Preparation, pyrolysis, and photolysis of three sterically crowded alkenes, 3,3,3′,3′,5,5,5′,5′-octamethyl-3,3′,5,5′-tetrahydro-4,4′-bi(4H-pyrazolylidene) 2,2,4,4-tetramethyl-3-(3,3,5,5-tetramethyl-3,5-dihydro-4H-pyrazol-4-ylidene)-thietane 1,1-dioxide and 2,2,2′,2′,4,4,4′,4′-octamethyl-3,3′-bithietanylidene 1,1,1′1′-tetraoxide, potential precursors to the octamethyltetramethyleneethane (TME) biradical