Puckered transition state

Assignments. - Electric modulation of vibrational rotational bands of polar molecules included a study of phosphine.120 Ringbending (puckering) transition frequencies have been measured for the phospholene (42) for the ground and excited states.121 The PD deformation band for the sulphide (43) has been assigned.122... 

Why does C4H8 adopt puckered D2d geometry As shown in Fig. 3.82, the skeletal carbon atoms twist out of planarity (with dihedral cccc = 17.9°), allowing each methylenic hydrogen to be distinguished as axial (with cccH(a) = 94.9°) or equatorial (with cccH(e) = 138.7°). The puckered equilibrium structure lies only 0.8 kcal mol-1 below the transition-state D4h structure. 

Calculations on two Wittig reactants, alkylidenetriphenylphosphorane (a non-stabilized ylid) and its benzylidene analogue (a semi-stabilized one), have been used to identify the origin of the product selectivities for the two classes. A planar transition state gives a trani-oxaphosphetane intermediate, while a puckered one leads to cis-. These two transition states were favoured by the semi- and un-stabilized reactants, respectively. 

The potential surface for die gradient path addition of ethylene to silene and the possible existence and stability of intermediates in the thermal decomposition reaction of silacyclobutane has been explored.38 The energy maximum of die multi-step process corresponds to a cyclic transition state leading on one side to a planar silacyclobutane transition state which falls to ground-state puckered silacyclobutane and on the other side to a trans diradical which fragments to ethylene and silene. 

The planar conformation is highly strained so the ring is puckered, making the two protons of each CH2 group inequivalent one is in a pseudo-equatorial position, and the other pseudo-axial. Interconversion through the planar transition state Me is slow on the chemical shift timescale. 

Currently accepted mechanism of the Wittig reaction of aldehydes with non-stabilized ylides involves the formation of oxaphosphetanes through a [2-I-2]-cycloaddition-like reaction . The oxaphosphetanes are thermally unstable and collapse to alkene and phosphine oxide below room temperature. Under salt-free conditions there is no formation of betaine intermediates. The salt-free ylides can be prepared by the reaction of phosphines with carbenes generated in situ. Vedejs etal proposed a puckered 4-centre cyclic transition state I for sy -oxaphosphetane and planar structure J for anff-oxaphosphetane. In general, the flnfi-oxaphosphetane J is more stable than the syn-oxaphosphetane I, and under equilibrium conditions (when stabilized ylides are used) the E-alkene product is favoured (Scheme 4.24). However, kinetic control conditions, which appear to dominate when non-stabilized ylides are used, would lead to Z-alkene. 

The effect of substitution is also evident in the epoxidation of verticillene (21)153 at the 7,8-double bond giving 22. Once again, the conformation of the transition state for the preferred mode of attack is, ideally, puckered. The additional three-carbon bridge prevents total ring inversion additional substitution of the double bonds limits conformational mobility. Thus, even for this 14-membered ring system high diastereoselectivity is predicted. Experimental evidence so far, however, indicates only that no second diastcrcomcr of the 7,8-cpoxidc 22 has been isolated from the reaction mixture. 

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