Spirobicyclic Derivatives

Extension of the principle of axial-equatorial ring placement to spirobicyclic systems leads to the following general structural representation for a trigonal bipyramidal geometry  

2) This acyclic trigonal bipyramidal molecule is included for comparison purposes. 

Rectangular pyramidal geometry is observed, as a result of the following complementary effects, in spirobicyclic derivatives of the type (C6H402)2PY, where Y is less or equal in electronegativity to oxygen  

822(11) A], whereas the P-0 distances are identical, though longer than in the rectangular pyramidal derivatives 1—5. 

Distorted trigonal bipyramidal geometry is observed for the extended spirobicyclic diazadiphosphetidines 17 and 18. This is to be expected, as the presence of two different P-N distances does not lead to additional ring strain in the planar di-phosphetidine ring system because of its Q symmetry. From Table 2.5.2. it may be seen that the extended spirobicyclic system leads to considerable angular distortion at phosphorus in these two derivatives. 

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Table Z5.1. Bond lengths and angles in spirobicyclic derivatives of the type (05114X2)2. Comparison with idealised >31, or C4V geometries... 

Fig. 2.5.3. Spirobicyclic derivatives with unsaturated ring systems containing heteroatoms bonded to phosphorus. Postulated order of increasing trigonal bipyramidal character...

Not included in this stable are the monocyclic betaine type structures (I) and (II), Fig. 2.4.1., the intermediate square pyramidal/trigonal bipyramidal spirobicyclic derivatives 6, 10,... 

The spirobicyclic derivatives 5-p-bromophenoxy-10-methyl-2,2,3,3-tetrakis-trifluoromethyl-1,4,6-trioxa-10-aza-5 -phosphaspiro [4.5 Jdecane151, 1,6-dioxa-4,9-diaza-5- s-phosphaspiro[4.4 jnonane15 2 and 2-phenoxy-2,2-o-phenylenedioxy-4,5-bis(trifluoromethyl)-2,2-dihydro-l, 3,2-dioxaphospholene1 s3. ... 

The continued interest in spirobicyclic derivatives indicated by the structures listed above is also reflected in theoretical contributions on the quantitative description of structural distortions of cyclic phosphoranes in terms of the Berry exchange coordinate157 and on computer simulation of phosphorane structures158. ... 

It has been found that the general structural features which stabilize a square pyramidal geometry (or rectangular pyramidal geometry for spirobicyclic derivatives) for phosphoranes23 may be extended to the isoelectronic pentacoordinate silicon anions. These requirements may be summarized as follows ... 

Scheme 37. Conversion of uronate rearrangement products to spirobicyclic derivatives [71,72]...

It has also been reported that treatment of the alkyl iodide 42 with activated zinc led to the spirobicyclic ketone 43. Due to the presence of an activated carbon—carbon bond, THF was a suitable solvent and kinetic studies strongly supported an anionic carbozincation process arising from an open-chain organozinc, although a small part of the cyclic product may derive from an initial radical pathway (equation 15)32. 

A pronounced chirality effect on the chemical reactivity of ylid complexes is to be expected from propeller-type ligands with a C2 rotation axis. In a first attempt to synthesize pertinent examples, a spirobicyclic ylide 1 was designed (11) and converted into a novel lithio-derivative 2, which on reaction with metal halides yields multi-spiro complexes of unusual geometries ... 

The preparation of four different types of oxaza spirobicyclic systems (143) by 1,6- and 1,7-hydrogen atom transfer promoted by phosphoramidyl radicals in carbohydrate models has been described. The N-radicals are generated by reaction of dibenzyl phosphoramidate derivatives of C-glycosides with (dia-cetoxyiodo)benzene and iodine through a homolytic fragmentation of iodoam-ide intermediates (Scheme 37). 

Fig. 2.5.2. Spirobicyclic pentacoordinate phosphorus derivatives which have been analysed by X-ray diffraction...

Square (rectangular) pyramidal geometry is observed for spirobicyclic systems with very strained small ring systems e.g. for derivatives of the type (C6H4X2)2PY,... 

In the same vein, Canesi s group also reported a method to prepare spiro[5.5] undecanyl ring systems 150 from phenols 147 para-tethered to a free alkyne moiety [98]. The phenoxenium ions 148 undergo carbo-spirocyclization into the vinyl cations 149, which are then trapped by solvent-derived nucleophiles to furnish the spirobicycles 150 in good yields (Fig. 37) [98]. 

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