Silylenes Subject

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

Base-free uncomplexed silylene complexes are so far only known as reactive intermediates which are generated at low temperatures and trapped by suitable reagents. Several publications related to this subject are known, but most of the work is now summarized in review articles [95]. 

Bimetallic silylene-bridged complexes have been known for a long time and numerous articles related to this subject have appeared. Several compounds have been characterized, some of them also by x-ray structure analysis [165-171], For instance, the complex Mn2(CO)8(Si(C6H5)2)2 shows a distorted (MnSi)2 four-membered ring with a Mn-Mn bond [169], In the following section selected examples which have been described recently or are of particular interest in the present context will be discussed. 

The chapter table of contents contains subjects that were either unknown or merely distant hopes a decade ago, such as persistent silylenes, the dissociation of disilenes to silylenes and terminal silylene-transition metal complexes. The kinetics and spectroscopy of silylenes and theoretical treatments of silylene structure and reactivity have made such gigantic strides in the intervening years that they represent new vistas in our understanding. 

Bridged silylene complexes are the subject of a recent comprehensive review by Ogino and Tobita338. These complexes can be classified into three types A, B and C (Scheme 9). In type A complexes there is no metal-metal bonding, the silicon is essentially tetravalent, and the bonding is similar to that in mononuclear metal-silyl complexes. In type C complexes, the bonding is best described as /j2-coordination of the Si—H bond to the metal, or alternatively as a metal-hydrogen-silicon 3-center 2-electron bond. 

Systems which contain cr(E-E)-7r conjugated systems (E = Si or Ge) have been the subject of many photochemical investigations. For the most part investigations have focussed on silicon systems, that first identified carbon-silicon double-bonded species as photoproducts . Later studies also indicated that arylsilanes undergo 1,3-sigmatropic shift reactions as well as extrusion processes yielding silylenes . ESR studies have... 

The majority of the reports included in this section are concerned with the photoreactions of organosilicon compounds. In particular, the generation and reactions of silylenes (silanediyls) have been the subject of much interest. A variety of organosilylenes have... 

Diketones often are protected as enol ethers or enamines and these selectively functionalized compounds may be subjected to complementary transformations (Scheme 94). Also silylenes can be prepared from diketones and -hydroxycarbonyl compounds by reaction with dimethyldicyanosilane. Naturally, these blocking groups are relatively sensitive to hydrolysis. On the other hand, partial solvolysis can open a route to monoprotected derivatives (e.g. 101), usually blocked at the sterically less demanding carbonyl function as 0-silyl cyanohydrins (see Scheme 95). Deprotection is finally achieved with silver fluoride in THF. 

Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review. Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc and the photoinduced electron-transfer reactions involving organosilicon compounds. ... 

Silylenes 113 are the silicon analogues of carbenes and formally contain a divalent silicon atom. We will use here the term silylene which is commonly used in the literature in preference to the systematic IUPAC term, silanediyl. Although silylenes of type 113 are the major subject of this section, other types of silylenes, such as the unsaturated 114 (1-silavinylidene) or 115 (2-silavinylidene) or cyclic silylenes, are possible reactive intermediates and we have discussed some of them in previous sections. In Section VII. A.4 silylenes other than 113 are mentioned. 

The reaction types known to produce disilenes are summarized in Chart 1 and apply regardless of the ultimate stability of the product. Historically, thermal 4 + 2 cycloreversion of complex l,2-disilacyclohex-4-enes, i.e. a retro-Diels-Alder fragmentation came first8 -14, followed by silylene dimerization15 -17. This early work produced only indirect evidence for the formation of disilenes as reactive intermediates, but in retrospect it is clear that these species were indeed produced. The early history of the subject is discussed in Reference 1. The first directly observable and isolable disilene resulted from the dimerization of photochemically produced dimesitylsilylene18, ushering in a new era in disilene chemistry. These more recent developments are described in Reference 2. 

When subjecting the disilane 48 to a newly developed pyrolysis technique, trimethylsi-lane is split off and 1-silacyclopropylidene 49 is formed as the most stable C2H2S1 isomer. Photolysis of this silylene with monochromatic light yields 50 which was identified by comparing its IR spectrum with a calculated spectrum. Whether 50 is a true cycloalkyne or a diradical, a singlet or a triplet must await further investigation as must the study of its chemical behavior. 

The Diels-Alder adduct formed by treating furan with hexafluorobut-2-yne has been subjected to a retro-Diels-Alder reaction acetylene is eliminated and the reaction provides a route to 3,4-bis(trifluoromethyl)furan. The corresponding cycloadduct (154) formed from l,l-dimethyl-2,5-diphenyl-sllacyclopentadiene and the butyne was decomposed in an attempt to generate dimethylsilylene by a symmetry-allowed cheletropic fragmentation (Scheme 50) 836 evidence for silylene formation could be obtained unless the decomposition was effected photochemically or in refluxing cumene, under which conditions the addition of tolan enabled dimethylsilylene to be trapped... 

A mixed di-f-butylsilylene has been prepared from (5)-5-hexen-2-ol and prochiral l,4-pentadiene-3-ol for synthesis of (25, 7S)-dibutyroxynonane, the sex pheromone of Sitodiplosis mosellana. The intermediate 43 was then subjected to ring closing metathesis to provide the diene 44 in 70% yield over two steps (eq 16). Deprotection of the nine-membered silylene was achieved using TBAF under refluxing condition in the presence of molecular sieves. Reduction with H2/Pt02 and diacetylation yielded the desired (25, 7S)-dibutyroxynonane in 22% overall yield. ... 

The electronic structure, including the spectroscopic aspects and the chemistry of silylenes, were the subject of some recent reviews. Here, our aim is to show, using silylenes as a case study, that it is possible to plan systematically new stable systems with unusual bonding, by using the methodology of computational chemistry in a systematic way. For other aspects we refer to the previous reviews, when appropriate. The procedure applied was the following ... 

The chemistry of stable silylenes has been the subject of considerable research since the first isolation of silicocene by Jutzi in the 80s (1). More recently, silylenes stabilized by coordination of a donating ligand on the silicon(II) atom have attracted much attention due to their high reactivity strongly related to the nature of ligands, and we have demonstrated that phosphine-stabilized silylenes display unique properties. Of particular interest, the phosphine-stabilized silicon(II)-hydride 5 immediately reacts with olefins and alkynes via [2-1-1] cycloaddition reactions. In the case of diphenylacetylene, the corresponding pentacoordinate... 

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