposition, and reaction with P4, sulphur and tellurium. In order to avoid excessive redundancy the interested reader should consult the two reviews by Driess on this subject376,377. A number of theoretical treatments on unsaturated silicon phosphorous compounds, apart from those cited in the above-mentioned reviews, has been published394-398. 

The reaction of methylenecyclopropanes with transition metal complexes is well known to promote a catalytic a-ir cycloaddition reaction with unsaturated compounds, in which a trimethylenemethane complex might exist71-76. Recently, much interest has been focused on the interaction of strained silicon-carbon bonds with transition metal complexes. In particular, the reaction of siliranes with acetylene in the presence of transition metal catalysts was extensively investigated by Seyferth s and Ishikawa s groups77-79. In the course of our studies on alkylidenesilirane, we found that palladium catalyzed reaction of Z-79 and E-79 with unsaturated compounds displayed ring expansion reaction modes that depend on the (Z) and (E) regiochemistry of 79 as well as the... 

The influence of Lewis acids on the 4 + 2-cycloaddition of (2ft,2/ft)-A,iV/-fumaro-ylbis[fenchane-8,2-sultam] with cyclopentadiene and cyclohexadiene was investigated by IR studies of the sultam compexes with various Lewis acids.101 The first enantios-elective silicon Lewis acid catalyst (91) catalysed the Diels-Alder cycloaddition of methacrolein and cyclopentadiene with 94% ee.102 [A1C13 + 2THF] is a new and efficient catalytic system for the Diels-Alder cycloaddition of a,/9-unsaturated carbonyl compounds with dienes under solvent-free conditions.103 Dendritic copper(II) triflate catalysts with a 2,2 -bipyridine core (92) increased the chemical yields of Diels-Alder adducts.104... 

One silicon tethered example that is unique in its selectivity is the cinnamyl tethered silyl enol ether shown in Sch. 17. Unlike all of the other silyl tethered examples, this compound gives a photoadduct that is the result of a cross 2+2. However, it is the product expected if the cycloaddition is a stepwise process involving radical intermediates. It is also the product expected if the reaction pathway is controlled by 7i-stacking. 

In view of the preference of the tetrasilabuta-1,3-diene 139 for the s-cis form, it seemed worthwhile to examine its behavior in [4 + 2] cycloadditions of the Diels-Alder type. Since 139, like many disilenes, should behave as an electron-rich diene, we attempted to react it with various electron-poor and also with some electron-rich olefins. No reaction was detected in any case. Only in the presence of water did 139 react with quinones to furnish the unsymmetrically substituted disilenes 36 and 37 (see Section III.A). The effective shielding of the double bonds by the bulky aryl groups and, above all, the 1, 4-separation of the terminal silicon atoms of about 5.40 A appear to be responsible for these failures. Thus, it was surprising that treatment of 139 with the heavier chalcogens afforded five-membered ring compounds in a formal [4 + 1] cycloaddition (see below). 

Three additional rings systems containing endocyclic Si=Si double bonds were obtained from the tetrasilabutadiene 139. Since Diels-Alder products of this compound have as yet not been synthesized, probably because of the steric overcrowding and the large 1, 4-separation of the terminal silicon atoms, it was surprising to find that the action of sulfur on 139 resulted in a formal [4 + 1] cycloaddition to furnish the thiatetrasilacyclopentene 154 in high yield (equation 39)142. 

Diazoacetic acid silyl esters can be prepared by fra t-esterification of tert-butyl diazoacetate with trialkylsilyl triflate <1985JOM33>. Analogously prepared (alkenyloxy)silyl 203 and (alkynyloxy)silyl diazoacetates 206 underwent silicon-tethered 1,3-dipolar cycloaddition reactions as shown in Scheme 37 and Equation (38). Compound 205 resulted from a lateral criss-cross cycloaddition of the intermediate azine 204, which was formed from two molecules of 203 by diazo + diazo or diazo + carbene reaction <2000T4139>. On the other hand, when silyl diazoacetates 206 were kept in xylene at 142 °C for 1 h, bicyclic pyrazoles 207 were obtained (Equation 38). 

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